Read this before you use any number here. Every figure in this article is either quoted from a published source and attributed to that kind of source, or it is our own arithmetic with the assumptions written next to it. None of it is a specification for your order. Do not copy a number from this page into a drawing, a specification or a purchase order. Use the method, not the values.
One disclosure, up front. Our own folding door category page is empty. We make thermal break casement windows, sliding windows, tilt-and-turn windows and sliding doors. We have written this because comparing folding door quotes is a problem we watch buyers lose money on, and because the discipline described here is the same discipline we apply to our own quotes. Where the honest answer to a folding door brief is a sliding door, we say so in Section 16.
Two folding door quotes for the same 6 metre opening will usually arrive priced per square metre, and both will be priced correctly. They can still be quotes for two entirely different assemblies. The reason is simple enough to state in one sentence: a square metre does not contain panels.
A folding set is the only common door type where the number of moving leaves is still a free variable after the opening size has been fixed. A sliding door in a 6 metre opening with three tracks has a panel count that is largely forced by the tracks. A folding set in the same opening can be four leaves, five, six, eight or ten, and every one of those is a legitimate answer to the same brief. Each leaf you add brings its own glass unit, its own hinge line, its own carrier, its own pair of sash corners and one more vertical joint that has to keep the weather out. None of that appears anywhere in a square metre price.
One published buying guide for aluminium glazing puts the problem in a single sentence and then moves on: two quotations for what appears to be the same three-panel bifold door may not be equivalent at all. It then lists what has to be matched before the totals mean anything: the number of panels, the direction they fold, the location of the traffic door, and whether the set opens in or out. That list is correct and it is the whole of most advice on this subject. This article takes it as the starting point and goes one level down, into the numbers those choices move.
Before you compare totals, force both quotes onto the same panel schedule. A panel schedule is just a list: leaf one, its width, its height, its glass build-up, its hardware. One supplier guidance page describes it as a schedule that ties each leaf to its width, height, glass type and hardware arrangement. Ask for it. If a factory will not produce one, that is itself the most useful piece of information in the whole exercise.
Here is the arithmetic, done on one opening so you can repeat it on yours. We take a 6000 mm by 2400 mm structural opening, which is 14.40 square metres, and we divide the width by the panel count. We have ignored the frame deductions that a real fabrication drawing would apply, because this is a normalisation exercise, not a cutting list; the point is the shape of the relationship, not the last millimetre.
| Leaves | Leaf width | Area per leaf | Glass only per leaf (28 mm unit) | Inside the widest published leaf width? | Clear opening left when fully open |
|---|---|---|---|---|---|
| 4 | 1500 mm | 3.60 m2 | 108.0 kg | No. The widest maximum leaf width we found published by any folding hardware maker is 1300 mm. | About 5708 mm |
| 5 | 1200 mm | 2.88 m2 | 86.4 kg | Yes, exactly at the limit. 1200 mm is the figure published independently by four separate sources. | About 5635 mm |
| 6 | 1000 mm | 2.40 m2 | 72.0 kg | Yes, comfortably. | About 5562 mm |
| 8 | 750 mm | 1.80 m2 | 54.0 kg | Yes. | About 5416 mm |
| 10 | 600 mm | 1.44 m2 | 43.2 kg | Only on the smallest published system, whose maximum leaf weight is 20 kg. 43.2 kg of glass does not fit inside 20 kg. | About 5270 mm |
Two things in that table should change how you read a quote. First, the four-leaf version is not a cheap option, it is an unavailable one: 1500 mm leaves exceed every maximum leaf width we could find published. A quote that quietly assumes four leaves in a 6 metre opening is quoting something that cannot be built as drawn, and you will find that out at the drawing approval stage, not at the price stage.
Second, look at the last column. We have estimated the stack using a leaf depth of 73 mm, which is the sash section depth published by two separate aluminium folding door sources (one gives 73 mm, the other 72 mm). Ten leaves stacked take about 730 mm of your opening; five take about 365 mm. The panels do not disappear when you open the door, they park inside the hole. The quote with more panels sells you more hardware and gives you less opening. Any comparison that only looks at price per square metre cannot see this, because both quotes describe 14.40 square metres.
This is the part of the comparison that nobody does, and it is the most valuable thing in this article.
