Ask a window supplier whether a sliding window can be an egress window and you will get a confident answer. That confidence is the problem. The honest answer is that the question is unanswerable in the form it is usually asked, because egress is not a property of a window. It is a property of a room, an installed opening and a finished floor, and it is measured by somebody with a tape measure after the window is in the wall.
No factory holds an egress approval. No series name carries one. What exists is a set of conditions applied to one opening in one room, and the reason a sliding window makes the conversation harder than a casement does is simple: the code measures the hole, and a slider only ever has half of one.
In the family of codes derived from the International Residential Code, five conditions apply at the same time, in the fully open position:
| Condition | Commonly published figure | What it actually governs |
|---|---|---|
| Net clear opening area | 5.7 sq ft (0.530 sq m) | The hole, not the glass, not the frame, not the rough opening |
| Minimum net clear height | 24 in (610 mm) | Measured at the narrowest point, with the sash fully open |
| Minimum net clear width | 20 in (508 mm) | The dimension a sliding window struggles with most |
| Maximum sill height | 44 in (1118 mm) | To the bottom of the clear opening, from the finished floor |
| Operation | No key, no tool, no special knowledge | Applies to the window and to anything placed over it |
Four of those five are numbers. The fifth is a sentence, and it is the one that disqualifies more installations than any dimension, because it is the one that follows the window for its whole life. A lock changed in year six, a screen fixed in place in year three, a grille added after a break-in: none of these changes the glass, and all of them change the answer.
Here is the single most useful thing to know about egress sizing, and it is true on both sides of the Atlantic.
Take the two minimum dimensions from the table above. Twenty inches wide and twenty-four inches high. A window that opens to exactly that is not an egress window. Twenty times twenty-four is 480 square inches, which is 3.33 square feet, and the requirement is 5.7. The window satisfies both minimum dimensions and fails the area test by a wide margin.
The area requirement is 820.8 square inches. That single number defines a curve, and the two minimum dimensions sit well inside it:
| If one dimension is at its minimum | The other must be at least | Comment |
|---|---|---|
| Height 24 in (610 mm) | About 34.2 in (about 870 mm) of width | Roughly 14 in more than the published width minimum |
| Width 20 in (508 mm) | About 41 in (about 1040 mm) of height | Roughly 17 in more than the published height minimum |
| Both at minimum, 20 x 24 in | 3.33 sq ft | Fails. Extra area cannot be traded against short height, and vice versa |
This matters more for a sliding window than for anything else, because a slider has less width to give away. It is already spending half of it.
Most egress arguments happen because two people are quoting two different numbers and both believe they are talking about the size of the window.
| The number | Where it comes from | Who uses it |
|---|---|---|
| Nominal callout, for example 1200 x 1200 mm | The catalogue name for the unit | Nobody who has to pass an inspection |
| Overall unit size | What actually arrives on the truck | The framer |
| Rough opening | Unit size plus the installation gap | The builder, and it is already larger than the window |
| Net clear opening | The hole, measured fully open, at the narrowest point | The inspector. This is the only one that counts for egress |
| Openable sash area | The area of the sash that moves, however far it moves | Ventilation rules in some codes. A different measurement entirely |
The last row is not a footnote. It is the subject of its own section below, because the gap between row four and row five is where a sliding window lives.
A casement sash swings out of the opening and the whole frame becomes the hole. A sliding sash parks itself on top of the other half of the window, so the hole is at most half the frame width, minus whatever the meeting stiles take.
Below is the arithmetic. The deductions are illustrative, not ours, and not a product claim. For a two-panel unit with one fixed and one moving panel, take the net clear width as roughly half the frame width minus about 75 mm for the interlock, and the net clear height as roughly the frame height minus about 100 mm for the head and sill rails. ｒｅｐｌａｃｅ both numbers with the manufacturer's published net clear opening for the exact size you are ordering, or measure it yourself with the sash fully open.
| Frame size (mm) | Estimated net clear (mm) | Area | Against 5.7 sq ft, 24 in, 20 in |
|---|---|---|---|
| 900 x 900 | 375 x 800 | 0.30 sq m, 3.2 sq ft | Fails width and area. Not close |
| 1200 x 600 | 525 x 500 | 0.26 sq m, 2.8 sq ft | Fails height outright. 500 mm is well under 610 mm |
| 1200 x 900 | 525 x 800 | 0.42 sq m, 4.5 sq ft | Fails area. A very common bedroom proportion |
| 1200 x 1200 | 525 x 1100 | 0.58 sq m, 6.2 sq ft | Passes area, but the width margin is about 17 mm. One revised section drawing away from failing |
| 1500 x 1200 | 675 x 1100 | 0.74 sq m, 8.0 sq ft | Passes with working margin |
| 1800 x 1500 | 825 x 1400 | 1.16 sq m, 12.4 sq ft | Passes comfortably. This is the shape a slider wants to be |
Once the opening is tall enough, the width minimum governs rather than the area, and the required frame width stops falling. Below that point, every 100 mm of height you give up costs you frame width.
