Cavity tray or not? Confused by contradictory builders

19 May 2012
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United Kingdom

I have been given (by two builders) completely contradictory advice.

I am building a single-storey, flat roof extension, and where the roof joins the back wall of the house I have been advised on two methods.

1. Cut a narrow slit across the brickwork to push lead flashing into.

2. Remove rows of bricks and fit cavity trays.

Builder 1 said there was no need for cavity trays. Builder 2 said they were essential!

Can anyone shed any light on who might be right, or if it's a matter of opinion?
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Doing it by the book it’s (1) and (2)
If the wall is deemed in good condition/a limited amount of exposed brickwork area/non exposed area, (2) may be omitted normally on the say so of the building control officer
"Builder 1" should be referred to as just "1", as the word does not apply to him.

However you are not referring to two methods for the same thing. The slot is required for flashing to seal the roof against the wall, the cavity tray is required to stop the wall in your new room from getting soaked.
No 2 is correct

but....It depends on a number of things like how exposed is your house, how much brickwork is above the extension, how much house roof overhang there is, whether the brickwork is non porous etc

However what you have to remember is that the external brickwork that is above the extension roof is external and lower down it becomes internal. If the inside part of the wall is going to be finished with a 2 coat plastering system then bear in mind it is very hygroscopic and any damp getting into the cavity might make it damp. Ive seen it happen on a number of occasions, so it does happen.

So if you want to mitigate any risk then go for the cavity tray -building control may insist anyway. Its certainly part of NHBC requirements.
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Thank you all for the quick replies. You have confirmed what I suspected all along, that cavity trays are the way to go!
For cavity wall only,If the wall is fully rendered above the lintel i believe even nhbc allows the omission of trays and weep vents, otherwise it's a case of good luck you might well get away without.

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