Accurately calculating window rough openings during wall framing ensures aluminium and timber windows slide in square, sashes operate freely without binding, and sub-sill flashing trays maintain continuous weatherproofing. Use this technical reference guide and sizing charts to plan first-fix stud openings and cripple stud schedules to AS 1684 and AS 2047.
What Is a Window Rough Opening in Australian Building?
A window rough opening is the framed opening created in a timber stud wall to receive an external window unit.
In Australian residential construction (governed by AS 1684 and AS 2047), windows are typically supplied with pre-fitted primed pine timber reveals (usually 19mm thick). The framed stud opening must be sized large enough to accept the outside reveal dimensions while leaving a 5mm to 10mm perimeter gap for leveling, plumbing, and installing compliant sill flashing trays.
Framing an opening too tight prevents the installer from squaring the window frame, resulting in binding sashes, air leaks, and water ingress. Framing it too wide leaves excessive gaps that require oversized packing and make internal architraves difficult to fix.
Australian Standard Window Codes & Sizing Chart
Australian window manufacturers (including Bradnam's, Stegbar, AWS (Vantage / Elevate), Dowell, and Wideline) designate window sizes using a standard 4-digit code: HHWW (Nominal Height in hundreds × Nominal Width in hundreds).
| Window Size Code | Nominal Aluminium Size ($H \times W$) | Outside Reveal Frame Size | Stud Rough Opening | Typical Australian Application |
|---|---|---|---|---|
| 0606 | 600 × 600 mm | 638 × 638 mm | 648 × 648 mm | WC, Powder Room, Obscure Glazing |
| 0609 | 600 × 900 mm | 638 × 938 mm | 648 × 948 mm | Laundry, Small Bathroom |
| 0912 | 900 × 1200 mm | 938 × 1238 mm | 948 × 1248 mm | Standard Bedroom, Study |
| 1015 | 1000 × 1500 mm | 1038 × 1538 mm | 1048 × 1548 mm | Bedrooms, Dining Area |
| 1218 | 1200 × 1800 mm | 1238 × 1838 mm | 1248 × 1848 mm | Master Bedroom, Standard Living Room |
| 1224 | 1200 × 2400 mm | 1238 × 2438 mm | 1248 × 2448 mm | Wide Living Room, Open Plan |
| 1824 | 1800 × 2400 mm | 1838 × 2438 mm | 1848 × 2448 mm | Feature Window, Low-Sill Living |
| 2124 | 2100 × 2400 mm | 2138 × 2438 mm | 2148 × 2448 mm | Alfresco Sliding Window / Door Height |
For complete wall framing material take-offs including studs, plates, and noggings, use our Wall Framing Calculator.
The +50mm Reveal & Shimming Formula (AS 2047)
The standard rule of thumb for framing residential aluminium windows in Australia is to add 48mm to 50mm to both the nominal height and nominal width:
Window Stud Rough Opening Formulas (with 19mm Timber Reveals):
Rough Width = Nominal Width + (2 × Reveal Thickness) + (2 × Shimming Clearance)Rough Height = Nominal Height + (2 × Reveal Thickness) + (2 × Shimming Clearance) Example for 1218 Window (1200mm H × 1800mm W): Width = 1800 + 38 + 10 = 1848 mm | Height = 1200 + 38 + 10 = 1248 mm
- 19mm Primed Pine Reveals (38mm total): Factory fitted to the aluminium frame on head, sill, and both jambs.
- 5mm Perimeter Clearance (10mm total): Provides essential tolerance for leveling the sill on packers, squaring the frame, and sliding sub-sill flashing trays into place.
Window Wall Framing Anatomy: Sills, Jacks & Cripples
Under AS 1684.2, a window opening requires specialized timber framing members to maintain load paths around the opening:
- Sill Trimmer: The horizontal timber member directly beneath the window. It spans between the jack studs and supports the bottom reveal of the window unit.
- Bottom Cripple Studs (Sill Studs): Short vertical studs spaced at standard 450mm or 600mm centres beneath the sill trimmer. They transfer window and sill loads down to the bottom wall plate.
- Jack Studs (Trimmers): Vertical studs on either side that support the lintel header. For window widths greater than 1500mm, use double jack studs.
- King Studs: Continuous full-height studs running from bottom plate to top plate on each side of the opening to resist wind loads.
- Top Cripple Studs: Vertical studs placed above the lintel up to the top plate to maintain uniform stud spacing for fixing plasterboard and external cladding.
- Lintel Header: The structural beam spanning the top of the opening. Calculate required timber and steel beam spans with our Lintel Calculator.
Standard Sill Heights by Room Type
Sill height is measured vertically from the finished floor level (FFL) to the top of the horizontal sill trimmer:
| Room / Location | Standard Sill Height (from FFL) | Rationale & NCC Compliance |
|---|---|---|
| Bedrooms & Living Rooms | 900 mm | Matches standard desk height and chair backs; comfortable viewing level while seated. |
| Kitchen Benches & Sinks | 1050 mm – 1200 mm | Clears 900mm standard kitchen benchtop height plus 150mm tile or stone splashback. |
| Bathrooms & Ensuites | 1500 mm | Provides visual privacy above 900mm vanity basins and mirrors; satisfies wet area regulations. |
| Low Feature Windows | 300 mm – 500 mm | Contemporary living designs. Must use Grade A Toughened Safety Glass under AS 1288. |
| Full Height Glazing | 0 mm (Floor Level) | Sliding stackers and fixed glass panels; requires recessed concrete slab rebates. |
Weatherproofing, Sill Trays & AS 2047 Installation
Water penetration around windows is the leading cause of building defects in Australia. Ensure full compliance with AS 2047 and the National Construction Code (NCC):
- Sub-Sill Flashing Trays: Always install a continuous extruded aluminium or folded metal sub-sill flashing tray beneath the timber reveal with a minimum 10mm rear upturn (back-dam) and side end-dams.
- Building Wrap / Sarking: Wrap the pliable building membrane into the rough opening at least 100mm on all sides before fixing the window frame.
- Solid Corner Packing: Place rigid, non-compressible plastic packers directly under the outer bottom corners and mullion junctions. Never over-pack the centre of the sill trimmer, which causes upward bowing and binds the sliding sash.
Step-by-Step On-Site Framing & Fastening Guide
Follow this proven framing sequence when assembling window openings on-site:
- Set Out Bottom Plate: Mark the window centerline on the bottom plate, then measure out half the rough opening width in each direction plus jack stud thickness.
- Cut Jack & King Studs: Cut king studs to full ceiling frame height (Wall Height minus 135mm for top and bottom plates) and jack studs to rough opening height plus sill height.
- Install Sill Trimmer: Fix the horizontal sill trimmer between jack studs, nailing through king studs with two 75mm framing nails per end.
- Fit Bottom Cripples: Cut bottom cripple studs to exact sill height and nail into bottom plate and underside of sill trimmer at 450mm centres.
- Check Diagonals: Cross-measure both diagonals with a tape measure to verify squareness before fixing lintels and top plates.