Read this before you copy any number out of this article
Every dimension, weight, cycle count and deflection figure below is quoted from a publicly published source and labelled as such. None of it is a product claim by us. Do not paste any of it into a drawing, a specification or an order without checking it against your own opening, your own panel sizes and your own authority having jurisdiction.
One more thing up front, because it changes how you should read everything after it: we build sliding doors and windows. Our folding door, swing door and entry door category pages are empty at the time of writing. Everything in this article about folding doors and about hinged French pairs comes from published public sources and from arithmetic we show you, not from our own catalogue.
Ask ten people what a bi fold French door is and nine of them will describe a mechanism. Ask them what makes it French and they will describe a picture. That split is not a quibble. It is the reason the phrase is so hard to buy from.
The word French in this context has moved. A published door-type reference traces the form back to Renaissance Italy, spreading through France in the sixteenth and seventeenth centuries, and states plainly that the defining feature historically was the use of multiple small glass panes, called lites, divided by muntins. The same source then says what most people now mean: today the term refers to the pair of hinged leaves regardless of glazing. Another published patio-door glossary keeps the older half of that meaning alive and splits hinged patio doors into two names: those with large undivided panes are usually called garden doors, and French doors are the ones whose panes are divided by grids.
So the word has done something unusual. It started as a description of glass and became a description of hardware. Both senses are still in circulation, in the same sentence, on the same spec sheet. When someone writes bi fold french door, they are almost always using the second sense, the pair-of-leaves sense, and bolting a mechanism onto it.
The word bifold has drifted the opposite way. It has no agreed referent at all. In one published folding-door guide it is an exterior patio set of two to seven leaves on a track. In another it is an interior closet door that trims by half an inch a side. In a third it is an accordion room divider. In a fourth it is a barn door that folds. These are not variations of one product. They are four products with four load ratings, four hardware sets and four sets of rules.
Throughout this article, when we say standard French door we mean the hinged pair: two leaves, one on each jamb, meeting near the middle, one of them the active leaf that carries the handle and the latch, the other held shut by bolts at the head and the sill. That is the thing most people are comparing against, and it is the thing whose behaviour we can actually reason about.
The standard answer to this question is one sentence: one folds and one swings. It is true. It is also useless, because it describes the thing you can already see in the photograph, and it tells you nothing about the five decisions that actually have to be made before anyone can price it.
Here is the answer we would put in its place, and it is the sentence the rest of this article unpacks.
A standard French door hands its weight to the two walls on either side of the opening. A bifold French door hands its weight to the beam above the opening, or to the floor beneath it, and which of those two it is changes almost everything downstream.
Everything else follows. Why a bifold asks a question of your lintel that a French pair never asks. Why the number of vertical lines you look at when the door is shut is not a style decision but a panel-count decision. Why the joint in mid-air is a compression joint on one door and a contact joint on the other. Why the flat threshold everyone wants and the weather seal everyone wants are the same knob turned two ways.
This table is the rest of the article in compressed form. It is also the table we have never seen a competitor publish, because the comparison articles in this category almost without exception run three rows: opening style, price, energy efficiency.
| The question | Standard hinged French pair | Bifold French door | What that decides for you |
|---|---|---|---|
| Where the weight goes | Into the jambs, through hinges, then into the studs either side | Up into the header if top hung, or down into the slab if bottom rolling | Whether your existing lintel has to be checked or replaced |
| Vertical lines in the view when shut | One, in the middle | One fewer than the panel count | What you look at for the many hours a day the door is closed |
| How the joint in mid-air seals | Compressed: the active leaf is pulled against an astragal carried on the inactive one | Contacted: every leaf-to-leaf joint is also a hinge and cannot be compressed | How many sealing lines you own, and of which kind |
| What it takes out of the room when open | Two quarter-circle floor areas, radius equal to one leaf width | A strip along one or both jambs, depth equal to the stacked pack | Which piece of furniture, radiator or switch you lose |
| How flat the floor can be | A sill and a stop; can be low, but the meeting joint needs something under it | Top hung can run near flush; bottom rolling runs on a raised floor track | Whether accessibility and weather sealing fight each other on your project |
| Whether it needs adjusting later | Occasionally, usually hinge and lock point geometry | On a published schedule, with a published force threshold | Whether you have a maintenance budget or an eventual complaint |
| What failure looks like | Binding, a latch that no longer lines up, a leaf dropping at the corner | Panels dragging, rising operating force, a pack that no longer stacks square | Whether you call an adjuster or order a part number |
This is the difference that gets discovered latest and costs most, because it is invisible until someone opens the wall.
