Wall/roof abutment detail

Road-Hog

Doing a small extension to the existing house where I have a roof butting up against an existing wall so there is going to be soakers and lead flashings etc involved. I've been told by a plasterer that the best way to seal the interface is to cut a 3-4mm channel along the existing wall and then fit the lead counter flashing into this channel sealing it in with a silocone type product and then to bend down the lead over the soaker. Which sounds fine but is silicone a durable enough type product, will it not just degarde over time leading to leaks. Can anyone advise me if this is an acceptable detail or is there an alternative.

PaleRider

Hi,

It's probably be fine if you wash out the dust along the joint.

The plasterer could also fix a Stainless Steel 12mm Stop Bead along the lead, with about 20mm covering over the lead. But still cut the track.

Both ways will work.

Mellor

I think you are gettign confused.
The silicon isn't there is seal anything. It's there to close the joint and help keep the lead in place. The concrete wall itself isn't water tight, so silicone could even be bypased anyway.
The lead is there to seal the where the roof meets the wall, not seal the wall.

Road-Hog

Mellor wrote: »

I think you are gettign confused.
The silicon isn't there is seal anything. It's there to close the joint and help keep the lead in place. The concrete wall itself isn't water tight, so silicone could even be bypased anyway.
The lead is there to seal the where the roof meets the wall, not seal the wall.

I acknowledge what you are saying but silicon no matter what its function still degrades over time...........would it not be better to use something that won't...........e.g. a non-shrink type cementious grout that won't react with lead?

Mellor

Of course, non shrink grout would be perfect. Harder and more expensive, but ultimately a better job.
I was just pointing out that its not the seal that important

Road-Hog

Mellor wrote: »

Of course, non shrink grout would be perfect. Harder and more expensive, but ultimately a better job.
I was just pointing out that its not the seal that important

Many thanks for advice.......just one last question, if you didn't use any silicon or other type of product and just 'wedged in' the lead into the recess is there not a possibility that with driving rain against the wall that water (albeit in minute amounts) could make its way in behind the lead and down the face of the wall?

Mellor

Some could, but prob not a whole lot more than what lands on the face of the wall and soaking in. Its getting underneath the lead thats a problem.
That said, I'd always seal it with something. edging it in mgiht work, but would eventually slip out.

mickdw

Just an additional concern on this. If this extension was built as part of original house build, there would most likely have been a cavity tray built into the house wall to catch any water travelling downwards in the outer block (assuming cavity wall).
If you house wall is exposed to driving rain, it is possible that you would still have problems with the detail you are using as the entire outer block is allowed to be soaked through. This being the case, your lead cut partially into the outer block is not going to stop this possible downward movement of water through the blocks or down the inner surface of the outer block. This would be of particular concern if there is a large area of wall surface above this new roof. If the new roof in coming in only a few inches under an existing roof, well there would never be a problem.
I know of cases where the existing wall above the new extension had to have the entire wall coated in a bituthene product, mesh & new plaster.

dathi

dont forget to insulate the old house wall on the extension side with insulated plasterboard or you will have a very large thermal bridge. in effect you will have an outside wall in the middle of your house sucking the heat out of the extension. this can lead to mould growth at the junction of the ceiling and the wall as it is the coldest spot, and people thinking that they have a leak.