Folding door literature always publishes a maximum panel size. It is almost always three numbers sitting next to each other: a maximum leaf width, a maximum leaf height, and a maximum leaf weight. Read together, they look like a rectangle you are allowed to build. Read against each other, they usually are not.
Here is the only physical constant you need, and it is not in dispute: glass weighs about 2.5 kg per square metre for every millimetre of glass thickness. A standard 28 mm double glazed unit, built up as 6-16-6, contains 12 mm of glass in total, so it weighs about 30 kg per square metre. That figure appears across the industry and we have used it in our own previous work; we restate it here so you can redo every line below yourself.
Now take a published folding hardware range that lists eight systems side by side, with a maximum leaf width, a maximum leaf height and a maximum leaf weight for each. Multiply the two dimensions, multiply the result by 30 kg per square metre, and compare that with the weight limit on the same row. This is pure arithmetic on published numbers, and this is what comes out.
| System | Max leaf width | Max leaf height | Max leaf weight | Area at both maxima | Glass alone at 30 kg/m2 | Verdict |
|---|---|---|---|---|---|---|
| E2 | 1100 mm | 3000 mm | 60 kg | 3.30 m2 | 99.0 kg | 39.0 kg over, before any frame |
| E2i | 1000 mm | 3000 mm | 40 kg | 3.00 m2 | 90.0 kg | 50.0 kg over |
| E3 | 1200 mm | 3600 mm | 115 kg | 4.32 m2 | 129.6 kg | 14.6 kg over |
| E4 | 1300 mm | 4000 mm | 160 kg | 5.20 m2 | 156.0 kg | 4.0 kg left for the entire sash, its hardware and its seals |
| A9F | 620 mm | 3000 mm | 20 kg | 1.86 m2 | 55.8 kg | 35.8 kg over |
| A14F | 820 mm | 3000 mm | 40 kg | 2.46 m2 | 73.8 kg | 33.8 kg over |
| F2 | 900 mm | 2400 mm | 40 kg | 2.16 m2 | 64.8 kg | 24.8 kg over |
| F3 | 1000 mm | 2700 mm | 80 kg | 2.70 m2 | 81.0 kg | 1.0 kg over |
Seven of the eight fail on glass alone, before a single gram of aluminium has been added. That is the headline, but the more useful observation is the eighth.
The one system that survives leaves 4 kg for the whole sash. A 1300 by 4000 sash has a perimeter of 10.6 metres. Four kilograms spread over 10.6 metres is under half a kilogram per metre, for a section that has to carry a 4 tonne-per-square-metre-equivalent glass load and hold a thermal break. Ask any folding doors manufacturer whether their sash section is that light, and watch how the conversation changes.
The same test run on the three complete aluminium folding door systems we found publishing all three numbers gives a softer but equally instructive result. All three publish a maximum leaf width of 1200 mm, a maximum leaf height of 3000 mm and a maximum leaf weight of 120 kg. The rectangle is 3.60 square metres, the glass alone is 108.0 kg, and that leaves 12.0 kg for the sash. The perimeter of that sash is 8.4 metres, so 12.0 kg is about 1.4 kg per metre of section.
We are not going to tell you that 1.4 kg per metre is impossible, because we have not weighed their section and we are not going to pretend we have. We are going to tell you what to do with the number: put it in an email and ask. "Your spec sheet says 1200 by 3000 and 120 kg. On a 28 mm unit that leaves 12 kg for the sash over an 8.4 metre perimeter. What does your sash section actually weigh per metre?" A supplier who answers that question properly has just told you more about their product than any brochure in your inbox.
The general rule this produces is worth memorising, because it applies to every folding door quote you will ever read: a maximum panel size is not a rectangle, it is three separate ceilings, and on most published systems you can only stand under two of them at a time. The three numbers are each true. They are just not true together.
A folding door is a collection of hardware wearing a door. The profile is the part the factory makes; the mechanism is the part that decides whether the door works in year five. Yet most quotes name the profile series and not the hardware system, or name a hardware brand without naming which system in that brand's range.