A hinged French pair hangs each leaf on hinges fixed into the jambs. The load path runs sideways into the studs on either side of the opening. The header above carries the frame and not much else. That is why generations of French doors have been fitted under headers nobody ever calculated.
A bifold has two possible load paths and they are opposites. A published hardware guide for folding doors describes them this way: a top hung system hangs the panels from an overhead head track bolted to the header, with a pin or channel at the floor that only guides and holds no weight, and the whole load travels up into the beam. A bottom rolling system puts the load bearing rollers in a floor mounted track, so the weight goes straight down into the slab or subfloor and the upper track only keeps the panels lined up.
Now the number that makes this concrete. The same published source gives a header deflection limit of L over 500, typically three to four millimetres, for the top hung case, and notes that a steel lintel or an engineered beam is required. Another published guide puts it in plain language: a top hung bifold hangs its entire weight from the header, and a four panel aluminium and glass set can pull three hundred to four hundred and fifty pounds down on that beam, which is why a plain two by ten rarely does the job over an eight foot opening.
Here is the part we think nobody has written down. Residential framing headers are sized to deflection conventions of their own, written for floors and roofs, not for a track that has to stay straight to within a few millimetres. So the bifold is asking a question of a member that was never asked that question before, in a unit nobody used when they sized it. A French door does not care whether your header deflects. A top hung bifold is a machine that only works if it does not.
A published folding door specification gives a worked table of leaf weights against roller ratings. We reproduce the shape of it, not as our data but because it shows the method:
| Leaf size, millimetres | Glazing build-up | Leaf weight, kilograms | Minimum roller rating, kilograms |
|---|---|---|---|
| 800 by 2200 | 5 plus 12A plus 5 | 58 | 93 |
| 900 by 2400 | 6 plus 12A plus 6 | 78 | 125 |
| 1000 by 2700 | 6 plus 12Ar plus 6, low E | 96 | 154 |
| 1100 by 3000 | 6 plus 16Ar plus 6, laminated | 132 | 211 |
| 1200 by 3200 | 8 plus 16Ar plus 8, laminated | 168 | 269 |
Leaf weights and roller ratings as published by one folding door supplier. The rating column carries roughly a sixty percent margin over the static leaf weight, because dynamic load as a panel leaves the track can exceed static weight. Different suppliers publish different numbers for the same leaf size.
Read the last two rows against the four hundred and fifty pound figure quoted earlier and you will see two different public sources describing a four panel set at noticeably different totals. That is not a contradiction to be resolved. It is the point. Published weights belong to published leaf sizes. Change the leaf size or the glass and every number in that table is someone else's number.
Two more published constraints sit behind the same question. One public guide gives maximum panel weight of one hundred and twenty kilograms for top hung and one hundred and sixty kilograms for bottom rolling, and explains that very tall glass therefore needs bottom rolling. Another quotes named hardware ranges: one premium residential system rated to one hundred kilograms a leaf, another to one hundred and fifteen, and entry residential systems sitting around forty to fifty-five. The word bifold has more than one weight ceiling, and the one you are buying is whichever system your supplier actually quotes.
