extension cavity tray

RME

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Hi

I'm designing a single storey lean to extension for the rear of my house. The extension will envelope 2 french doors which currently lead into the back garden.
The roof will meet the main walls just under the first floor windows.
Because I have an external wall, which will now become internal, do I need to break out the outer leaf and install a cavity tray where the roof meets the wall or could I just chase the flashing 30mm or so into the joint in the outer leaf?

Thanks
 
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This is a pig of a job but generally BC will insist on it and rightly so IMO. The cost/hassle/remedial costs of retrofitting one later should damp occur will be considerably more.
 
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Strictly, you would need a cavity tray, but if the brickwork above is in good condition and well-pointed, it may not be necessary.
If a large part of the wall is occupied by the window frame, that would lessen the chance of any water ingress.
Your Inspector may have his own view.
 
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I agree with Tony in that many lean-to roofs terminate under a window sill meaning whatever masonry is left is either sheltered by the sill or is covered with lead.

It helps also if there is an eaves soffit above i.e. not on a gable end.
 
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Thanks for quick replies gents.
House is coming up to 6 years old so brickwork is still in good condition, extension is not on the gable end and windows account for about 1.8m of 4.5m width, so maybe a few things in my favour.
 
I agree with Tony in that many lean-to roofs terminate under a window sill meaning whatever masonry is left is either sheltered by the sill or is covered with lead.
And what about the exposed brickwork with holes in it each side of the window? I've seen many a skin of brickwork that you can see through where the mortars been poorly applied.
 
I agree with Tony in that many lean-to roofs terminate under a window sill meaning whatever masonry is left is either sheltered by the sill or is covered with lead.
And what about the exposed brickwork with holes in it each side of the window? I've seen many a skin of brickwork that you can see through where the mortars been poorly applied.
Agree.

And there is the question of interstitial condensation too.

However, the conditions that lead to a cavity tray actually being of any use are rare.
I will often argue that the damage and disruption of retrospectively fitting one does not outweigh the benefits unless you live in a high exposure area.
 
Hands up, if you have ever seen water coming out of a weep vent?

The only thing i have ever seen coming out of one is a spider. Most are occupied by dust and cobwebs.

Incidentally, 'Dritherm' ensure that no water will get past their cavity batts.
 
Hands up how many times someone comes on here after their conservatory was thrown up without a cavity tray and they are having damp coming in under the abutment.
 
If the OP's house is 6 years old, it is likely that the pointing will be in reasonable condition.
 
Yes you are right of course, developers employ the finest brickys they can who can afford to take time and pride in their work and QA is their highest of their prioritys.
 
Hands up how many times someone comes on here after their conservatory was thrown up without a cavity tray and they are having damp coming in under the abutment.
Yes but i would bet that this is when a connie has been fitted to a gable end and where the plastering has gone right up to the glazing bars?

I would say that this is more likely to be either saturation seepage or cold bridge condensation.

I still argue that it is rare.
 
Going back to the subject of nit-picking by inspectors; a few weeks ago I submitted plans to a council for a small (3.5 sq m) utility room. The BCO came back with 2 A4 sides of amendments/additional notes.
I bow to your superiority! :p
 

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