Cavity Trays

We are currently building an extension for a client.

Its a pitched roof single storey extension meeting a vertical existing wall to the rear of the property. Inside there will be two openings from existing house to extension (ie two spots where dampness would be a problem in the cavity above)

The drawings mention cavity trays by means of 'cavity trays' hand written with an arrow pointing to the proposed roof. There is no further spec but they are mentioned.

Our usual sub contracted bricky has refused to fit them as he believes the "integrity" of the existing building would be breached. He has fitted them before but on this one the sloping roof either side of the extension come very close to existing upstairs windows. He also reckons they shouldn't be necessary as the back of the house is not exposed to the elements.

Spoke to BCO last week on the phone and without even seeing the property yet he has stated that they are required as they are specified on the drawings.

That leaves me needing to look into ordering them and finding someone to fit them.

What kind of trays should I be looking for?

horrid job and can be a weakener if a slap dash builder installs them.

if the remaining brickwork above the new roof is minimal, and there is a decent soffit and the extension is on the leeward side, then i can't see the point in cav' trays.

argue the point to b.c or the designer.

i see you are 'high up' in derby.

Strictly, a cavity tray should be inserted wherever an external wall becomes an internal wall, an in this case you need a stepped tray.

You can fashion them out of 450mm DPC or you can get some purpose made ones from the likes of www.cavitytrays.co.uk - or screwfix have them

Your brickie is wrong to refuse to fit them, wrong about the inegrity and wrong about not needing them. Your brickie may have a point about the wall not being exposed, but its a risk if you don't fit them and really its not up to you to make the call - if the plans specify them them then you should fit them (specifically under your contract terms) or get the designer to alter the specification.

Even if the BCO says don''t fit them, if water does come through in the future, its your responsibility.

i have to say, here in the mid's, i have only encountered one case of damp ingress due to the lack of a cavity tray.

we fit them as part of the building process.

i think retro-fitting them would depend upon location and geography.

leaving them out carries a risk. but not a calamitous risk. the worst that could happen is a damp patch or a few drips on a windy rainy day. should this happen then fit 'em.

if it does not - then don't.

noseall said:

i have to say, here in the mid's, i have only encountered one case of damp ingress due to the lack of a cavity tray.

we fit them as part of the building process.

i think retro-fitting them would depend upon location and geography.

leaving them out carries a risk. but not a calamitous risk. the worst that could happen is a damp patch or a few drips on a windy rainy day. should this happen then fit 'em.

if it does not - then don't.

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Sounds good to me

They will be fitted. BCO stated clearly that they are required.

I wont question the bricky either, maybe he just dont fancy it, but he is a good guy and his team are damn fine brickys too.

Personally I've never encountered them so I've been reading up on how they work. I can see how much easier to fit them in the build stage than retro fit them



In this case we have removed an existing window and extended the opening and will fit doors into what will be the new extension. Would a system where a cavity tray is fitted above where the opening is (in this case above the new Type C) not be much easier? This imaginary tray would be wider than the opening and would divert any water back into the cavity which would obviously be shut either side. Kinda similar ideas to the ones over windows.

Seems madness having to remove and reinstate existing brickwork to fit these things.