Corrugated Roof Sheets Round Velux Windows

An Eccentric Anomaly: Ed Davies's Blog

After I abandoned my first attempt at fitting the roof sheets under the south-roof Velux windows it took me most of September to come up with an alternative plan I was not too unhappy with.

There are various constraints. The simplest is that the sheets need to be long enough to be overlapped by the Velux flashing (“skirt”) below the window by a wide-enough margin that it isn't immediately a leak but not so far that it pushes the skirt up at too high an angle (see the last picture in the September blog post if that doesn't make much sense). It turns out that 1600 mm is a more suitable length for this than the 1550 mm I'd originally ordered.

Slightly harder, the sheets below the window need to go under the skirt but the sheets up each side of the window need to go over it and over the foam strips and gutters each side of the window while at the same time we want a good weather-tight seal between those two sheets. Fortunately corrugated roof sheets are stiff lengthwise but fairly flexible across so the lower sheet can be curved a bit to do this.

While we're about it, there also needs to be fixing points for the solar thermal panels which are to go along the bottom edge of this roof (with gaps under the two bedroom windows which should be left free for means of escape). I had taken a bit of a “cross that bridge when I come to it“ view on that for the first attempt but on further consideration I unpacked the frame for one of the panels and found that they'd be far simpler to mount if there was something to screw to on 600mm centres vertically. Some sort of rail (Unistrut?) will be needed across the roof to actually bolt the panels to but some battens directly under where they'll go will help.

Accordingly, I replaced the middle batten below the window with three shorter fresh ones. Starting from the bottom batten the middle one is on the normal 850mm centres for screwing the roof sheets to. It has a 22mm batten (painted brown in this case) across the gap for the lower part of the roof sheet to screw to. Above and below are new battens on 600mm centres, also from the bottom, for the solar thermal panel rails. All end just outside the end of the window skirts.

Where the ends aren't over roof rafters I added a short “stub” of 12mm counter batten nailed through with a 61mm nail to go through the 35mm batten, the 12mm counter batten and part way in to the 22mm sarking boards behind. Hardly a major contribution to the structural strength but it'll help with any twist on the battens.

With one of the underneath sheets screwed in place nicely showing the curve up on to the full-depth batten under its right side. (The batten tied across is just to stop the far side of this sheet getting damaged by wind before the side sheet is screwed on top of it).

After putting both those sheets on it was time to add the firestop…

…and DPC up the verge then the full-length (2850mm) sheet to the right of the window.

And some more sheets to the right…

…and a couple of slightly shorter sheets across the top.

One of the problems with this construction is that the gutters round the Velux windows aren't really designed to deal with the torrent of water which will come down the roof with this sort of length of “fetch”. I have some thoughts on getting some “eyebrows” machined (or 3D printed) out of some UV-resistant plastic to screw above the windows to deflect the bulk of the water running down to the sides of the windows. Getting a reasonable fit to the shape of the corrugation could be fun as the sheets bend a bit as they're screwed in place.

Finally, finally two to the left and the first two sheets right at the top.

This was all done interleaved with adding some of the remaining sheets on the north roof depending on such factors as which way the wind was blowing on a particular day. However, lifting the next top sheet on the south roof will be more awkward as it needs to go up by the side of the window then move sideways to its final position. I have a scheme using both my pulleys but I'm not sure how it'll work in practice.