Too Long; Didn't Read
- A rake window follows a roofline angle — it is either a triangle or a trapezoid, never a rectangle, and cannot be measured with a single width-by-height.
- You need five measurements minimum: the base, both legs, the vertical height at the tallest side, and the angle — or enough dimensions to calculate the angle using trigonometry.
- You do NOT need a protractor. A 48-inch level, a tape measure, and basic rise-over-run math get you within one degree of the true angle.
- Custom shaped window replacement adds 20–40% to unit cost and 6–10 weeks to lead time — measure twice before you order.
- If your numbers don't close (base + angles don't sum to 180° for a triangle), stop and re-measure before sending specs to the fabricator.
Answer First: To measure a rake window, you need at minimum the base width, the tall-side height, the short-side height, and the rake angle in degrees. If you don't have a protractor, calculate the angle using rise-over-run: divide the height difference between the two sides by the base width, then take the arctangent. On most smartphones, that's atan((tall height − short height) ÷ base) × (180 ÷ π). Round to the nearest half-degree. That number — along with the three linear dimensions — is everything a fabricator needs.
Rake windows are one of those things that look straightforward until you're standing on a ladder with a tape measure, trying to figure out which corner is the reference point and why the angle you got doesn't match what the framing carpenter wrote on the drawings.
They are not rectangles. They cannot be measured like rectangles. And if you hand a fabricator a width-and-height spec for a trapezoid window, one of two things will happen: they will call you back asking for the angle, or — and this is worse — they will assume an angle and fabricate the wrong unit.
This article walks through exactly how to measure a rake window, how to calculate the pitch angle without a protractor, and what to hand off to a manufacturer when you're ordering a custom shaped window replacement.
What Is a Rake Window, Exactly?
Rake window — a fixed specialty window unit whose frame follows the slope of a roofline, producing a shape that is either a right triangle, an isosceles triangle, or a trapezoid depending on the geometry of the gable.
The word "rake" in carpentry refers to the inclined edge of a gable — the sloped portion of the roof overhang. A window installed in or below that rake line takes on the same angle.
You will find them in three common configurations in GTA homes:
Right triangle — one vertical side, one horizontal base, one sloped hypotenuse. Common in the peak of a simple gable above a front door. The vertical side is plumb, the base is level, and the slope mirrors the roof pitch.
Trapezoid (parallel sides) — a rectangle with one or both top corners cut to follow the roofline. One side is taller than the other, but both are vertical. The base is horizontal. This is the most common rake configuration you will encounter in Etobicoke bungalows with cathedral bump-outs, or in North York post-war semis that had dormer additions put on in the 1980s.
Parallelogram — both sides are angled, neither is vertical. Less common. You see it occasionally in contemporary builds in Vaughan or Oakville where the architect was making a statement. Measuring a parallelogram window is more involved and beyond the scope of this article.
[Image Idea: Three side-by-side diagrams showing right triangle, trapezoid, and parallelogram rake window shapes with labeled dimensions]
Why Standard Measurement Rules Don't Apply
For a regular rectangle replacement window, the measurement protocol is simple: width at three points (top, middle, bottom), height at three points (left, middle, right), take the smallest of each. Done.
Rake windows break that logic in two ways.
First, the frame is not parallel to anything plumb or level. The sloped side follows the roof pitch, which could be anywhere from a gentle 4-in-12 (18.4°) on a wide bungalow to a steep 12-in-12 (45°) on a Victorian gable in Riverdale. You cannot assume — you have to measure the actual angle in the actual opening.
Second, the shape has more information embedded in it than three points can capture. A trapezoid has four sides. If those four sides don't close correctly when you calculate the geometry, the fabricated unit will have a gap at one corner. Even a one-degree error in the reported angle produces a visible misalignment when the glazier installs the unit. At a 36-inch base width, one degree of error equals roughly 5/8 inch of gap at the apex corner.
The Five Measurements You Need
For any trapezoid rake window, record these five dimensions before you do anything else. Use a tape measure, read to the nearest 1/8 inch, and take each measurement twice.