There is also a rating trap here that recurs across every moving door we have written about. The rating is per leaf, not per set. A published hardware guide states it directly: each leaf is carried by its own top roller and bottom pivot, and the rating on the component is per panel, not per set, and hardware failure traces back to specifying rollers against total opening weight rather than per panel weight.
This one is pure arithmetic, which is why we like it: you can check it yourself and it involves no trusting anybody.
Take a three thousand six hundred millimetre opening. Divide it three ways.
| Configuration on a 3600 mm opening | Approximate width of each leaf | Vertical meeting lines you look at | Published clear opening, where quoted |
|---|---|---|---|
| Standard French pair, two leaves | 1800 mm | 1 | Near full width when both leaves are released |
| Bifold, four panels | 900 mm | 3 | About 85 percent, per one published table |
| Bifold, six panels | 600 mm | 5 | About 90 percent, per the same table |
Leaf widths are our own arithmetic: opening divided by panel count, before allowances for outer frame, fixings and joints, so finished leaves are slightly narrower. Clear opening percentages are published figures from one folding door guide and are quoted as that source's figures, not as a universal value.
On the same wall, the standard French pair gives you a leaf three times the width of a six panel bifold, and one vertical line instead of five. And those lines are there all the time you are not walking through the door, which is nearly all the time. A published comparison of folding, sliding and French doors says the quiet part out loud from the other side of the argument: the gaps between the panels in folding doors can break up your view somewhat.
Notice also that the two things move in opposite directions. More panels means a bigger clear opening when you open it, and more lines in the view when you close it. You cannot have both, and there is no version of this decision in which the panel count is merely a styling choice.
If you were hoping the word standard in the question would pin down a panel width, it will not. Here are four public sources, all reputable, all different:
The smallest permitted leaf varies by more than a factor of one and a half between three brands that all sell the same product name. So do not ask what panel width is standard. Ask which panel width this system's tested limit allows, and then ask what your opening divides into.
Both doors have leaves that meet each other away from any wall. How that joint is sealed is, we would argue, the real main difference, and it is the one that determines what you will be complaining about in year five.
On a hinged French pair the joint is a compression joint. A published aluminium door technical page describes the mechanism precisely: the astragal is a movable post attached to the edge of the inactive leaf, usually T shaped, with one flange lapping over the gap between the leaves and the stem sitting inside the gap, and that flange carries the weatherstrip so the astragal seals the joint as well as locking it. The order of operation matters: the inactive leaf is bolted top and bottom first, which turns it into a fixed post for that moment, and then the active leaf closes against it and drives its own bolts. A moving-industry guide adds the practical number that the astragal is one to two inches wide, and that when a pair feels loose in service, a shoot bolt short of full engagement is the first thing installers check.
Two leaves, one joint, and it is squeezed.
On a bifold, every leaf-to-leaf joint is also a hinge. That single fact has a long tail. You cannot compress a hinge. A hinged joint has to stay free to rotate, so the only thing you can put in it is a gasket that touches. The mid-air joint on a French door is a compression problem; the same joint on a bifold is a contact problem. And there are not one of them. Four panels gives you three. Six gives you five. Each one is simultaneously a sealing interface and a moving interface, which are two jobs that pull in opposite directions.
This is the same physical argument that explains why sliding doors need contact seals rather than compression seals, and why mechanisms that lift or press a panel out of its sliding plane exist at all. On a folding door there is no such escape: the hinge cannot be removed from the equation, because the hinge is the product.
There is a second consequence of the same geometry, and it is usually presented as a convenience option. It is not.
A standard French pair already has a solution for everyday use: one leaf is active, one is inactive, and the inactive one is held by bolts at the head and the sill until you release them. Published moving-day guidance puts numbers on what that means in practice. On a sixty inch unit of two thirty inch leaves: one panel open with the astragal present gives about twenty-seven to twenty-eight inches clear; both panels open with the astragal present gives about fifty-four to fifty-six; both open with the astragal removed gives about fifty-seven to fifty-nine; a modern pair with no astragal at all gives about fifty-eight to sixty.
