How to specify aluminium windows: the builder guide

Most specification errors are not wrong values. They are values decided in the wrong order: a face width chosen from a brochure before the wind load is calculated, a glass build fixed before the acoustic requirement is known, a threshold detailed before the floor level is set.

Every part of how to specify aluminium windows follows from that order. Performance first, because it is the only step that can veto the rest. Glass second, because it carries most of the cost and most of the performance. Frame and finish third, operation fourth, hardware fifth, and the interface with the wall last, because it is where the warranty is actually decided.

A window is rated against a standard, and the standard is set by where the building stands. North America uses NAFS, and Canada adds the CSA A440S1 supplement, which supplies the Canadian climate figures and the Canadian air and water levels. Europe and the UK use EN 14351-1. Australia uses AS 2047, with AS 1288 for the glass. The same physical unit can pass one and fail another, because the design pressures and the acceptance criteria are different.

So the first line of the specification is not a number. It is the standard, the class, and the design pressure, written in that order: NAFS class and DP, or AS 2047 performance level, or the EN 14351-1 classification string. Everything downstream is sized against it.

In Canada the supplement also supplies the driving rain wind pressure by municipality, and that figure is not a fixed percentage of the design pressure. It can be higher. A specification that assumes a percentage has assumed the wrong number, which is the subject of its own piece in the collection above.

Glass is where the performance lives and where the money sits, and it is the second step of how to specify aluminium windows. Four decisions, in order: thickness from the load, coating from the solar and thermal brief, cavity and gas from the U-value target, lamination from the impact and acoustic requirement.

Thickness comes from a calculation, not from habit. On a large fixed pane the deflection at design pressure decides the thickness, and a pane that passes the stress check can still bow enough for the client to reject it. That is a separate acceptance line, and it belongs on the drawing before the glass is ordered.

The coating choice then decides the rest. A soft coat gives the better U-value and needs the cavity sealed properly; a hard coat is more tolerant of handling and of the double-sided exposure that a corner unit creates. Solar control in a hot market is a different calculation from winter heat retention in a cold one, and a build that does both is a compromise with a number attached.

The frame has three jobs: carry the glass, break the thermal bridge, and return a sightline the architect will accept. The order matters. The structural depth and the wall thickness come from the wind load and the pane size; the thermal break comes from the U-value and the condensation risk; the face width is what is left over.

In practice, a slim face is an outcome of a deeper profile, not a design starting point. A series that advertises a 35 mm face gets there with a deeper chamber and a structural section, and a factory asked to hit the face width on a shallow profile will thin something that should not be thinned.

The opening type decides the hardware, the threshold, the drainage and the maintenance schedule. A fixed light has none of those problems. A casement needs a friction stay sized to the leaf. A tilt-turn needs a gasket set that seals in two positions. A lift-slide needs a track, a drainage path and a leaf weight the hardware can carry. A bi-fold lives or dies by the top track and the stacking space.

The choice is usually made on the elevation drawing, by the way the room is meant to be used. That is the right instinct, and it needs one check before it is locked: whether the required clear opening width, the leaf weight and the threshold height are achievable in the series being specified.

Hardware is specified by cycle count and by class, not by brand loyalty. A multipoint lock, a hinge class and a handle are three separate decisions, and the security standard of the market decides how much of that is mandatory: a South African house asks a different question from a Queensland one.

The useful line on a schedule is the standard the hardware is tested to, with the class, plus the operation count. A schedule that says heavy duty has said nothing that can be verified.

The finish decision is two decisions: the process and the class. Powder coating and anodising are different products with different repair routes; within powder, the AAMA 2604 and 2605 classes describe different levels of colour and gloss retention under UV, and a coastal or high-UV site is where the difference shows after five years.

Batch consistency is the part that gets missed. A sample is one powder lot and one oven run. A project of forty units may span three lots, and a colour match that looked perfect on the sample can read as two shades on a long elevation. Ask how the batch is controlled and how a touch-up is specified.

This is where warranties die, and it is the step most often left to the site. Three numbers belong on the drawing: the rough opening tolerance, the flashing sequence, and the threshold detail.

The rough opening has to be measured to a stated method and stated tolerance, because the frame is built to a tolerance too, and the two have to leave a shim gap that can be sealed. The flashing has to be shown in sequence, with the sill pan under the frame and the membrane lapping over the flange, not the other way round. The threshold has to show the finished floor level, the drainage path and the height above the external paving.

When those three are on the drawing, the installer has a target. When they are not, the installer improvises, and the improvisation is what the first storm finds.

• The standard, the class and the design pressure, with the negative pressure stated separately.
• The water test pressure, and the air leakage class the market requires.
• The glass build, written as thicknesses and cavity, with the coating named by surface.
• The frame series, the face width and the wall thickness.
• The operation type per opening, with handing and the clear opening required.
• The hardware standard and class, with the operation count.
• The finish process, the class and the colour reference.
• The rough opening size, the tolerance and the fixing method.
• The flashing and threshold details by drawing number.
• The test report reference for the configuration, with its date.

• A performance claim with no report behind the exact configuration.
• A glass substitution approved late, which quietly changes the U-value and the weight.
• A face width fixed before the load calculation, which forces a thinner wall than the span needs.
• An operation chosen for the elevation and never checked against the leaf weight.
• A threshold and flashing detail left to the site, discovered at the first rain test.