| Frame height | What governs | Minimum frame width, two-panel slider | Comparable casement frame width |
|---|---|---|---|
| 900 mm | Area, about 663 mm of clear width needed | About 1475 mm | About 815 mm |
| 1200 mm | Width minimum, 508 mm | About 1165 mm | About 660 mm |
| 1500 mm | Width minimum, 508 mm | About 1165 mm | About 660 mm |
The casement column uses the same illustrative method with a smaller width deduction, because the sash leaves the opening instead of parking in it. The gap between the two columns is roughly 500 mm at a 1200 mm frame height and roughly 660 mm at 900 mm. That gap is the real cost of choosing a sliding window for an escape opening, and it is paid in wall, not in money. In a masonry opening or a foundation wall, that is the difference between using the hole you have and cutting a new one.
This is the part that surprises people, and it is worth its own section because it is a pure own goal.
Our TS130 side press window is published in two configurations: one fixed plus one movable, and middle fixed with two sides movable. The second configuration is a genuinely good product for daylight and for cross ventilation. For escape it is worse, because each moving panel is about a third of the frame instead of about a half.
| Frame width | Two panels, one moving | Three panels, two moving | What the three-panel version buys you |
|---|---|---|---|
| 1600 mm | About 725 mm of clear width | Not applicable at this width | The reference case |
| 1800 mm | About 825 mm of clear width | About 525 mm of clear width | Two escape holes instead of one. The code counts one |
| 2400 mm | About 1125 mm of clear width | About 725 mm of clear width | More glass, and a hole 400 mm narrower than the two-panel at 1800 mm |
The same logic applies one level up in door sizes. Panel count on a patio slider is a look, a daylight decision and a ventilation decision. It is also an escape decision, and the three are not aligned.
Where two sliding sashes meet, the opening loses a pair of stiles, not one. Our published sash widths are 75 mm on TS130 and on SL126/182 in the 75 variant, and 96 mm on SL126/182 in the 96 variant. On paper the difference is 21 mm. Against a 508 mm requirement that is not much, but the margin at 1200 x 1200 mm in the table above was about 17 mm, so 21 mm is the difference between passing and failing in the most ordinary bedroom proportion there is.
The practical instruction is not "choose the narrower sash". It is ask for the section drawing with the interlock dimensioned, and compute the hole from that drawing rather than from the frame size. A sash width is a sightline number. The interlock is the number that closes on your clearance.
On a sliding window, the bottom of the clear opening is the top of the bottom rail, not the frame sill. The 44 in limit is measured to the bottom of the clear opening, so a deeper bottom rail pushes the measurement up even though the frame has not moved.
At door scale this stops being a measurement question and becomes a physical one. SL126/182 is published with two bottom track options, 43 mm high and 8 mm low. If that door is the escape route from a bedroom to a terrace, 43 mm is a step you have to lift your leg over in the dark, and 8 mm is very nearly flush. Both are legitimate products; they are not interchangeable in this room.
Codes permit bars, grilles, covers and screens over an escape opening on one condition: releasable or removable from the inside without a key, a tool, special knowledge, or force greater than normal operation of the window itself. One city amendment adds that such bars, grilles or grates shall weigh no more than 30 pounds, about 13.6 kg, which tells you how literally this clause gets read.
On a sliding window the mesh panel usually sits in the outer track. So the question to put in writing, before you order, is: can this screen be lifted out from inside the room, by someone who has never seen it before, in the dark, without a tool? A permanently fixed screen with no inside release is one of the recurring findings at inspection, and it reduces the effective clear area even though the window itself is fine. TS130 lists an inward-opening mesh among its extended functions. That configuration is the one that answers the question directly, so if the room needs escape, that is the configuration to ask about.
In the IRC family the limit runs one way: the bottom of the clear opening not more than 44 in, about 1118 mm, above the floor. The measurement is to the finished floor. A code-interpretation source makes the consequence explicit: if the finished floor is tile or hardwood that adds thickness, the rough sill has to be set lower to keep the finished sill inside 44 in. That is a sequencing problem, not a product problem. Somebody chooses the floor after somebody sets the window, and the margin disappears in between.
England runs it in both directions. Approved Document B guidance puts the bottom of the escape opening between 800 mm and 1100 mm above the finished floor, with rooflights in a loft conversion allowed between 600 mm and 1100 mm. So there is a floor as well as a ceiling: a cill set very low is not automatically better, and on period properties with high cills this is the condition that fails most often.