1. Base width (B) — the horizontal distance across the bottom of the opening, measured from the inside face of the left jamb to the inside face of the right jamb. This is your reference dimension. Everything else is calculated relative to it.
2. Tall-side height (H1) — the vertical distance from the sill to the top of the frame on the taller side. For a right-triangle window, this is the only height — the other side is zero (or meets the peak).
3. Short-side height (H2) — the vertical distance from the sill to the top of the frame on the shorter side. In a right triangle, this is zero. In a trapezoid, it is some positive number less than H1.
4. The rake length (R) — the actual measured length of the sloped top edge, from the top of the tall side to the top of the short side. This is your cross-check. Once you calculate the angle, you can verify: does R equal √[(B²) + (H1 − H2)²]? If your tape measurement matches the calculated hypotenuse within 1/4 inch, you have clean numbers. If not, one of your first three measurements is off.
5. The angle (θ) — the pitch of the rake in degrees. This is what fabricators need and what most homeowners don't know how to extract.
[Image Idea: Labeled diagram of a trapezoid window with all five dimensions marked — B, H1, H2, R, and θ — showing measurement points at the frame's interior face]
How to Calculate the Angle Without a Protractor
This is the part most DIY guides gloss over with "use a protractor" — which is unhelpful when your opening is 8 feet up a gable wall and the angle needs to be accurate to a fraction of a degree.
You already have what you need: B, H1, and H2.
The rise is the vertical difference between the two sides: Rise = H1 − H2
The run is the horizontal base: Run = B
The pitch ratio is Rise ÷ Run. A 6-in-12 roof, for example, has a pitch ratio of 0.5.
The angle in degrees is the arctangent of that ratio:
θ = arctan(Rise ÷ Run) × (180 ÷ π)
On a phone calculator in scientific mode: enter (H1 − H2) ÷ B, hit =, then press the tan⁻¹ (or atan) button. The result is in degrees.
Example: Tall side H1 = 48 inches. Short side H2 = 24 inches. Base B = 36 inches.
- Rise = 48 − 24 = 24 inches
- Run = 36 inches
- Pitch ratio = 24 ÷ 36 = 0.667
- Angle = arctan(0.667) = 33.7 degrees
Cross-check the rake length: √(36² + 24²) = √(1296 + 576) = √1872 = 43.3 inches. Measure R with your tape. If you get 43 1/4 inches, your numbers are clean. If you get 46 inches, go back and re-measure H1.
The 48-Inch Level Method (Field Alternative)
If the opening is awkward to measure and you want a quick field check before pulling out the math, use a 48-inch level.
Hold the level perfectly horizontal along the base of the sloped edge. From the end of the level, measure straight down to the sloped surface. That drop divided by 48 gives you the slope ratio. Multiply by 12 to express it as a roof pitch (X-in-12). Convert to degrees using a pitch-to-angle table or the arctan formula above.
This method is accurate to about ±1 degree for slopes between 15° and 55°. Outside that range, the geometry of the measurement introduces error and you should use trigonometry instead.
Pro Tip: Most GTA homes built before 1990 have roof pitches that fall into a small set of standard ratios: 4:12 (18.4°), 5:12 (22.6°), 6:12 (26.6°), 7:12 (30.3°), 8:12 (33.7°), or 12:12 (45°). If your calculated angle is within one degree of one of these numbers, it almost certainly matches. Note the standard pitch in your order specs — fabricators recognize it immediately and it serves as a built-in double-check.
