where are cavity trays required?

I am trying to ascertain where cavity trays are required. As I understand it they would be needed any time the wall cavity gets penetrated eg. above a window or an external door. However building control have said that I also need a cavity tray where my extension roof meets the existing wall. Is this actually a building control requirement? or have I just got an OCD building control officer and more importantly what would be the purpose of the cavity tray when the roof doesn't penetrate the cavity.

jonniejoejonson said:

I am trying to ascertain where cavity trays are required. As I understand it they would be needed any time the wall cavity gets penetrated eg. above a window or an external door.....

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...or where an external wall then becomes an internal wall such as abutted extension rooms etc - like yours.

Required and needed are two different concepts in my view. I have only ever seen one occasion where a retro-fitted CT would have been of use. Most walls above abutted extensions are protected by a decent soffit overhang above or are unlikely to have the prevailing wind drive rain in that direction or the masonry is in good order and impermeable etc.

However these are not valid enough arguments to refuse. Water could still find its way into a cavity by other means even though it is equally as rare as driven rain penetration. I think many coastal towns insist on them or at least render the wall etc. None insist in staffs however. I'm of the opinion that the disruption to the masonry is not worth it around here and would strive to find other means to mitigate.

Just to add - we always fit CT's (where needed) as part of a normal building process.

Wherever water might run down a cavity and end up inside a room, you need a tray.

Firstly thanks both for your responses, however I’m still confused to the need. I assumed the lead flashing would stop rain penetration from water running down the outside wall or getting into the cavity. And even if it doesn’t any moisture that does get inside of the cavity is still going to run down to the ground below DPC. The only time it wouldn’t is if there’s a window or door below.

jonniejoejonson said:

I’m still confused to the need

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The concept is that wind driven rain, or heavy rain, can get *through* the bricks and mortar and run down the inside of the cavity. Tony just gave a lovely history lesson which is highly relevant : https://www.diynot.com/diy/threads/cavity-fill-rockwool-or-eps.530834/page-2