North America says "not higher than". England says "not higher than and not lower than". Both are trying to describe the same person: somebody woken by smoke, who has to get a leg over the sill without a stool.
"Without the use of a key, tool or special knowledge." Every dimension in this article can be satisfied and the window can still fail on that sentence.
The ventilation rule and the escape rule are written about the same physical window and they do not measure it the same way. This is not a subtlety. It is the reason a window can be right and wrong at the same time.
The Australian National Construction Code requires a ventilating area of not less than 5 percent of the floor area of the room for Class 1 to 4 buildings, rising to 10 percent for many Class 5 to 9 buildings. Its explanatory information then says something that egress codes would never say: the ventilating area of a window is measured as the size of the openable sash, and this is the case regardless of the type of window, whether awning, casement or sliding, and irrespective of any restriction on the openable sash.
| Rule | What it measures | For a casement | For a sliding window |
|---|---|---|---|
| Escape and rescue opening | The hole, fully open, at the narrowest point | Very nearly the whole frame | About half the frame, minus the interlock |
| Natural ventilation | The area of the sash that can open, however far it opens | The sash, which is nearly the same number as the hole | The moving sash, credited at full area even though half of it is never uncovered |
A national advisory note on openable windows makes the gap explicit by drawing it: the correct ventilation area for an awning window is the full openable sash, not the smaller boxed-in opening created by a restrictor. That is a defensible way to write a ventilation rule. It is not a defensible way to write an escape rule, and the escape rule does not do it.
So the practical sentence to carry away is: a sliding window can satisfy the ventilation percentage and fail the escape requirement on the same drawing, because the ventilation rule never asks how big the hole is. If your schedule shows one compliance tick per window, it is the wrong schedule.
New-build egress failures are design errors and get caught at plan review. Replacement failures are quieter, because everybody involved believes they are putting back what was there.
The lesson is procedural rather than technical: measure and photograph the existing opening before the old window comes out. Once it is gone there is nothing to compare against, and "the old one was bigger" is not evidence.
Where the bottom of the clear opening is below the adjoining ground, a window well is required, and it brings its own numbers: horizontal area not less than 9 sq ft, horizontal projection and width not less than 36 in, about 914 mm, and the well has to allow the opening to be fully opened. Where the vertical depth exceeds 44 in, a permanently affixed ladder or steps are required, usable with the window in the fully open position, with rungs at least 12 in wide, projecting at least 3 in from the wall, spaced not more than 18 in on centre. The ladder may encroach into the required well dimensions by up to 6 in. Drainage is required unless the foundation sits on well-drained Group I soils.
The sliding-specific trap is in the phrase "allow the opening to be fully opened". A casement sash swings outward into the well. A sliding sash travels sideways, and the end of its travel finishes inside the well's side wall. A well sized to the hole is not necessarily a well sized to the travel. If the sash cannot finish its run, the window is not fully open, and a window that is not fully open does not have a net clear opening to measure. Also check the well against a deck or porch above: escape openings under decks must be fully openable and provide a path of not less than 36 in in height to a yard or court.
We make aluminium system windows and doors. We do not hold an egress approval, and neither does any other manufacturer, because there is no such thing to hold. The approval belongs to your building, granted by your authority, after your window is installed. What a factory can honestly do is give you the inputs, and then get out of the way.
What we can supply: the section drawing with the interlock dimensioned; the sash and frame dimensions; the track heights; the glass build-up; the hardware set; the material and test evidence we publish. What we cannot supply: the determination of which rooms in your plan require an escape opening, the certificate, or the sign-off.
One more thing to keep out of the conversation. The 240-hour salt spray result and the 5,000 N shear figure belong to one series in our catalogue, do not transfer to others, and are not air, water or wind ratings. The 30,000-cycle figure is a rig test on hardware, not a service life, and a handle cycle is not a loaded sash cycle. No series in our catalogue publishes air, water or wind ratings at all, which means that for those properties there is no declared threshold to deviate from. None of these is an egress number either. Bringing them into an egress discussion is how a simple measurement turns into an argument.
