Galvanised steel window sills
A roofer can fold a sill in pre-painted galvanised steel on site from the same material as the parapet coping or the flashings. It is cheap and matches in colour. But it has one weakness worth knowing about – the steel core is protected only by a thin layer of zinc and paint.
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- Galvanised steel sills
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How galvanised steel is protected
The steel sheet is coated with zinc on both sides. The designation Z275 under EN 10346 means 275 g of zinc per m² in total on both sides, which is about 20 µm per side. Pre-painted (coated) sheet – in Slovak lakoplastový plech – has an organic coating over the zinc: polyester, or thicker finishes.
Zinc is slowly used up in the atmosphere. ISO 9223 gives a first-year corrosion rate for zinc of 0.7–2.1 µm per year in category C3 environments (towns, moderate pollution). On that basis a 20 µm layer would be used up in such an environment in roughly 9–29 years; paint extends this. This is a rough calculation, not a guarantee – more in the article How long does galvanised steel last?
Strengths and weaknesses
| Strengths | Weaknesses |
|---|---|
| low material cost | cut edges have neither zinc nor paint protection |
| folded to measure from ordinary sheet | damaged paint must be touched up or it corrodes |
| same material and colour as the other flashings | a folded sill must be bedded or it drums |
| stiffer than aluminium of the same thickness | corrodes quickly below copper |
What to watch
- cut with snips, not an angle grinder – Ruukki and Lindab both exclude damage caused by grinders from their warranties,
- touch up cut edges and scratches with the manufacturer’s touch-up paint,
- sweep away metal swarf from drilling straight away, or it leaves rust spots,
- fold the nose with a drip and a 30–40 mm projection beyond the finished facade.
The domain’s original product range
The original website on this domain offered window sills made from pre-painted galvanised sheet 0.5–0.6 mm thick. This thickness is still common for sills today; for deeper sills, the choice of thickness and bedding becomes more important.
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 | EN 10346 Continuously hot-dip coated steel flat products – table of coating masses (Z275 = 275 g/m² total on both sides, ~20 µm per side) | ArcelorMittal, catalogue E20 (quoting the table from the standard) | catalogue E20 | 14 October 2026 |
| 02 | ISO 9223:2012 Corrosivity of atmospheres – categories C1 to CX and first-year corrosion rates of zinc | ISO; table summarised by the American Galvanizers Association | 2012; summary 2 December 2021 | 14 October 2026 |
| 03 | Ruukki warranty certificate – GreenCoat RWS Pro rainwater system 30 years; half the periods above 900 m above sea level (in Slovak) | Ruukki Slovakia s.r.o. | template 09/2026, for products sold after 1 September 2026 | 14 October 2026 |
| 04 | Lindab a.s. warranty certificate – conditions: min. gutter fall 5 mm/m, no contact with copper (in Slovak) | Lindab a.s. (Slovakia) | valid for products bought after 15 April 2019 | 14 October 2026 |
| 05 | Archived snapshots of the original klampiarskevyrobky.sk website (2012, 2013, 2016, 2018) (archived site in Slovak) | Internet Archive – Wayback Machine | 2012–2018 | 14 October 2026 |
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