The published folding hardware table we used above also carries four rows that never appear on a door brochure, and each of them is a legitimate reason for two quotes to differ by a lot.
| System | How the load is carried | External use shown on the published table | Minimum leaf thickness | Maximum leaves |
|---|---|---|---|---|
| E2 | Top hung | Yes | 35 mm | 8 left plus 8 right |
| E2i | Top hung | Yes | 35 mm | 4 left plus 4 right |
| E3 | Top hung | Yes | 35 mm | 8 left plus 8 right |
| E4 | Top hung | Yes | 45 mm | 8 left plus 8 right |
| A9F | Top hung | Not shown for this system | 35 mm | 4 left plus 4 right |
| A14F | Top hung | Not shown for this system | 35 mm | 7.5 left plus 7.5 right |
| F2 | Bottom rolling | Not shown for this system | 35 mm | 8 left plus 8 right |
| F3 | Bottom rolling | Not shown for this system | 35 mm | 8 left plus 8 right |
Look at the spread. Maximum leaf weight runs from 20 kg to 160 kg, an eightfold range, inside one manufacturer's own catalogue. Maximum leaf width runs from 620 mm to 1300 mm, more than two to one. Maximum leaf height runs from 2400 mm to 4000 mm. If two factories quote you the same opening using different systems from this one table, they are not slightly different products. They are different products.
Note also the half leaf. One system publishes 7.5 left plus 7.5 right. A half panel is a real thing in folding door geometry, and it exists because the folding stack needs a terminating leaf that is not a full width. If your quote does not say whether the set contains a half leaf, the panel schedule does not match the drawing.
This is the single most consequential line that is missing from most folding door quotes, and it is the one that arrives as a surprise on site.
A top-hung folding set hangs from the head. The entire weight of every leaf is delivered into the beam above the opening, through the head track, and the track has to be fixed into something that can take it. A bottom-rolling set puts the weight on the floor, through rollers running on the sill track. Same opening, same square metres, same glass, and a completely different structural question for your building.
This is not a preference. If your opening has a steel that has already been sized for a top-hung set and you buy a bottom-rolling one, the floor has to take a load nobody calculated, on a track that has to be perfectly level along its whole length because the panels roll on it. If the reverse happens, your lintel is now carrying a door it was never asked to carry. A quote that does not state which of the two you are buying has not told you which part of your building is holding the door up.
There is a second reason it belongs on the quote: the floor. Bottom-rolling systems put a loaded track at the threshold, and a loaded track is not the same object as a drained threshold. That interacts directly with the threshold decision in Section 8, and it is the reason the two cannot be chosen independently.
Here is a sentence buried in a hardware manufacturer's own product description, and it explains something that confuses buyers constantly: their offset hinge system allows for equal-sized door panels regardless of the configuration.
Read the implication. Without that specific hinge geometry, the leaves in a folding set are not all the same width. Fold a chain of panels and the ones next to the hanging point and the ones at the trailing edge end up different, because each hinge line consumes a little of the opening and the geometry has to resolve somewhere. So a quote that says six panels has not told you six of what.
This matters for three reasons that all land on cost. Glass is cut to size, so six different leaf widths is six different glass sizes and a lower cutting yield from each stock sheet. The meeting stiles between leaves of different widths carry different loads. And the elevation looks different, which matters if you have already shown a drawing to a client. One supplier guidance page makes the related point that odd and even panel counts leave different handle and meeting stile arrangements: the hardware layout is a function of the count, not an afterthought to it.
Ask one question: are the leaves equal width, and if not, what is the width of each one. If the answer is a single number, ask again.
A folding set is opened as a set perhaps thirty times a year. It is opened as a single leaf, to let a person through, perhaps two thousand times a year. Those two motions use different hardware, different seals and different locks, and only one of them is the door you actually live with.
One published aluminium folding door specification makes the distinction explicit: it rates the main door, and only the main door, for 50,000 open-close cycles. That is a serious number for a single leaf, and it is also a very precise piece of information about what was tested. It was not the set. It was the leaf you walk through.
So when a quote mentions a cycle figure, the first question is which leaf it belongs to, and the second is whether the same hardware is on the other leaves. Three things follow.
If the drawing says a 3 panel folding door with a traffic leaf and the quote prices a plain three-panel set, the two documents are describing different products and neither of them is wrong. That is exactly how this goes wrong.
A folding door supplier's own published guidance names the two most expensive mistakes on a folding door project, and they are not about the door. They are a stack in the wrong place, and a threshold that cannot be installed at the finished floor level.
