Dormer Details

jackofairbrother · 2026-02-01T17:34:55+0000

Hi all,

Just framed a small flat roof dormer. Would appreciate a bit more info on the waterproofing details between the new grp/rubber roof and tiled roof.

I have attached a picture below of the area.

Roof is set lower than the ridge. So will have lay board going up and under the ridge.
I have installed soffits to the side and front.
At the moment I have carried the roof joists either side of the frame in to existing roof on to a noggin to support it.
On the lay board above, how far do I extend it sideways where it meets the tiles. Does it just go up against the tiles and lead covers the gap and goes down over the side of the soffit?

Cheers

Jack

noseall · 2026-02-01T20:41:23+0000

jackofairbrother said:
Hi all,

Just framed a small flat roof dormer. Would appreciate a bit more info on the waterproofing details between the new grp/rubber roof and tiled roof.

I have attached a picture below of the area.

Roof is set lower than the ridge. So will have lay board going up and under the ridge.
I have installed soffits to the side and front.
At the moment I have carried the roof joists either side of the frame in to existing roof on to a noggin to support it.
On the lay board above, how far do I extend it sideways where it meets the tiles. Does it just go up against the tiles and lead covers the gap and goes down over the side of the soffit?

Cheers

Jack

It needs to travel far enough past each side to collect the lead flashing that will be travelling upwards from the dormer sides. You will need to lay the roof tiles and dormer side flashings up to this junction before doing the flattie.

jackofairbrother · 2026-02-02T08:19:14+0000

Thanks. So you dress the lead over the top of the Osb. Would you then go back over the lead with the roof finish ie GRP?

noseall · 2026-02-02T10:09:01+0000

jackofairbrother said:
Thanks. So you dress the lead over the top of the Osb. Would you then go back over the lead with the roof finish ie GRP?

Are you not adding insulation on top of those roof joists?

wasson · 2026-02-02T12:29:11+0000

You can buy these? https://www.leadworx.com/shop/corner-flashings/lead-dormer-corner-pair/

noseall · 2026-02-02T12:32:18+0000

chriskernow said:
You can buy these? https://www.leadworx.com/shop/corner-flashings/lead-dormer-corner-pair/

You sit them at the bottom of the dormer upright exit, either side ('L' & 'R').

The faff at the top requires a bit more 'tailord fabrication' and usually a lot of bossing and bashing.

jackofairbrother · 2026-02-02T13:59:07+0000

noseall said:
Are you not adding insulation on top of those roof joists?

No it’s going to be a cold deck with insulation
between and underneath

noseall · 2026-02-02T14:43:04+0000

jackofairbrother said:
No it’s going to be a cold deck with insulation
between and underneath

You know.that means a venting headache, don’t you?

jackofairbrother · 2026-02-02T15:21:30+0000

50mm clearance above insulation. vented soffits on both sides of house and all sides on dormer.

On the lead detail mentioned above.

The lead dresses on top of Osb and goes over the GRP? Wouldn’t imagine the other way round as it wouldn’t adhere to lead?

noseall · 2026-02-02T15:41:11+0000

jackofairbrother said:
50mm clearance above insulation. vented soffits on both sides of house and all sides on dormer.

On the lead detail mentioned above.

The lead dresses on top of Osb and goes over the GRP? Wouldn’t imagine the other way round as it wouldn’t adhere to lead?

The GRP will adhere to the lead and go on top.

jackofairbrother · 2026-02-02T16:04:45+0000

Oh okay. Thank you.
Been impressed by your previous jobs you’ve posted

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Dormer Details

jackofairbrother

Attachments

noseall

jackofairbrother

noseall

wasson

noseall

jackofairbrother

noseall

jackofairbrother

noseall

jackofairbrother

DIYnot Local

Similar threads