I'm planning to put a garage and porch extension on my house.
I understand I'll have to install cavity trays in the existing wall (1960s semi) above the roofline for the porch because the wall will change from being external to internal. (I do need to do this don't I?)
So I also need to do this above the roofline for the garage?
I'm keen to avoid doing this on the whole lot if possible as it'll be a lot of hard work!.
In an ideal world you should install cavity trays but as you say it is a lot of work. You could just fit lead flashing above the roof and see how bad water ingress is. If the amount of water entering is unacceptable treat the wall above the extension roof with a silicone liquid - this will need re-doing every couple of years
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