Read that first row again. The daily clear opening of a standard French door is about half its width, because the other half is bolted shut, and the astragal is carried on the leaf rather than left on the wall. The theoretical clear opening is everything. The one you actually walk through on a Tuesday is one leaf.
The bifold's answer to exactly this is the traffic door: one panel with its own handle and multi point lock that opens on its own. And here is the arithmetic that catches people out. A published UK guide states it clearly: an odd number of panels folding one way includes a traffic door as standard, while an even number usually gets one only by splitting the set, three folding one way and one the other. A second published source confirms the pattern and adds that odd counts of three, five or seven allow a traffic door while even counts of two, four or six give a symmetrical fold but no easy single door.
Which means: if you buy a four panel bifold because four looks balanced, you have bought a door whose everyday opening is created by splitting it three and one. You paid for four panels and you use one. That is not a criticism of folding doors. It is the honest version of a decision most people make on symmetry.
| Configuration | How it splits | Is there a panel you can open alone | What you give up |
|---|---|---|---|
| 2 panels | Both fold one way | Usually not, per published guidance | You fold the whole set to step outside |
| 3 panels | 3 plus 0, or 2 plus 1 | Yes, as standard on an odd count | One stack, all at one end |
| 4 panels | 3 plus 1, or 2 plus 2 | Only if split unevenly | Symmetry, if you want a traffic door |
| 5 panels | 4 plus 1, or 3 plus 2 | Yes | Four lines in the view when shut |
| 6 panels | 3 plus 3, or 5 plus 1 | Only if split unevenly | Five lines in the view, and a strong lintel |
The lock count follows the same logic. A French pair has one active leaf with a multi point mechanism running up its edge. A bifold has a traffic door with its own multi point lock plus, on the remaining leaves, shoot bolts or linkage into the head and the sill. A four panel folding set is not one door with four times the hardware. It is two mechanisms, one of which you use every day and one of which you use on the four days a year the weather is good.
Both doors take something away. They take different things from different places, and the difference decides which of your possessions has to move.
A published door planning reference gives the French door case directly: floor area reserved is two quarter circle arcs, each with radius equal to one leaf's width, and a sixty inch French door of two thirty inch leaves reserves two arcs of about five square feet on the swing side. Another published framing guide puts the same idea as a rule for a seventy-two inch unit: a thirty-six inch radius clear on the swing side.
The bifold takes a different shape. A published UAE guide states that each folded panel adds about one hundred to one hundred and twenty millimetres of stack depth, and to allow about one hundred and fifty millimetres per panel at the stacking end; a single stack setup may need up to a metre of side clearance. Split stacking needs roughly half that.
Put the two on the same three thousand six hundred millimetre opening, using our own arithmetic:
| Door | Shape of floor it claims when open | Approximate area, our calculation | Where it sits, which is what actually matters |
|---|---|---|---|
| French pair, two 1800 mm leaves | Two quarter circles, radius 900 mm, opening inward | About 1.27 square metres | Out into the room, in front of the opening, where you would put a table |
| Bifold, four panels stacking one way | A strip about 900 mm long and about 440 mm deep, folded flat to the jamb | About 0.40 square metres | Along the wall at one end, which is where radiators, curtains, sockets and switches live |
Our own arithmetic, shown so you can redo it. French case: two quarter circles of radius 0.9 m gives pi times 0.9 squared, divided by two, which is about 1.27 square metres. Bifold case: four panels at the mid point of the published one hundred to one hundred and twenty millimetre range gives about 440 mm of depth over a length of about one panel width, 900 mm. Stack length varies by configuration.
The bifold claims roughly a third of the floor area. But the French pair claims the middle of the room, where furniture goes anyway and where a gap reads as normal, while the bifold claims the strip against the wall, which is the one strip in a room you cannot avoid using. Both doors take something away. Ask which one takes the thing you were not planning to use.
