Lead flashing under cills timber frame, timber cladding

[Hannah82w]

We have built a timber frame house and mostly cladding it vertically in British larch. The window frames and cills will be aluminium, and the glazers have suggested we need to add lead flashing under the cills, which isn't detailed in our drawings etc.

Our architect has said this isn't strictly necessary, but might help keep driven rain out. We live in the south, so not in extreme weather conditions.

 

We're wondering if lead is actually necessary (what is it protecting that isn't already protected by breather membrane e.g. the timber frame, ply etc), where would it actually be fitted (over the batten under the cill but under the cladding? I don't want to see it! Does it need to wrap around the corner of the frame?), can we fit it ourselves using either rolls of lead flashing or preformed trays?

 

Would an alternative be to add another layer of breather membrane that wraps the window frame fully and down over the bottom batten to keep rain out of the timber?

 

We're in the middle of cladding so don't want to hold up progress but may need to get this in under the cladding.

 

Photos attached of one of the window openings (the bottom batten will be chamfered to send water away from the frame) and the cladding style for info.

[Hannah82w]

Our architect has also just suggested using DPC as an alternative lead, which would certainly be easier for us to do ourselves. Does anyone have any images of fitting dpc like this?

[TonyT]

Does the cill have a drip detail?

[Kelvin]

This has come up on mine too but only on the dormer window where it joins the pitched roof. My joiner has said typically you see a lead flashing come from under the window and under the standing seam roof. Also not detailed in our drawings. 

Edited May 19, 2023 by Kelvin

[Hannah82w]

To update, our BC inspector has just made a visit to site (planned but unplanned!) and didn't think we need to worry about anything under the cill as the timber cladding will come right up under it.

 

We might still add an extra bit of membrane to cover the batten just for good measure.

[Hannah82w]

2 hours ago, TonyT said:

Does the cill have a drip detail?

Yes, I'm pretty sure it does.