The second one is worth dwelling on, because it is invisible on a quote. A threshold is not a product you ｓｅｌｅｃｔ from a list; it is a relationship between three numbers that the factory does not have: the structural floor level, the finished internal floor level, and the external drainage level. One published system offers four threshold options. Another states plainly that a low threshold only suits an indoor condition, and that a supplier should tell you where a flush-looking detail is inappropriate rather than letting a visual preference override drainage.
We can show you what this looks like from our own catalogue. Our thermal break sliding door series publishes two track heights, 8 mm and 43 mm, and a separate high-ｄｒｏｐ bottom track at 45 mm with an optional 85 mm water barrier. Those are three different answers to the same opening, and they cannot be chosen after the floor is poured. The same is true of a folding door threshold, and the quote must state which one is priced and what finished floor level it assumes.
The line to put in your enquiry is not "low threshold". It is: internal finished floor level, external finished level, the difference between them, and which of the supplier's published threshold options is priced against those numbers.
Glass on a folding door quote is usually one line: double glazed, or triple glazed, sometimes with a coating named. It is never one line in reality, because the glass is the largest single contributor to leaf weight, and leaf weight is what selects the hardware.
Published figures converge tightly here, which makes them usable. One system publishes glazing options at 28 mm, 30 mm and 44 mm. Another publishes a maximum glazing of 45 mm. A third offers up to 44 mm triple glazing, with 36 mm on stepped sashes. The ceiling is around 44 to 45 mm, and that is a real convergence across independent sources.
Now the arithmetic on one leaf, 1200 mm by 2400 mm, which is 2.88 square metres, against the most commonly published leaf weight limit of 120 kg. Our arithmetic, using 2.5 kg per square metre per millimetre of glass.
| Build-up | Total glass thickness | Weight per square metre | Glass on this leaf | Against a 120 kg leaf limit |
|---|---|---|---|---|
| 28 mm double glazed, 6-16-6 | 12 mm | 30 kg/m2 | 86.4 kg | Fits, with 33.6 kg to spare for the sash |
| 44 mm triple glazed, 6-16-6-16-6 | 18 mm | 45 kg/m2 | 129.6 kg | 9.6 kg over, before the sash |
| 44 mm triple glazed, at the largest area the 120 kg limit allows | 18 mm | 45 kg/m2 | 120.0 kg in 2.67 m2 | The leaf has to shrink to about 1200 by 2220 mm |
The triple glazing upgrade that looks like a one-line change on a quote is not a one-line change. On this leaf it costs about 180 mm of height, or it moves you to a heavier hardware system, or it moves you to more panels. All three of those are priced differently, and none of them is visible if the quote only says "triple glazed".
There is a second glass row that buyers never ask about and should: the glazing rebate. The maximum glazing figure tells you how thick a unit the sash will physically accept. It does not tell you which unit is in the quote. Ask for the build-up written out in millimetres, glass by glass: 6-16-6, or 6-16-6-16-6, with coatings and gas fill named separately.
Put the whole argument in one table. Let n be the number of leaves. Here is what the factory has to buy, cut, assemble and pack as n changes, with the opening held constant.
| Item | How many for n leaves | What it does to the quote |
|---|---|---|
| Glass units | n | More, smaller units cost more per square metre to cut and more to handle. |
| Sash corner joints | 4n | Four times any corner defect rate. Ask what the corner construction is. |
| Hinge sets | About n | The single largest hardware multiplier in a folding set. |
| Carriers or rollers | About n | Rated per leaf, not per set. A set rating does not exist. |
| Vertical meeting stile seals | n minus 1 | Every one is a potential air and water path. This is the maintenance load. |
| Handles and levers | At least one on the traffic leaf, plus per meeting stile | A visible cost difference that is easy to leave off a quote. |
| Head track | Constant, one length | The only major item that does not grow with n. |
| Bottom track and sill | Constant, one length | Constant, but its drainage duty grows with the number of joints above it. |
| Opening area | Constant | The number the price is divided by. It is the one thing that never changes. |
There it is in plain sight. Everything that costs money in a folding door scales with n, and the number the price is divided by does not. That is the mechanism by which two honest quotes for the same wall can differ substantially, and it is why price per square metre is the wrong unit for this product.
The unit to ask for instead is price per leaf, alongside the leaf schedule. Then the two quotes become directly comparable, because the thing that was hidden is now in the denominator.
We put this number in the first table, and it deserves its own section because it is the one buyers never think to ask about until the door is installed.