One more thing that only applies to the folding door: you must also choose which side it folds to. Outward folding keeps the room clear and is the usual choice for gardens; inward folding suits balconies and any elevation where nothing may project outside. Both are published as legitimate. But folding outward puts the running gear and the hinges on the weather side, and it means that to clear a jam you walk outside.
This is where the folding door has a genuine advantage over a hinged pair, and where that advantage quietly costs you something else.
Published figures for the two load paths differ on the threshold. One supplier table gives a flush or five millimetre low profile threshold for top hung against a fifteen to twenty millimetre track for bottom rolling, and states plainly that top hung suits accessible thresholds. A UK and EU guide offers three options: a standard weathered threshold standing about fifty millimetres proud that gives the best seal, a low threshold of about twenty millimetres, or a flush drained track that gives level access for wheelchairs and prams but needs a drainage channel.
A top hung bifold with no floor running track is, as far as we can tell from the published material, the only way to get a genuinely flush floor across a very wide opening. You cannot do that with a hinged French pair: the meeting joint between the two leaves needs a sill under it to catch what comes down the joint, and the leaves themselves need a stop.
Now the cost, and it is stated very directly in a published patio door comparison: folding doors have no threshold options that eliminate a tripping hazard, but they are not as energy efficient as those with thresholds, and for that reason they are recommended for interior use such as a house to sunroom transition.
That sentence deserves to be read slowly. It comes from a company that sells folding doors. The configuration that gives you the flat floor is the configuration with the least sealing at the bottom. Accessibility and weather tightness are not two separate options on this door. They are two ends of one knob.
There is a smaller version of the same trade on the bottom rolling option. A published Canadian guide describes the floor channel as a place that collects grit, leaves and meltwater, notes that standing water in the track can freeze and lock the rollers in place, and says the channel is also a path for wind driven rain so drainage and sealing have to be right. The same source adds a bottom guide in a top hung system holds no weight and only keeps the panels from swinging in the wind.
If you have read anything else we have written about tracks, you will recognise the principle: a track holding water is usually by design, and the failure is the water crossing the inner upstand, not the water being there. On a folding door in a freezing climate the same water becomes a different problem entirely, because the door stops moving rather than the room getting wet.
A hinged French pair, once hung and adjusted, mostly stays adjusted. Hinge geometry and lock point geometry are the two things that drift, and both are usually correctable with a driver.
A folding door has a published maintenance schedule and a published number attached to it. One supplier's technical data gives cycle test standards of twenty-five thousand open close cycles for the residential class and one hundred thousand for heavy duty commercial, an operating force that should stay below forty newtons per leaf after cycling, and a threshold of fifty newtons above which the set is out of adjustment. The same source sets a twelve month service interval for track cleaning and roller re greasing.
That is a remarkable thing to have in public. It means the question is not whether your folding door needs adjusting. It is whether anyone has written down who does it, when, and what number they are checking against. We would rather you asked for that in writing than discovered it by pushing harder each month.
| Symptom | Most likely cause | Usually a defect, or usually adjustment | First thing to do | What people do instead, which makes it worse |
|---|---|---|---|---|
| Panels dragging along the floor | Track out of level, or a carrier that has dropped | Adjustment | Check header deflection and carrier height before touching anything else | Forcing it, which bends the linkage and turns one adjustment into a part order |
| Operating force rising over months | Debris in the track, or rollers past their service interval | Maintenance, not defect | Measure it. Published guidance puts out of adjustment above fifty newtons per leaf | Lubricating over grit, which makes a grinding paste |
| Panels will not stack square | Hinge wear, or unequal panel weights | Depends on cycle count and age | Photograph the pack from above, and check whether the traffic door is the panel that drifted | Shimming the floor, which moves the symptom to the head |
| Water standing in the bottom track | By design. A channel has to take water and drain it | Not a defect until it crosses the inner upstand | Confirm the drainage slots are clear and unobstructed | Sealing the outer slot, which traps the water with nowhere to go |
| Frozen rollers in a cold month | Standing water in a floor channel, published as a known risk | A design consequence, not a manufacturing defect | Thaw, clear, then look at whether the drainage is adequate for the exposure | Pouring hot water in, which refreezes and can reach the seals |
| Leaf-to-leaf joints letting air through | Contact seals on a hinged joint, which is what a hinged joint can have | Neither. It is the mechanism | Compare against a compression joint door before you buy, not after | Adding more gasket, which raises operating force and wears the hinges faster |
This is the section we would least like you to take from us, so we will frame it as a question to take elsewhere.
