Cavity tray dilemma

[henrygw]

Hi all,

Looking for a bit of advice on cavity trays above a roof abutment. I've had an extension built and the builders skipped the cavity tray. They've gone bust so I'm left to try and find a solution.

The building inspector has picked up on it, and said it really needs to be there. The plans say cavity tray or Vandex BB75 to waterproof, which is a cement slurry, and it seemed like he would accept that as a less optimal solution, but I'd probably have to hack all the render off to apply it.

The wall is rendered finish and there are 3 windows and an internal corner above the abutment, presumably the cavity tray wouldn't need to extend under the windows, so it feels like a lot of stop/starting bits of cavity tray to insert, with the sloping/corner abutment being the most difficult.

If I go down the cavity tray route, what would be the best way to cut the block out? I don't think there is enough clearance to get a stihl saw in due to the low pitched roof.

Part of me thinks the waterproofing option is better because it's not disturbing the integrity of the walls.

Thanks,

Henry

 

[noseall]

or Vandex BB75 to waterproof, which is a cement slurry......
The wall is rendered finish

Tell the BCO that the rendered finish is protection enough and that he should accept that.

 

[freddiemercurystwin]

Tell the BCO that the rendered finish is protection enough and that he should accept that.

'What about all the brickwork? Or the lead flashing cut into the vertical bricks? Or have you never seen an air gap in the mortar between bricks? Pitch looks a bit low for those rooflights too.

 

[noseall]

'What about all the brickwork?

The bits that are sheltered under the sills?

 

[freddiemercurystwin]

Yea the bits that are sheltered under a tiny overhanging bit of plastic. I suppose there's never any wind driven rain where the OP live either?

 

[noseall]

Yea the bits that are sheltered under a tiny overhanging bit of plastic. I suppose there's never any wind driven rain where the OP live either?

The exposed brickwork can easily be remedied with taller lead rather than some sh1tty slurry all over the front of the house....roof tiles....windows...

 

[Notch7]

If it was me, I’d avoid traditional cavity trays, it would make a right mess.

I’ve had trays fitted by cavitytrayman they developed a tray which goes through a slot cut - the tray is a flat sheet with a lip at the back. The thinking is 95% of water in a cavity runs down the outside skin.
http://www.cavitytrayman.co.uk/


I’ve certainly had jobs where the house is rendered and water got in the cavity - usually around the windows.

if you have a big opening and a steel - BC might want a cavity tray.

if its mostly wall, I would say fit timber battens and use foil backed plasterboard. Avoid 2 coat wet plaster direct on the wall - they absorb water and show damp very easily.

 

[mrrusty]

Unless you are in a hurry to get a sign-off, and if you believe the render is protection enough as an alternative to the tanking slurry, why not just delay a year or so with the sign off and get a couple of winters under your belt. If there is no sign of any problem after that, then a strong argument to BC to accept the render as adequate protection.

If there is a problem, you know you need to remedy.

 

[henrygw]

Thanks all,

Yes I am aware they should have really brought the lead up around the bottom of the window. The lead work is pretty terrible to be honest, and seems to be the main source of water getting in at the moment.

The roof pitch is pretty low, probably somewhere around 15 degrees, about the limit of Velux I think. They aren't leaking yet, bonus.

I think what it really needs is a render drip bead adding above the leadwork, just I didn't want to redo all the leadwork and then find BC won't sign it off.

I am in South West England in the very severe exposure zone. There is a 4 meter steel below. Might just have to bite the bullet and put it in, maybe stitch drill the wall and slowly get the cavity tray in. It's going to take forever and make a big mess.