Single story gable extension and existing cavity

[Srp]

Hi all,

We are having a single story gable extension added to the back of our 1950’s house. There are two steel beams supporting the new openings between old and new.

The old house has a cavity wall, and walls are rendered with painted pebble-dash type render.

I’ve read the for flat roof extensions cavity trays are required above steels, but with a pitched roof the rises above the steels, what is best practice?

Planning on talking through with builder (who comes highly recommended but has not been particularly confidence inspiring) and architect next week.

 

[Alastairreid]

Stepped cavity trays would be installed above the roofline over the lead apron with weep vents.
This should be on the plans for building regs. Your builder should know.

 

[23vc]

BCO may (rightly or wrongly) allow them to be absent though if the wall is rendered (ie fairly well protected) and isn’t a huge area also protected by the soffit overhang.

 

[Notch7]

For peace of mind I say get it done.

LBC might not insist on cavity trays on rendered faces, but in my experience rendering doesnt guarantee a dry cavity, Render cracks and I’ve seen instances where water gets in around the cill of a window and runs down the cavity.

I would also make sure you get the parapet wall properly detailed and insist the builder does it correctly. Parapet walls have a transition, they are external walls at the top which then become internal so damp proof detailing is vital

 

[Srp]

BCO may (rightly or wrongly) allow them to be absent though if the wall is rendered (ie fairly well protected) and isn’t a huge area also protected by the soffit overhang

The render is some sort of pebble dash-ish stuff. The main roof soffit overhangs ~30cm.

Would cavity trays be a problem so close to windows?

 

[Srp]

For peace of mind I say get it done.

LBC might not insist on cavity trays on rendered faces, but in my experience rendering doesnt guarantee a dry cavity, Render cracks and I’ve seen instances where water gets in around the cill of a window and runs down the cavity.

I would also make sure you get the parapet wall properly detailed and insist the builder does it correctly. Parapet walls have a transition, they are external walls at the top which then become internal so damp proof detailing is vital

Thanks. That’s helpful. What would be best practice for parapet wall? They have installed some sort of flexible membrane above roof on one side. The rest of parapet doesn’t have anything yet.

Original plan was for standing seam roof and parapet cap, but have changed to tile roof due to cost. I think the plan is to put coping stone on top now.

Would they need to put a cavity tray or membrane across entire wall?

 

[23vc]

I know others will disagree, but particularly given so much of the wall is taken up by window, and it’s dashed, I’d say fitting them may not be worth it in this case. Any water ingress is going to be fairly unlikely, and easy to locate and fix. But that’s me from the perspective of the person doing the work and assessing the risk, you can specify that the builder does whatever you want

 

[^woody^]

Window openings are a significant source of water ingress to cavities. Eaves also, along with ..... cracked render.

All cavity walls require cavity trays.

The plan monkey should have specified them, and if so the builder would have priced for them.