A published technical page on French door locking states that the International Building Code caps a single swinging leaf at forty-eight inches nominal on an exit door. Standard French leaves are published in the twenty-four to thirty-six inch range, and a published rough opening table gives fifty and a half inches of opening for a pair of twenty-four inch leaves, sixty two and a half for a pair of thirty, and seventy four and a half for a pair of thirty-six. So a seventy-two inch French pair is two thirty-six inch leaves, both inside that quoted cap.
But the cap is about the leaf, not about what you have to do to get it open. On a French pair, the inactive leaf is held by bolts at the head and the sill, and a published moving guide notes that homeowners may not know how to release them. On a bifold, the panels have to be unlatched and then folded, and the folded pack itself sits in one end of the opening.
Both doors have a gap between what they can open to and what they open to in the first ten seconds. Whether that gap is acceptable is a question for your authority having jurisdiction, for the occupancy, and for the hardware configuration actually supplied. We are not going to answer it for your building. We will say this much: ask it before the opening is framed, in writing, and get the answer filed with the drawings, because it is the one question on this list that cannot be fixed by adjusting anything.
Earlier we said the word French started life as a description of glass and became a description of hardware. There is a practical consequence for anyone who wants both words to be true at once.
The classic French look is divided lites: small panes separated by muntins. A published door glossary still splits hinged patio doors on exactly this line, with undivided glass called a garden door and gridded glass called French. Product naming keeps the old vocabulary too, which is why you will still see listings built around a count of lites, such as a 4 lite entry door.
Now put that grid on a folding door and do the arithmetic from the earlier table. On a three thousand six hundred millimetre opening, a French pair gives you leaves eighteen hundred millimetres wide to draw a grid into. A four panel bifold gives you nine hundred. A six panel gives you six hundred. The grid does not get smaller by the same amount the leaf does, because you are now drawing it six times and cutting it with five meeting lines. The thing that made the grid read as elegant is the leaf proportion, and the leaf proportion is exactly what the folding mechanism takes away.
There is a neat way to say this: the phrase bifold French door is most true at two panels and least true at six, and six is where it is usually sold. At two panels you have two wide leaves, one meeting line, and a shape that genuinely reads as a French pair that happens to fold. At six you have a glass wall with a folding mechanism and five vertical lines, and calling it French is describing the finish rather than the form.
That matters for one more reason. If what you actually want is the French look, there are cheaper ways to buy it than a folding mechanism. A wide two panel configuration, or a slider with applied grids, will both give you the picture. Published material in this category is quite honest that the gridded French look is the most restrictive of the three for views. Buy the mechanism because you want the wall to open, not because you want the grid.
Here is where we stop pretending to be neutral, because we are not: we make sliding doors. Our 28 folding door category page, along with our swing and entry door pages, is empty. We do not build any of the doors in the comparison above.
