A casement window rough opening is not a number you look up in a size chart. It is the outcome of four separate decisions: how much clearance the frame needs, how deep the reveal actually is, which way the sash swings, and how far out of square the wall has drifted. Get three of those right and miss the fourth, and the unit still will not fit. This guide walks through each one in the order a project actually encounters them, from first tape measure to signed-off shop drawing.
Most sizing errors on a casement project are not arithmetic errors. They are vocabulary errors. Four different dimensions all get called “the window size,” and only one of them is what you frame:
A catalogue line such as “600 × 1500” may be a nominal call-out rather than a true frame size, and nominal call-outs are rounded for convenience. Before a single stud is cut, ask the supplier which of the four numbers the figure refers to, and ask for it in writing on the drawing.
The gap between frame and structure does three jobs. It gives the installer room to plumb, level and shim the unit. It leaves space for backing rod, foam and sealant. And it gives the frame somewhere to move.
That third job matters more for aluminium than for any other frame material. Aluminium has a markedly higher coefficient of thermal expansion than timber or uPVC. A wide frame on a dark-coloured façade in direct sun grows measurably across its casement window width over the course of a summer afternoon. In a tight opening that growth has nowhere to go, and the sash starts to bind on the hinge side, which is usually reported months later as a “faulty window” rather than as a framing decision.
This is why commonly published aluminium installation guidance calls for roughly 10 mm of clearance per side and at the head, against the 5 to 6 mm per side typical of a timber window. The same guidance also warns against going much beyond 15 mm per side: past that point the shims lose bearing surface and the perimeter sealant becomes the only thing holding the unit in position.
In North American light-frame practice the same idea is expressed imperially, with the rough opening commonly framed 1/2 inch larger than the window in both width and height, leaving about 1/4 inch of shim space on all four sides. Both conventions describe the same thing: enough gap to adjust, not so much that fixing and sealing stop being reliable.
The rule that overrides all of the above: do not adopt a clearance figure from a blog, including this one. Adopt the figure your window supplier specifies for that specific profile, wall build-up and finish colour, and get it written onto the shop drawing. Dark colours run hotter, and hotter profiles expand more.
Width and height get measured three times each. Depth, if it is measured at all, gets measured once — and it is the dimension that most often gets a unit rejected on site.
An aluminium casement is not a flat panel. The frame has real depth, the sash sits inside it, and where an integrated insect screen is specified the assembly gets deeper still. If the reveal is shallower than the frame, the unit stands proud of the wall, internal trims and reveals will not close, and the head detail has to be rebuilt around a window that has already been paid for.
On a thermal-break system the frame depth is driven by the insulation chamber, so a deeper profile generally means better thermal performance. Depth is a performance decision as much as a fitting constraint, which is exactly why the deeper frames below exist rather than everything being built shallow.
| ALPES model | Frame wall width | Glass sash thickness | Opening method | Thermal K-value |
|---|---|---|---|---|
| C120 Outward Opening System Window | 119 mm | 62 mm | Outward | 1.6 |
| C116 Outward-Open Window | 116 mm | 81 mm | Outward | 1.7 |
| C113 Outward-Open Window | 113 mm | 57 mm | Outward | — |
| AW90 Passive Window (tilt and turn / inward open) | 90 mm | 90 mm | Inward / tilt and turn | 1.3 |
Read that second column as a minimum reveal depth, not a target. Measure the reveal at left, centre and right, and use the smallest reading — the same discipline you already apply to width and height. On a retrofit, remember to subtract any render, cladding or insulation that has not been applied yet, because those layers change the finished depth after the window has been manufactured.
The hole in the wall is the same size whichever way the sash swings. Everything around it is not.
When a single rough opening is split into two or more sashes, the vertical divider takes width out of the daylight. Across the ALPES casement range, mullion widths run from 75/31 mm on the C120 and 75/21 mm on the C113, down to 58/22 mm on the C116 and 72/4 mm on the AW90. A 22 mm slim mullion and a 75 mm structural one give very different sightlines and very different structural capacity.
A wide opening on an exposed elevation with a high design wind pressure needs the structural mullion whether or not the architect prefers the slim one, and the rough opening has to be sized around whatever is chosen. Settle the mullion on the shop drawing rather than on site, where the only remaining options are expensive ones.
Work through these six steps on every opening, including the ones that look identical to the last. Identical-looking openings on the same elevation routinely differ by several millimetres.