When a folding set is fully open, the leaves are parked against one or both jambs. They occupy a strip of the opening equal to roughly the number of leaves multiplied by the sash depth, plus the hinge gaps. Using the published sash depths of 72 and 73 mm, a five-leaf set parks in about 365 mm, and a ten-leaf set parks in about 730 mm. On a 6 metre opening, that is the difference between keeping about 5635 mm and keeping about 5270 mm of clear passage.
So the more-panels quote is worse in three separate ways at once: more hardware to buy and maintain, more glass units to cut, and less clear opening when the door is doing the thing you bought it to do. It may still be the right answer, because heavier leaves need more of them. But it should be a decision you make with these three numbers on the table, not a decision the factory makes for you on a square metre price.
Ask where the stack parks, on which side, and how much clear opening is left. Then ask whether the furniture, the railing or the kitchen island on your plan drawing is in that strip. This is a plan-view question, and front elevations hide it completely.
Here is the worksheet. Fill it in yourself and send the same filled version to every factory. Do not send them an empty opening size and wait to see what comes back, because what comes back will be three different products and you will spend two weeks discovering that.
| Line | A good entry looks like | If you leave it blank |
|---|---|---|
| 1. Opening and its basis | ____ mm wide by ____ mm high, stated as structural opening, frame size, or masonry opening | Each factory assumes a different basis and the set does not fit. |
| 2. Leaf count | ____ leaves, plus or minus a half leaf | The whole quote becomes uncomparable. This is the big one. |
| 3. Leaf widths | Equal at ____ mm, or a list of each leaf width | Glass is cut to the wrong sizes and the elevation is not what you drew. |
| 4. How it carries load | Top hung, or bottom rolling | Nobody knows whether the beam or the floor is carrying the set. |
| 5. Traffic door | Yes or no, which leaf, which side, and its cycle rating | You get either a set you cannot use daily, or an unbudgeted extra. |
| 6. Glass build-up | ____ mm, written glass by glass, with coating and fill named | Leaf weight is unknown, so the hardware may be undersized. |
| 7. Threshold and floor levels | Threshold option named, internal finished level ____ mm, external finished level ____ mm | The classic failure: a door that cannot be installed at the floor you built. |
| 8. Hardware system | Named brand and named system within that brand, with its leaf weight rating | Two quotes can differ eightfold in capacity and look identical on paper. |
| 9. Stack side and clear opening | Stack parks left, right or both; clear opening ____ mm when open | The stack lands on a railing, an island or a doorway. |
Beyond the numbers, there are four phrases that routinely carry two different meanings in two different quotes. None of them involves bad faith. They involve a word doing more work than it can.
| The phrase | One meaning | The other meaning | How to force it out |
|---|---|---|---|
| Supply only | The set, crated, at your port | The set plus hardware packs, glazing gaskets and fixings, in separate packages | Ask for the packing list, and whether it can be checked against the panel schedule. |
| Aluminium bifold | A named system house profile with published test data | A house-brand extrusion with no published system data | Ask which profile system is quoted, by name, and for its published section drawing. |
| Tested | The configuration you are buying, at your size | A reference configuration, at a reference size, from a different leaf count | Ask for the report and read the specimen description before you read the result. |
| Hardware included | A named system with a published leaf rating | A brand name, with the system chosen later to suit the weight | Ask for the system code and its maximum leaf weight in kilograms. |
| Ask | A useful answer contains | Sounds reasonable, carries no information |
|---|---|---|
| How many leaves, and how wide is each | A count and a schedule of widths | "Standard configuration" |
| What is the maximum leaf weight, and is it top hung or bottom rolling | A figure in kilograms and the load path | "Heavy duty hardware" |
| What is the maximum leaf size, and which of the three limits governs mine | Width, height and weight, plus which one binds at your size | "Up to 1200 by 3000" |
| Which system is the hardware, and what is its cycle rating | A brand, a system code, and which leaf was tested | "German hardware" |
| Which threshold, against which floor levels | A named option and an assumed level difference | "Low threshold available" |
| What glazing rebate does the sash have | A dimension in millimetres from the section drawing | "Takes up to 44 mm" |
| Is the set rated for external use | A yes or no against the named system's own published table | "Suitable for patios" |
| Which standard is the test report against | A standard number with its year, and the specimen description | "Meets international standards" |
| How is the set packed and identified | A packing list that can be checked against the panel schedule, leaf by leaf | "Export standard packing" |
| What date is the price based on | A named commodity reference and a named base date | "Valid for 30 days" |
We said at the top that our folding door category page is empty, and we would rather say that plainly than let you build a schedule on it. What we can do is show you how we handle the same problem in the products we do make, because the discipline is identical and it is the part of this that survives into any order you place with anyone.