So rather than tell you which to choose, we will tell you what each one costs you, and let you decide which cost you can live with. A published comparison gives clear opening as up to about ninety five percent for bifold, fifty to sixty six percent for sliding, and around one hundred percent of a small span for French.
| Option on a 3600 mm opening | It gives you | It takes away | It never asks your structure |
|---|---|---|---|
| Standard hinged French pair | One compressed meeting joint, wide leaves, the classic proportion, near full clear opening when both leaves are released | About 1.27 square metres of floor inside or outside the opening, and a daily width of only one leaf unless you release the bolts | Anything about the header. The load goes sideways into the studs |
| Four panel bifold | About 85 percent clear opening, no swing arc, a genuinely flush floor if top hung | Three vertical lines in the view, a strip of wall at the stacking end, and contact seals on every leaf joint | Nothing. It asks the most. Published guidance wants a checked header and a deflection limit |
| Three track thermal break slider | One interlock line, no swing arc, no stack in the room, no demand on the header, and the movement stays in one plane | Clear opening. About two thirds on three tracks, and the panel that stays put is always in the view | Anything. The load goes down through the track into the floor |
Each of the three gives you a different version of slightly less. The French pair takes your floor. The bifold takes your view and a strip of wall. The slider takes your clear opening. Pick the one you least need.
If you take nothing else from this article, take this: the three options are not ranked. They are three different shortages, and the one that hurts is whichever shortage lands on something your room did not have to spare.
Since we do not build either door in this comparison, the honest version of this table has a third column that is mostly one answer. We think that is more useful than filling it with numbers borrowed from somebody else's catalogue.
| What you need | Why it decides the outcome | Can we supply it | What you should do instead |
|---|---|---|---|
| Panel weight and roller rating per leaf | Both are per leaf, and both change with glass build-up | No. We do not build folding doors | Ask for the rating per leaf and the glass build-up it was rated with, in the same sentence |
| Header deflection limit for the supplied track | Published limits cluster around L over 500, which framing is not usually sized to | No | Get it in writing from the door supplier, and hand it to whoever sizes the beam |
| Air, water and wind test classes | A class without a standard name and version is an adjective, not a result | No folding or hinged door test data of our own | Ask for the report, the standard, and whether the specimen was the opening configuration you are buying |
| Cycle class and operating force after cycling | It is the only published number that tells you when the door is out of adjustment | No | Ask for both. A cycle count on its own does not tell you what to check on site |
| Which panel is the traffic door, and does it have a lock | On an even panel count it only exists if you split the set unevenly | No | Mark it on the elevation drawing before anyone orders |
| Threshold height and drainage route | The flattest option and the best sealed option are not the same option | No | Ask where the water goes, in writing, before the floor is finished |
| Whether it is permitted as a required exit door | Nothing in the product answer settles this | No, and nobody should answer it for you | Your authority having jurisdiction, in writing, before the opening is framed |
| A wide opening with no swing arc and no header question | This is the one row where the answer is not a folding or hinged door | Yes. This is what we build | Send us the opening in millimetres and the wall build-up layer by layer |
Fill these in and send them. The fourth column is the one that gets ignored, and it is the one that costs money later.
| Field | A good answer looks like | A poor answer looks like | If you leave it blank |
|---|---|---|---|
| Opening width and height, millimetres | Three widths, two heights, both diagonals, smallest value wins | A single round number | The panel count is guessed, and so is everything that follows from it |
| Wall build-up, outside to inside | Every layer named with its thickness | Two by four, or two hundred mil | Frame depth is ordered blind and the reveal is discovered on site |
| Load path required | Top hung, or bottom rolling, stated | Whichever is standard | You inherit a structural requirement you did not know you had |
| Header condition | Existing member described, or noted as new | Not mentioned | The heaviest single risk in the whole order goes unpriced |
| Panel count and split | Three plus one, or two plus two, named | Four panels | You may receive a set with no panel you can open on its own |
| Which panel is the traffic door | Marked on the elevation | Left to the factory | The everyday door ends up behind the sofa |
| Glass build-up per panel | Millimetres, cavity, gas, coating, laminate, named | Double glazed | Every weight rating in the quote is rated for a different door |
