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How to measure openings and size aluminium doors and windows

Aluminium fenestration products can add significant value to buildings. Measuring and product sizing errors can lead to expensive delays and remanufacturing of wrongly sized products.

By applying the following guidelines and principles the sizing of your aluminium products should be a smooth and accurate process.

Building openings

A building opening consists of a reveal which is the top and side surfaces to which the aluminium product will be fixed. The bottom of an opening is the floor in case of a door and the sill in case of a window. Building openings can have raw (unplastered) or plastered reveal finishes at the time of measurement and installation. Ideally the state of the opening when measuring should be as it would be when the products will be installed.

Window and door openings are hardly ever perfect: They can be out of square with unparallel and non-linear sides. The opening can even be bowed. Aluminium windows and doors are only made square with parallel sides. These two conflicting states (imperfect openings and perfect windows and doors) can cause sizing problems if care is not taken.

Arched opening

Bowed opening

Unparallel opening

Non-linear sides

How to measure openings

Openings are measured vertically and horizontally. For each, three measurements are typically taken (right, centre and left or top, middle and bottom). On very wide openings (eg. For doors wider than 4 metres) more measurements should be taken.

Measurements are taken from the reveal, sill and floor surfaces using a tape measure or a laser measuring device. Take care to identify and measure from extremities protruding into the opening to ensure that minimum dimensions are incorporated in your measurements.

For larger openings it is important to ensure that the top and bottom of the openings are measured relative to a level reference line (datum line).  Without this precaution measurements could be hopelessly wrong.

Wrong way to measure

Right way to measure

Adding the smallest measurements above and below the datum line (green arrows above) together, will give the correct measurement to size the smallest feasible product.

From measurements to product manufacturing size

The measurement of the opening and the manufacturing size of the product will never be equal. Several allowances need to be made to the measurements taken.

Fitment tolerance

Aluminium products need to be made slightly smaller than building openings to allow for easy fitment and allow for opening tolerances and imperfections.

The tolerance applied depends on the finish of the reveal. A pre-plastered reveal will be less forgiving than a reveal to be plastered after the product has been installed. For plastered reveals a 5mm (2.5mm per side) fitment tolerance reduction from the measurement is appropriate for fairly square and straight-edged openings. For raw (unplastered) openings a 10m (5mm per side) fitment tolerance reduction from the measurement is adequate.

Product bottom interface

A product’s interface with the floor or windowsill can impact the required size of the product.

For windows, the impact is only affected if a sloped windowsill is going to be added after the window is installed. The window size should thus be reduced to allow space for the sill to be built by the builder after installation of the window. The reduction depends on the type of windowsill envisaged and should be checked with the builder. In some cases, the windowsill is preinstalled but generally this is not the case.

For doors, the size is impacted by the interface of the threshold with the floor. The bottom aluminium frame of a door is called the threshold. Ideally a threshold should be level and flush with the top of the floor finish level (FFL). This prevents a tripping hazard and reduces the likelihood of water ingress underneath the door threshold.

Allowing for the threshold to align with the top of the FFL has an impact on the size of the product and depends on three factors as follows:

1. The thickness of the product threshold

Product Type Threshold Thickness (mm)
Hinged door with threshold 30
Hinged door without threshold 0
Standard Patio Door 30
Palace Sliding door 50
Vistafold stacking/folding door 45

2. The relative position of the FFL to the measurement surface.

Floor element Description Typical Thickness (mm) Typical Vertical Measurement below FFL (mm)
Floor finish Top of the FFL like tiles, carpet, laminated flooring. Tiles: 10 -20 Lam. flooring: 10 – 15 Vinyl: 5 -10 0
Screed Typically the surface onto which the floor finish is placed. 30-50 Floor finish thickness
Floor Slab Rough first concrete pour onto which screed is placed. 100-300 Floor finish thickness PLUS Screed Thickness

3. The bottom surface to which the height measurement was taken.

The lower point or level to or from which the height measurement is taken determines how the overall height measurement is adjusted

Bottom Measurement Surface Height measurement adjustment required
FFL Add product threshold thickness to measurement
Screed Subtract floor finish thickness and Add threshold thickness to measurement
Slab Subtract screed thickness Subtract floor finish thickness and Add threshold thickness to measurement

Floor standing windows and shopfronts

Windows and shopfronts that extend to the floor are not affected by aligning thresholds or bottom profiles with FFL. It is however desirable to embed the product into the floor to create a better finish and improve water ingress prevention.

It is thus advisable to oversize the product manufacturing height (add to the opening height measurement after allowance for fitment tolerances). A 5- 10mm “bite” into the FFL is desirable.

Measuring before breakout of existing wood/steel windows and doors

In breakout type projects aluminium windows and doors are intended to replace existing to steel or wooden products. Measurements are typically taken before existing products can be removed. In these instances, openings are measured from existing plastered reveals which will be altered during removal of the old fenestration elements.

The manufacturing sizes of the aluminium products will be larger than the measured opening sizes. An estimate of the addition to the measurements has to be made. It takes some judgement and experience, but the trick is to rather add less than more – typically 10mm in total (5mm per side).

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