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DPC cavity tray

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I’m looking for some advice on cavity trays at DPC on an extension I’m building, really, I want to know if/when they are a requirement.


It’s shown on the BR drawings but the diagram used seems to be fairly generic.


The construction will be brick/90mm T&G PIR/block. Not in a high radon area.




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They are required to catch any damp in the cavity and push it to weep vents in the outer brick/blockwork, it usualy slopes towards the outside , also required above windows and doors .

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These details send brickies round the bend.


Ideally the PIR should be cut at an angle to facilitate the cavity tray to be sloping towards the outside skin allowing water to flood out of the weep vents.


In the real world it is difficult to achieve without creating voids in the insulation between cuts.

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Thanks for the comments. I’ve not had much to do with BC besides the foundation inspection and just before I poured the slab. I did ask the question by email but seem unable to get a simple yes/no 🤣


The attached pic is a typical  drawing.  You will need to fit cavity trays where there are any door or window openings.”


I’ve no real issue adding the tray at DPC, as pointless as it seems, given the rest of the house does not. Pretty much all the Youtube/Instagram builds I scoped out prior to the project don’t seem to include it which is what makes me question it.

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7 hours ago, Iceverge said:

A tray at the DPC, what exactly is it achieving?


We don't have any, just a DPC on each leaf.



it negates the need for external paving to be 150mm below the outer DPC

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