How to choose an exterior window sill
When choosing a window sill, people most often decide by colour. What matters more is whether it will be the right depth, whether it is stiff enough and how it is finished at the ends.
- Sheet
- Choosing a window sill
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- Sources
- 4 items
- Type
- advice article
Five questions
| Question | What it affects | Recommendation |
|---|---|---|
| How deep is the reveal after the insulation goes on? | sill depth | reveal + ~10 mm under the frame + 30–40 mm projection (ZOFI) |
| How deep and how exposed will the sill be? | stiffness | for large depths and on the ground floor, consider an extruded profile |
| Does the sound of rain bother you? | type and bedding | extruded, or pressed and fully bedded |
| What other metals are on the facade? | material | aluminium or pre-painted galvanised steel; never below copper |
| How will the ends be finished? | durability of the render | end caps with an expansion gap, worked into the render |
The types in brief
- pressed aluminium – versatile, many depths, lower price; retailers give a thickness of about 0.8 mm and depths of 70–400 mm,
- extruded aluminium – thicker (1.2–2.45 mm depending on depth), stiffer, quieter,
- pre-painted galvanised steel – folded to measure by a roofer, matches the roof flashings.
In detail: Aluminium sills, Galvanised steel sills.
Colour
Dark colours heat up more in the sun and expand more (aluminium 0.24 mm/m per 10 K according to KJG). With a dark sill, the expansion gap at the end caps matters even more.
What to ask for in a quote
- the type, material, thickness and colour,
- the depth and the lengths, piece by piece,
- the type of end caps (plastic / aluminium, fitted before / after rendering),
- how it will be fitted and bedded,
- who measures up and who is responsible for the dimensions.
Next: How to measure a window sill · window sill calculator.
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.
| Item | Source | Published by | Version / date | Checked |
|---|---|---|---|---|
| 01 | Measuring a window sill correctly before fitting it to the facade (projection 30–40 mm, +10 mm under the frame, end caps, expansion) (in Czech) | ZOFI fasády s.r.o. | undated | 14 October 2026 |
| 02 | Exterior extruded aluminium window sills – technical data (Al Mg3 alloy, 1.2–2.45 mm thick depending on depth, 25 mm nose, depths 110–360 mm) (in Slovak) | najparapety.sk (retailer) | product page | 14 October 2026 |
| 03 | Exterior pressed aluminium window sills – technical data (Al-Mg-Mn alloy 0.8 mm, depths 70–400 mm, max. length 6,000 mm, end caps) (in Slovak) | Widoor s.r.o. (retailer) | product page | 14 October 2026 |
| 04 | Installation guide – KJG half-round rainwater system 2025 (bracket fall, expansion joints, thermal expansion) (in Slovak) | KJG a.s. | 2025 | 14 October 2026 |
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