| Item | Our published position | Why the gap exists |
|---|---|---|
| Net clear opening, in mm, for the size you order | Not published | It is a function of the size and configuration you order, not of the series name. Ask per line item, not per catalogue |
| Which panel moves | TS130: one fixed plus one movable, or middle fixed with two sides movable | Two options is not a default. Confirm which one the quotation is for |
| Sash width and interlock | TS130 75 mm; SL126/182 96 mm and 75 mm; SL155 44.5 mm | Sash width is a sightline. The hole loses the pair where sashes meet |
| Bottom track height | SL126/182: 43 mm high, 8 mm low | It sets the bottom of the clear opening and it is the step you climb over |
| Screen type and inside removal | 80 mesh stainless options; inward-opening mesh listed as an extended function | The clause is absolute. Nominate the mesh for the room that needs escape |
| Operating sequence from inside | Not published as an egress statement | Count the motions. A lift-and-slide has more of them than a plain slider |
| Glass build-up behind the maximum size | Maximum sash sizes published per series | Published maximums assume a glass configuration. Heavier glass moves the maximum and the hardware duty |
| Rated load per sash | TS130 200 kg; SL155 400 kg; SL112/167 220 kg | Per sash, not per unit, and dominated by glass weight. Not an egress figure |
| Frame wall width | TS130 130 mm; SL126/182 126 and 182 mm; SL155 155 mm; SL112/167 112 and 167 mm | This is the wall depth you need. It is a series property, not a function of track count: the four-track 155 mm is shallower than the three-track 182 mm |
| Air, water and wind ratings | Not published for any series | A defect is a deviation from a declared threshold. With no declared rating, complaints become arguments rather than measurements |
Send these ten lines with the enquiry rather than after it. The right-hand column is the part that saves money, because it tells you when you have been answered without being told anything.
| Line | Fill in | Sounds reasonable, tells you nothing |
|---|---|---|
| 1 | Room use: sleeping, habitable, below grade, and the storey height above ground | "It is a bedroom" |
| 2 | Code edition and the name of the authority that will inspect | "Meets international standards" |
| 3 | Net clear opening in mm, for this exact size and configuration | "It is a standard size" |
| 4 | Which panel moves, and how many panels | "Both are available" |
| 5 | Interlock dimensioned on the section drawing | "Slim profile" |
| 6 | Bottom track height, and the finished floor build-up at the sill | "Low threshold" |
| 7 | Written confirmation that the mesh removes from inside without a tool | "Screens included" |
| 8 | The full operating sequence from inside, motion by motion | "Easy to open" |
| 9 | Glass build-up assumed by the published maximum size | "Standard double glazing" |
| 10 | Window well width, if below grade, measured to the sash travel | "The installer will handle it" |
| Situation | Why it goes wrong | Specify instead |
|---|---|---|
| A narrow wall in a room with a normal ceiling | Width is the one thing a slider cannot conjure, and the area rule punishes a short opening twice | A casement or an inward-opening sash in the same opening |
| A high cill you cannot lower | No amount of width fixes a sill above the limit, and the bottom rail makes it worse | Revisit the room layout or the escape route before revisiting the window |
| Below grade with a well sized to the glass | The sash cannot finish its sideways travel, so the window is never fully open | A wider well, or an opening type that swings rather than slides |
| A screen or grille that cannot be released from inside | The clause admits no exceptions and no partial credit | Inward-opening mesh, or a release that needs no tool and no key |
| The panel count was chosen for the view | Three panels give a narrower hole than two in the same wall, and the code counts one opening | Two panels on the escape room, three where the view matters and escape does not |
| A replacement where the old window was the escape route | Changing type can remove the like-for-like path and pull the whole opening into current code | Measure and photograph the existing opening before anything comes out |
Yes, in the sense that no common code bans the type. The sliding window has to produce the required net clear opening on its own, from the panel that moves, in the fully open position. It usually does this by being considerably larger than the casement that would have done the same job.
No. That opening is 3.33 sq ft against a 5.7 sq ft requirement. It satisfies both published minimums and fails the area test. At 24 in of height you need about 34.2 in of width; at 20 in of width you need about 41 in of height.
It is permitted if it is releasable or removable from the inside without a key, a tool, special knowledge, or more force than the window itself needs. If it cannot be removed from inside, it reduces the effective clear area and can fail the inspection on its own. On a sliding window, where the mesh normally sits in the outer track, this is worth asking about in writing.
In the code text we have seen quoted by jurisdictions, where a door is provided as the required emergency escape and rescue opening it shall be side-hinged or sliding, so a sliding door is explicitly a permitted type. The distinction is not sliding versus hinged, it is exterior versus interior: a sliding patio door to the outside can be an escape route, while a barnwood sliding door on an interior track is a room divider and never an escape route. The sill question disappears at door scale, and the bottom track height takes its place.
You need a bigger window to get the same hole. Three panels at 2400 mm leave about the same clear width as two panels at 1600 mm, because each moving panel is a third of the frame rather than a half. The same arithmetic is why the panel configuration is worth checking on any multi-panel unit, including a 3 panel entry door, where the side panels are usually fixed and the moving leaf is narrower than the overall unit suggests.
Because the honest answer depends on your size, your configuration, your wall, your floor finish and your authority, and all five of those are yours rather than the manufacturer's. A supplier who answers yes immediately has told you nothing about your project. Shortlisting on that basis is why some buyers end up comparing catalogues instead of comparing numbers, and why the phrase best entry door company produces a shortlist but never produces a net clear opening. Ask for the number in millimetres for the line item you are buying.