Ordering: What to Put on the Spec Sheet
Once you have clean numbers, here is exactly what to write on the specification form. Different fabricators use slightly different templates, but every Canadian custom window manufacturer needs these fields to produce an accurate unit.
| Field | What to Enter | Notes |
|---|---|---|
| Shape | Right Triangle or Trapezoid | Not "triangle window" — be specific |
| Base Width | e.g., 36 1/4" | Measure at sill, interior face to interior face |
| Tall-Side Height | e.g., 48 1/8" | State which side — left or right |
| Short-Side Height | e.g., 24 1/4" | For right triangle, this is 0 |
| Rake Angle | e.g., 33.7° | Always degrees, never "6:12" unless asked |
| Rake Length | e.g., 43 3/8" | Include as cross-check only |
| Reference Corner | e.g., "Bottom-left is square corner" | Critical — without this, units get fabricated mirrored |
| Frame Depth | e.g., 3 1/2" | Match to wall framing depth |
| Glass Type | e.g., LoE-272, Argon-filled | Match to adjacent windows in the home |
The reference corner deserves its own paragraph. A trapezoid window is not symmetrical. The fabricator needs to know which corner is the 90° square corner (where the vertical jamb meets the horizontal sill) and which corner has the angle cut. If you describe a window with the tall side on the left and the slope going down to the right, but the shop floor reads it as tall-side-right, they build a mirror image. It happens. The words "tall side is LEFT when viewed from the exterior" prevent the problem.
What Happens at the Factory
Understanding the fabrication process helps you understand why your numbers matter so much.
Vinyl frames for specialty shapes are either extruded-and-welded or fabricated from mitered sections depending on the manufacturer. For rake windows, the sloped top member is cut at the rake angle, typically on a programmable saw accurate to 0.1 degrees. The glass unit is then cut to fit the frame, not the other way around.
Most Canadian specialty fabricators — including shops in the GTA that serve contractors like us — use CNC frame welding that eliminates the manual squaring errors common in older shops. But the input coordinates still come from your spec sheet. Garbage in, garbage out.
Glass itself is cut on a CNC water-jet or score-and-break table. Insulated glass units (IGUs) for trapezoid shapes are fabricated with the spacer bar running along all four edges, including the sloped edge. The argon fill is injected through a port that is later plugged and sealed. The result is a sealed unit with the same thermal performance as a rectangular IGU — the shape does not compromise the seal or the gas fill.
In 2026, most GTA-area window fabricators are quoting 6–10 weeks for custom specialty shapes. Standard rectangular replacements ship in 3–4 weeks. That lead time difference is real, and it affects project scheduling on renovation jobs. If you're coordinating a residential window replacement with a general contractor, place the shaped window order first. And if you're unsure how the rough opening dimensions for your rake window relate to standard framing sizes, Canadian window sizing conventions explain the 1/2-inch tolerance rules that apply to custom units too.
Common Mistakes That Cause Rejected Orders or Bad Fits
Measuring the exterior rather than the interior. The exterior face of the frame includes the brickmould or trim, which overhangs the actual rough opening. Always measure the interior opening — the clear dimension inside the frame members.
Not accounting for the rough opening tolerance. Like any replacement window, a rake unit needs a 1/4-inch gap on all sides for shimming and sealing. Order the window 1/2 inch narrower than your measured interior opening width, and 1/2 inch shorter on each vertical side. Your fabricator may apply this tolerance automatically — confirm with them before specifying.
Reporting the angle in roof-pitch notation. "6:12" means something to a framing carpenter. It means less to a glazing fabricator's CNC programmer. Convert to degrees. It takes 10 seconds with a calculator and eliminates ambiguity.
Forgetting to specify the glass package. A rake window in a gable end often faces south or west, making solar heat gain a real concern — especially in GTA homes where a sunlit gable can drive summer cooling loads significantly. If the adjacent windows in that room use LoE-180 or LoE-272 glass, match the coating. Mismatched glass packages create visible colour differences between the shaped window and its rectangular neighbours.
Ordering a venting unit by mistake. Most rake windows are fixed — they do not open. This is both practical (venting hardware on an angled frame is complicated) and structural (fixed units transfer lateral loads more predictably). Confirm with your installer whether the opening position and wall structure support a venting option. If not, specify fixed glass and save yourself a 15% cost premium on hardware you cannot use.