Everything we publish is a performance number. Not one of them is a consumption number, and not one of them tells you what a panel or a square metre costs. Here is the honest inventory.
| What we publish | What it actually measures | What you still have to ask | Why the gap exists |
|---|---|---|---|
| K values of 1.3, 1.6 and 1.7 | System values for named series. Not whole-window values, not NFRC. | The calculated value for your size, your glazing, under your standard | A thermal number belongs to a specimen, and your opening is a different one. |
| 240 hour salt spray and 5000 N shear | Coating and substrate on one series, and the composite bond of the insulation strip. They do not transfer to other series. | Which series they belong to, and what your exposure class requires | They are results for a tested construction, not a family characteristic. |
| 30,000 cycles | A rig test on a hardware component. | Which component, on which rig, and which leaf of which product | A cycle rig is not a door, and a component is not an assembly. |
| 200 kg, 220 kg and 400 kg | Per sash. Two of them are driving capacity and one is load capacity, and those are not the same measurement. | Which measurement, per sash, at which glass build-up | Putting them in one column and sorting descending is the most common misreading in our own catalogue. |
| 6060-T66 across the range | An alloy and temper declaration. It is not a performance grade. | Wall thickness at the thinnest point of your section | The same temper at 1.8 mm and at 2.5 mm is two different doors. |
| Insulation strip 34 to 44 mm | The distance the heat has to travel around the break, on one series. | The strip material and the width on the series you are buying | It is a path length, not a thickness of insulation. |
| Maximum single sash sizes | Published against an assumed glass build-up that we do not print. | The maximum at your glass build-up, in writing | Change the glass and the maximum moves. This is the same trap as Section 3. |
| Glass sash thickness | The stack-up of the sash. It is not the glazing rebate. | The rebate dimension from the section drawing | A sash can be thick and still accept a thin unit. |
| Track heights of 8 mm, 43 mm and 45 mm | The upstand you step over. Not a drainage performance figure. | Your finished floor levels, inside and out, in millimetres | The number that decides it is on your floor, not in our catalogue. |
| Nothing on the folding door category | Nothing. There is no product there. | Whether a sliding door answers the same brief better | We would rather leave a page empty than fill it with a product we do not make. |
We are going to do the thing that a folding door supplier will not do, and we are going to do it with our own numbers rather than theirs.
Every complication in this article comes from the same source: a folding set has many leaves sharing one opening, and every joint between them is a seal, a hinge, a weight and a maintenance item. A sliding door has two or three leaves and no hinge lines inside the opening at all. That is not a better door. It is a door with a different set of compromises, and for some briefs those compromises are smaller.
Our thermal break sliding doors publish a maximum single sash of 2500 mm by 3000 mm on one series and 2800 mm by 2600 mm on another. Read those against the folding hardware table in Section 3 and you will see the point immediately: the largest leaf any of the eight folding systems publishes is 1300 mm wide, and the sliding series publishes leaves more than twice that. Fewer leaves, fewer joints, and the opening is not eaten by a stack when the door is open.
None of this makes a folding door the wrong choice. It makes it a choice with a specific cost structure that a square metre price cannot show you, and it means the first question to ask is not which factory is cheaper but how many leaves each factory is actually selling you. Most enquiries for china folding doors arrive with an opening width, a height, and nothing else, which is exactly why the first three quotes are never comparable and the fourth round finally is.
