The size on the drawing is not the size that gets installed
When buyers ask for made-to-measure aluminium windows, they usually think the critical job is recording the opening width and height. The real job is harder: deciding how much room the frame needs to be installed, squared, packed, sealed, and allowed to move over time.
That gap between the raw opening and the finished frame is the part most suppliers gloss over. Yet it is the difference between a window that slides into place and one that fights the wall from day one.
A custom aluminium frame is not meant to be the exact same size as the hole. It is meant to be the right size for the hole once the building's imperfections, finishing layers, and installation tolerances are all accounted for. Treat the opening as the final dimension and the project becomes vulnerable to the same problems over and over: tight fits, bowed frames, uneven sealant joints, sticking sashes, and visible patchwork around the perimeter.
Why a true opening is rarely square
On paper, a window opening looks like a rectangle. On site, it is often a compromise made by framing timber, masonry, render, plasterboard, and years of settling. A wall can be straight enough to fool the eye and still be several millimetres out from top to bottom or left to right.
That is why experienced measurers never trust a single width and a single height. They check the top, middle, and bottom of the opening, then compare the diagonals. If those numbers do not agree, the frame has to be designed around the smallest practical fit, not the biggest number that happened to come off the tape.
The eye forgives. The hardware does not. A sash that is forced into a slightly tapered opening may look fine on installation day, then start rubbing, dragging, or failing to latch cleanly once the building settles or the temperature changes.
The measurements that actually control the outcome
A good survey is not just a width-and-height exercise. It is a record of how the opening behaves as a structure.
- Top, middle, and bottom width tell you whether the opening is parallel or tapered.
- Left, center, and right height show whether the head and sill are level.
- Diagonal measurements reveal whether the opening is square.
- Reveal depth determines how the frame sits in relation to the wall build-up.
- Sill condition affects how the window is packed, flashed, and drained.
Each of those numbers influences the final frame size in a different way. Ignore any one of them and the tolerances start disappearing fast.
A 900 mm opening can often hide a small error because the span is short and the frame has less distance to go before it hits the side. A 2,400 mm fixed panel is a different story. A slight bow in the wall becomes obvious, and the visible gap at one end may be too tight at the other.
Finish build-up is the hidden variable suppliers skip
The opening that exists during a rough measure is not always the opening that exists on install day. Render, plasterboard, tile returns, insulation boards, waterproof membranes, trims, and architraves all change the finished dimensions.
That matters most in renovations. A frame measured before render can arrive with no room left once the final coat is on. A frame measured before internal lining can leave an oversized reveal that looks unfinished and performs poorly. Either way, the problem is not the aluminium. The problem is that the frame was sized against an incomplete version of the building.
This is where many custom orders go wrong. A supplier may have accurate numbers, but if those numbers were taken before the adjacent trades finished their work, the fabricated window is being made to a temporary condition rather than the real one.
Why aluminium makes precision non-negotiable
Aluminium is one of the best materials for custom window work because it stays straight, handles large spans well, and supports slim profiles. That same precision is also why it punishes poor measurement.
A tight aluminium frame leaves less room for improvisation than timber. There is less material to shave, less flex to absorb error, and less visual bulk to hide a gap. The frame depends on geometry staying true so the sash can close evenly, the seals can compress properly, and the locking points can engage without strain.
If the opening is wrong, the failure shows up quickly:
- sashes that bind or scrape
- hardware that does not line up cleanly
- inconsistent seal compression
- drafts at corners
- visible packers or oversized sealant lines
- frames that look forced rather than fitted
That is why the best aluminium installers talk about tolerances before they talk about aesthetics. The appearance only works if the fit is right first.
The tolerance stack that actually matters
Tolerance is not one number. It is a chain of small allowances that all have to work together.
- the original error in the building opening
- the allowance for out-of-square conditions
- the space needed for packers and fixings
- the clearance needed for sealant or foam
- the movement allowance for thermal expansion and seasonal building movement
If any one of those is ignored, the rest of the system becomes fragile. The opening might be technically close enough on a tape measure, but not close enough for a clean installation.
This is why a quote that only asks for width and height is not enough. A real fabrication decision needs to know how the opening was measured, what condition it is in, what finishes are still coming, and how the frame will actually be fixed into the wall.
What a competent supplier asks before cutting aluminium
A supplier that understands custom fabrication does not jump straight to pricing. They ask questions that expose whether the site has been measured as a finished installation or just as an early estimate.
- Is this a structural opening or a finished opening?
- What are the top, middle, and bottom dimensions?
- What are the diagonal measurements?
- What finish layers are still to come?
- Where will packers, fixings, and sealant beads sit?
- Does the opening need to align with adjacent windows, doors, or cladding lines?
- Who is signing off the final site measure?
If those questions never come up, the job is being treated like a stock-order exercise. That is the wrong mindset for a made-to-measure product.
The practical test before you approve the order
Before a frame goes into fabrication, ask one simple question: how much clearance is being allowed, and why?
A serious answer should reference the opening condition, the installation method, and the finishing schedule. It should explain why the frame is being made slightly smaller than the opening, how the installer will square it on site, and what space is needed for packers and sealant.
If the answer is vague, the job is not yet fully understood. A custom window should never depend on luck, force, or on-site trimming to make it fit. The whole point of custom sizing is controlled tolerance.
That is the real difference between a window that fits in a way that looks good for a week and a window that fits in a way that still feels right years later. The best result is not the one that matches the tape measure exactly. It is the one that matches the building as it actually is, with every imperfection and finish layer already accounted for.