Aluminium window sills – pressed and extruded
Aluminium is now the most common material for exterior window sills. It does not rust, it is light and it can be finished in any colour. But two rather different products go by the same name.
- Sheet
- Aluminium sills
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- 6 items
- Type
- information guide
Pressed versus extruded
| Property | Pressed | Extruded |
|---|---|---|
| how it is made | folded from aluminium sheet | extruded profile of aluminium alloy |
| thickness | around 0.8–1 mm | 1.2–2.45 mm depending on depth (najparapety.sk) |
| stiffness | lower, needs bedding over its full area | higher, sags less |
| nose | folded, various shapes | profiled, with a drip groove |
| depths | wide range, easily made to measure | in steps set by the manufacturer |
| noise in rain | more noticeable if not bedded | less |
| price | lower | higher |
Thermal expansion
Aluminium expands with heat more than any other common sheet-metal roofing metal: KJG gives 0.24 mm per metre for a 10 K change in temperature (galvanised steel 0.12 mm). A two-metre sill changes length by about 2.9 mm over a 60 K range. That is why ZOFI recommends an expansion gap of 1–2 mm at the end caps and splitting sills longer than 5 m. Dark colours heat up more in the sun and expand more.
End caps
End caps are plastic or aluminium, and are fitted either before or after rendering. Aluminium end caps match the sill in colour and age in the same way; plastic ones are cheaper. What matters is that the cap allows the sill to move and is worked into the render.
Aluminium and other metals
Aluminium must not be below copper – water running off copper damages it (Lysaght, per AS/NZS 2179.1). RHEINZINK lists aluminium as harmless in combination with titanium-zinc. Be careful with stainless steel components above aluminium. Overview: material comparison.
Which to choose when
- extruded for deep sills, on exposed facades where someone may lean on the sill, and where noise is a nuisance,
- pressed for ordinary depths, unusual shapes and when price matters,
- for both, the correct projection and fitting matter more than the type.
Technical sources
The sources this sheet draws on. We checked prices and values from standards on the date shown; for prices, whatever the source currently states always applies. Sources marked “in Slovak” or “in Czech” are only available in that language.
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