An aluminium frame is an extruded rigid rectangle. It does not rack, twist or compress to accommodate an opening that is out of shape, so there is no “close enough” the way there is with timber. Fifteen minutes per opening is the highest-return task on site before delivery.
| Difference between the two diagonals | What it means | Action before ordering |
|---|---|---|
| Within 3 mm | Effectively square | Proceed. Pack and shim as normal. |
| 3 to 8 mm | Installable, but not square | Record the reading and tell the fabricator. Expect extra packing and possibly a marginally reduced unit size. |
| More than 8 mm, or a visibly bowed jamb | Structurally out of tolerance | Correct the opening: re-plaster, re-block or re-frame. Do not expect shims to absorb it. |
If the wall does not exist yet, the sequence is fixed. Mark the rough opening width on both the top and bottom plates and confirm the two marks are vertically aligned. Install full-height king studs that carry the load from the header down to the footing. Cut and fit jack studs, or trimmers, to the height that supports the header. Set the header sized for the span and the load above it. Then fit the rough sill with a slight outward fall so that any moisture reaching the cavity drains out rather than pooling behind the frame.
Two steps get skipped more than any others: the sill fall, and re-checking plumb and level after the header is fixed rather than before it. Confirm that the header size comes from the applicable span table or the engineer's specification for your jurisdiction, not from what was used on the last job.
A purchase order reading “1200 × 1500 casement, twelve units” is not a specification. A usable order carries the following for every single opening:
A manufacturer that offers both standard casement window sizes and custom dimensions should be able to take that list and return a dimensioned shop drawing for approval. If the response to a detailed schedule is a price and a lead time with no drawing, that is the point to slow down.
Use packers in assorted thicknesses and build up in small increments rather than forcing one thick shim that will compress under load. Place them beside the points where the frame will be fixed, so that driving a screw cannot pull the profile out of alignment.
Common practice is anchors no more than 600 mm apart, with one within about 150 mm of each corner, using fasteners suited to the wall: expansion bolts into concrete or masonry, screws into timber, self-drilling screws into steel stud. Tighten gradually and re-check level every few fixings. Uneven tightening is enough to bow an aluminium profile and undo the alignment work you just did.
Foam keeps expanding as it cures. A cavity packed full has bowed more than one aluminium frame, and the symptom — a sash that drags — is usually blamed on the hardware. Fill part of the depth, let it cure, then trim.
Acid-cure silicones release acetic acid while curing, which attacks aluminium over time. On a coastal project that is precisely the wrong place to introduce a corrosion mechanism. Use neutral cure, and use a backing rod so the bead has the right depth-to-width ratio to stay flexible.
Before handover, clear the drainage slots on the outer frame. Blocked drainage is one of the most common reasons water appears on the inner sill of a brand-new window that is otherwise perfectly sealed.
No. The rough opening is the hole in the wall and is always larger than the window frame, by whatever clearance the manufacturer specifies. Ordering a window to match the rough opening exactly leaves no room to shim, level or seal.
Use your supplier's figure. Where none is given, widely published aluminium guidance works on roughly 10 mm per side and at the head, and North American light-frame practice commonly frames 1/2 inch over the window in each direction. Either way, avoid exceeding about 15 mm per side, because the shims stop bearing properly.
Usually yes, but check the reveal depth first. Thermal-break frames are deeper than basic non-insulated profiles because the insulation chamber takes up that depth. A C120 at 119 mm or a C116 at 116 mm will not disappear into a reveal cut for a slim 50 mm section.
Only up to the point where structure takes over. Past a certain width the opening needs a mullion, and a mullion takes width back out of the daylight. A 58/22 mm mullion on the C116 removes far less glass than a 75/31 mm mullion on the C120, but the C120 mullion is there because the opening demands it.
The hole is the same. What changes is the clear space around it: outward sashes need exterior clearance for the swing, inward and tilt-and-turn sashes need interior clearance for both the swing and the tilt. Screens change the equation again, particularly on outward-opening units.
Up to about 8 mm of diagonal difference, generally yes, with packing and a note on the order. Beyond that, or where a jamb is visibly bowed, correct the opening first. If the frame has to be forced into a trapezoidal opening, the gaskets unseat and the hardware binds, and no amount of adjustment afterwards fixes it.
A rough opening is a specification, not a measurement. Once the clearance, the reveal depth, the opening direction and the squareness tolerance are all agreed and drawn, the rest of the project — manufacture, shipping, installation and commissioning — tends to go quietly. Where one of those four is assumed rather than confirmed, the cost shows up later, at the top of a scaffold.
If you are at the stage of turning measured openings into an order, send the schedule through with both diagonals and the reveal depth included. Whether you are working with a casement window contractor on site or ordering directly, the drawing that comes back should show the same four numbers you measured.