| Panel weight, and roller or hinge rating per leaf | Both numbers, both per leaf | Heavy duty hardware | You cannot tell whether the rating was per leaf or per set |
| Threshold height and drainage route | Height in millimetres plus where the water exits | Low threshold | Accessibility and weather sealing are decided by whoever shows up last |
| Fold direction and stack side | Inward or outward, and which jamb | To be confirmed on site | The stack lands on the radiator and cannot be moved |
| Test reports you require | Standard name, version, class, and specimen description | Tested to international standards | You have a class with no standard behind it, which is a word |
| Who adjusts it, and when | A named party and an interval | Fully adjustable | Nobody owns the twelve month check and the force creeps up unmeasured |
| Ask this | A real answer contains | Sounds reasonable, tells you nothing |
|---|---|---|
| Is it top hung or bottom rolling | One of the two, plus the consequence for your header or your floor | It runs on a track |
| What is the roller rating | Kilograms per leaf, and the glass build-up it assumes | It is heavy duty |
| What deflection does the track allow | A span ratio and a millimetre figure | It needs a solid lintel |
| What is the air and water class | Standard name, version, class, and whether the specimen opened | It is fully weather sealed |
| How heavy is each panel | Kilograms per leaf at your glass build-up | The system handles it |
| Where does the water go | The route from the channel to the outside, in order | It drains |
| How many cycles was it tested to | A number, a standard, and the operating force after cycling | German hardware |
| Can I open one panel on its own | Yes or no, and which panel, marked on a drawing | It opens fully |
| What spares come with the order | A list of part numbers, shipped in the original container | Spare parts are available |
| What is the panel width | Millimetres, derived from your opening and this system's tested limits | Standard panel size |
No, and this is the most useful thing in the article. Folding changes the load path, the number of sealing joints, the number of vertical lines in the view, the threshold options, and the maintenance schedule. The one thing it does not change is the word French, which is why the phrase is misleading. What survives the fold is the finish and the paired-leaf idea. What does not survive is almost everything that determines how the door behaves.
It depends whether you mean the widest it can ever be or the width you use on a normal day. Published figures put a bifold at up to about ninety five percent and a French pair at around one hundred percent of its span when both leaves are released. But published moving guidance puts the daily figure for a sixty inch French unit with one leaf open at about twenty-seven to twenty-eight inches. A bifold with a traffic door gives you roughly one panel every day without touching the rest. Ask both numbers.
If you are buying a top hung bifold, assume the question is live until an engineer says otherwise. Published guidance for top hung systems gives a header deflection limit around L over 500, typically three to four millimetres, and says a steel lintel or engineered beam is required. A bottom rolling system sends the load to the floor instead and is the common choice for retrofits for exactly that reason. A hinged French pair does not raise the question at all.
With a top hung bifold and no floor running track, roughly yes, and that is a genuine advantage no hinged pair can match. The cost is at the bottom seal. A published patio door comparison says no threshold options are not as energy efficient as those with thresholds and are recommended for interior transitions such as a house to sunroom. Decide which of the two you are buying before you choose the track.
Start from the traffic door, not from symmetry. Published guidance is consistent that an odd count folding one way includes a panel you can open on its own, while an even count only gets one if you split the set unevenly. Then check the panel width against the system's tested minimum and maximum, which varies by more than a factor of one and a half between three named brands. Then count the vertical lines you will be looking at when it is shut, which is one fewer than the panel count.
Because the question gets asked of us constantly and the honest answer is not a product page. Our folding, swing and entry door categories are empty. We would rather explain what each option takes away than pretend to sell you all three. If what you actually need is a wide opening with no swing arc, no stack in the room and no question asked of the header, talk to us about a thermal break sliding door. If what you need is a wall that folds away, buy it from someone who builds them, using the twelve lines above.
One last time
Every figure here is quoted from a published public source and labelled as one, or shown as arithmetic you can redo. None of it is a specification. Do not carry a single number from this page into a drawing, a submittal or a purchase order. Send your opening in millimetres, describe your wall from the outside in, and make the supplier put their own numbers next to yours in writing.