Copy this, fill it in, and send the identical text to every factory. The value is not in any single line. The value is that every factory is now answering the same twelve questions, so the answers can be put in adjacent columns.
| Line | Paste this | If it comes back empty |
|---|---|---|
| 1 | Structural opening: ____ mm wide by ____ mm high. This is a structural opening, not a frame size. | Every following number is built on a guess. |
| 2 | Number of leaves: ____. Please include a panel schedule listing each leaf width. | The quote cannot be compared with any other quote. |
| 3 | Are the leaves equal width? If not, give each width. | Glass sizes are wrong and the elevation is not the one you drew. |
| 4 | Top hung or bottom rolling? | Your beam or your floor is carrying an uncalculated load. |
| 5 | Hardware: brand ____, system ____. Maximum leaf weight ____ kg. Maximum leaf width ____ mm. Maximum leaf height ____ mm. | You cannot tell whether the set is sized for your leaves. |
| 6 | Which of those three limits governs my leaf? Show the working. | You get three ceilings and no idea which one you are standing under. |
| 7 | Glazing: ____ mm, written glass by glass. Glazing rebate of the sash: ____ mm. | Leaf weight is unknown, so the hardware rating means nothing. |
| 8 | Threshold option: ____. Assumed internal finished floor level ____ mm, external finished level ____ mm. | The most expensive mistake on a folding door project, and entirely avoidable. |
| 9 | Traffic door: yes or no. Which leaf? Cycle rating and which leaf it was tested on. | The leaf you use daily is the one nobody specified. |
| 10 | Stack parks: left, right or both. Clear opening when fully open: ____ mm. | The stack lands on something, and you find out on handover day. |
| 11 | Test report: standard number and year, and the specimen description including leaf count and size. | A result with no specimen is a number, not evidence. |
| 12 | Price basis: per square metre ____, per leaf ____, commodity reference ____, base date ____, validity ____. | The price has a date in it. If you do not choose it, it gets chosen for you. |
Line 12 is the one people skip, and it is worth one sentence of explanation. A folding door is a long-lead, high-metal-content item. Every quote contains an assumption about what aluminium cost on some date, and if you have not named that date in writing, the factory has named it for you. Ask for both units on the same line, per square metre and per leaf. The first lets you compare with your budget; the second lets you compare with the other quotes.
No. The square metre count is fixed by your opening and is identical in both quotes, while the number of leaves, which is where most of the hardware, the glass cutting and the labour sit, is a free variable. Ask for a price per leaf alongside the per square metre figure, together with a panel schedule. Once both quotes are expressed per leaf with the same leaf count, the comparison starts to mean something.
Because they have chosen different hardware. Published folding hardware systems differ in maximum leaf width from 620 mm to 1300 mm and in maximum leaf weight from 20 kg to 160 kg, within a single manufacturer's own range. A factory working to a lighter system has to use more, smaller leaves to span the same opening. Neither is wrong; they are different assemblies, and the leaf count is the clearest evidence you will get of which system each one is actually building.
Trust each number individually and distrust them together. A maximum width, a maximum height and a maximum weight are three separate ceilings. On seven of the eight folding hardware systems we examined, a leaf built to that system's own maximum width and maximum height would weigh more in glass alone, at a standard 28 mm double glazed unit, than the system's maximum leaf weight. The rule to take away is to ask which of the three governs your leaf, and to ask for the working.
Whether the set is top hung or bottom rolling. It determines which part of your building carries the weight of every leaf, and it interacts with the threshold decision. It is also the item that is most expensive to discover late, because by then the steel or the floor has already been built. The runner-up is the panel schedule, which is the document that makes every other line on the quote checkable.
Very often, yes. A 1200 by 2400 leaf carries about 86 kg of glass in a 28 mm double glazed unit and about 130 kg in a 44 mm triple glazed one, using the industry figure of 2.5 kg per square metre per millimetre of glass. Against the commonly published 120 kg leaf limit, the second build-up does not fit. You either shorten the leaf, add leaves, or move to a heavier hardware system, and all three are priced differently.
No. Our folding door category page is empty, and we would rather say so than put a sliding door behind a folding door heading. We make thermal break casement windows, sliding windows, tilt-and-turn windows and sliding doors, and we have published the same kind of honest inventory of our own numbers in Section 15. If your brief can be answered by a large sliding set, we would be glad to quote it, and you can see comparable work in the project archive or reach us through our contact page.
One last warning. Do not copy any number from this article into a drawing, a specification or a purchase order. The published figures belong to the sources that published them and were true for the systems they described; the arithmetic is ours and depends on the assumptions printed beside it. Take the method: normalise the leaf count, make the factory produce a panel schedule, and read the three maximum panel numbers against each other instead of beside each other. Then get every number you actually intend to order confirmed in writing, against the standard your market uses, by the people who will manufacture your set.