When to Hire the Measurement Out
The measurement process described above is within reach of a careful homeowner. But there are three situations where we recommend professional measurement before ordering:
The opening is inaccessible. If the rake window is at peak height in a two-storey foyer, measuring accurately from a ladder at the angle required for plumb readings is difficult and carries fall risk. A professional brings a scaffold board or an articulating ladder and takes the reading at the frame, not from six feet away.
The existing window is still in place. If you're replacing rather than installing new, you can sometimes pull the specs off the existing unit — but only if the original was manufactured accurately. Older windows in Etobicoke and Scarborough from the 1970s and 80s were sometimes site-built by glaziers and may not match a true geometric angle. Measure the opening, not the old unit.
The rough opening is out of square. Houses settle. A gable wall in a 1950s bungalow has had 70 years to move. If your level reads a twist in the rough opening — the sill isn't flat, or the vertical jamb isn't plumb — the opening itself needs to be assessed before a window is ordered. No amount of shimming corrects a 3/4-inch out-of-plumb condition. The frame may need to be re-squared first.
If any of these apply, the measurement visit is money well spent. An incorrect custom unit costs the price of a second fabrication run and 6–10 more weeks of waiting.
Cost Reality: What Shaped Windows Actually Cost in the GTA
Straight talk: shaped windows cost more than rectangles, and the premium is real.
A standard vinyl casement or double-hung in the GTA runs $600–$1,200 installed, depending on size and glass package. A trapezoid or triangle rake window in the same vinyl material, same glass, runs $800–$1,500 installed. That 20–35% premium covers the non-standard frame fabrication, the custom glass cut, and the additional installation time for fitting and sealing a non-square unit.
The cost does not scale linearly with size. A small 18×24 triangle costs nearly as much as a 36×48 trapezoid because the setup cost for the custom fabrication run is largely fixed. If you're replacing multiple shaped windows — say, a matched pair of rake units on a gable end — batch them in a single order. You pay the setup cost once.
Painted or wood-grain finishes add another $150–$300 per unit and push lead times to 10–12 weeks. Standard white vinyl is the most economical option for shaped units.
Frequently Asked Questions
What is a rake window?
A rake window is a fixed specialty window unit that follows the pitch of a roofline. It takes either a triangular or trapezoid shape depending on whether one or both sides of the frame are cut at an angle. Rake windows appear most often at gable ends, in cathedral-ceiling bump-outs, and above entry doors where the roofline slopes directly above the glazing.
Can I replace a rake window myself?
The measurement portion is achievable if you're comfortable with basic trigonometry. The installation is a different matter. Rake windows sit in structural gable framing, and the flashing and sealing around an angled frame requires experience with both carpentry and glazing to prevent water infiltration. An incorrectly sealed rake window in a Toronto winter is a fast path to rot in the gable structure above.
How long does custom shaped window replacement take in Toronto?
In 2026, most GTA-area fabricators quote 6–10 weeks for shaped windows in standard white vinyl. Painted finishes push that to 10–12 weeks. Plan accordingly — if you need the window in before fall, the order needs to be placed by mid-summer at the latest.
Do rake windows need to meet Ontario Building Code egress requirements?
No. Rake windows are fixed — they don't open. Egress requirements under Ontario Building Code Section 9.9 apply only to operable windows in sleeping rooms. A fixed rake window is classified as a skylight or a decorative glazing element depending on its position, and it is governed by energy performance requirements (OBC SB-12) rather than egress rules.
What information does a window manufacturer need to fabricate a trapezoid window?
At minimum: base width, tall-side height, short-side height, and the rake angle in degrees. Most manufacturers also request the diagonal (hypotenuse) as a cross-check. Always specify which corner is the 90° reference corner — without that, mirrored fabrication errors are common. If you're matching an existing glass package, include the glass specification (LoE coating type, gas fill, spacer type) so the new unit performs consistently with adjacent windows.
Not sure your measurements are right before you order?
We do on-site measurement visits for shaped windows across the GTA. One visit, complete spec sheet, ready to send to fabrication. No obligation to order through us — though most people do.
Book a Measurement Visit