Cavity tray

[Ay8452]

Hi folks,

 

I am having an extension built at the moment. It's double store side extension with a new front porch.l single storey. The front porch has a gable shape roof. My drawings have specified a cavity tray above the opening from the porch into the double storey extension. It's a standard build with a 100mm inner skin block and then 100mm cavity with 90mm PIR board then 100mm outer skin block and then a silicone render finish.

 

My builder / bricklayer forgot to put the cavity tray in when building up the inner skin but then reassured me when doing the outer skin he would use a preformed cavity tray. I've just checked and for the outer skin he has simply used DPM membrane on the outer skin which then going into the PIR board and then up against the inner skin. It's  not a preformed tray. Is this a problem? I guess a preformed tray is less likely to move around but having just the DPM is going to move around especially water was going to fall on it. Please see diagram for reference. He has reassure me he's always done it this way but just seems like he's forgotten to do it right. P.s. everywhere else the cavity tray has been done the "correct" way with the DPM starting in the inner skin.

 

Maybe I'm overthinking it?

 

[Big Jimbo]

Is this above a window ?

Edited Wednesday at 16:51 by Big Jimbo

[Ay8452]

27 minutes ago, Big Jimbo said:

Is this above a window ?

 

 

It's above an opening into the house from the porch.

[Barnboy]

If I were you I'd make your builder rectify this to as the original drawing shows.

If the dpc is just glued/taped, whatever to the blockwork, at some point you're going to get a leak and water ingress into your house.

Get it done now rather than leave it and wish you'd have done it once it's leaked. 

Your builder has messed up, it's up to him to rectify things.

[Gus Potter]

4 hours ago, Ay8452 said:

Maybe I'm overthinking it?

Ok on the face of it, does not look promising. 

 

My first question is how did you find this out? Do you have some photos?

 

Before I was an SE I used to be a builder and often we would form cavity trays with DPC , fold them at the ends correctly and provide weep vents. I often work on older buildings. In the 1950's to 1960's the cavity trays were often formed from heavy duty bitumen felt that is still often hanging in to this day and age.

 

My guess is that your builder has made a bollocks of it as the cavity tray is up a slope and stepped. 

 

I always smell shite here as cavity trays take a lot of time = labour cost to install. Remember I'm a poacher come gamekeeper! 

 

Ask the builder for an elevation detail showing how the cavity tray is stepped. My guess is that these will not be forthcoming, next ask for some photographs, they may not be forthcoming either. 

 

Now look at your contract. 

 

 

 

[LnP]

The builder on a parapet wall I was having built forgot to install a cavity tray. I found out about it when the  job was not yet finished and we had heavy rain and water appeared through the already plastered ceiling. We agreed he would fit a cavity tray - strip of DPM as described above tied into the mortar beds of the inner and outer walls, with weep holes. That meant dismantling the top 2 or 3 courses of brick and block along about 20 m of wall.

 

It was his mistake but I nevertheless felt stressed dealing with it with him. But It was actually not as big a deal to go back and fix it as I’d thought. Dismantling brick and block is not that difficult, depending what’s above it.

 

[Gus Potter]

On 25/09/2025 at 00:24, LnP said:

I found out about it when the  job was not yet finished

Catching things early is key.