Cavity trays

[nicola armstrong]

Hi, just a quick question...

For a block and block wall (it will be built above an existing kitchen), my QS has given me a breakdown which has included 'cavity tray using 600mm DPM - 1 roll'.

Does this mean the builder simply lays a layer of DPM at the beginning of the build, on top of the existing wall?

Also, does a block and block wall need weep vents?

 

[wwwebber]

A cavity tray I believe sits within the wall void and directs water away from the internal wall out through some small holes / vents in the exterior wall.

 

[^woody^]

Cavity tray goes above the windows, and so yes in your case the builder will lay it on the existing wall above the kitchen. Its a DPC not DPM, BTW.

Weep vents are required with cavity trays no matter what the wall is made of

 

[John D v2.0]

Weep vents are required with cavity trays no matter what the wall is made of

True although just to add my 2p, if the wall above is completely rendered, they can be omitted
Edit weep vents that is, not the cavity trays

 

[dishman]

Where an external leaf lintel is to be fitted, and therefore no brickwork is to be removed from the internal leaf. How is a cavity tray normally fitted to the internal leaf? Is it omitted in this scenario?

 

[^woody^]

True although just to add my 2p, if the wall above is completely rendered, they can be omitted
Edit weep vents that is, not the cavity trays

You can't have a cavity tray without weep holes. How will the water get out?

 

[^woody^]

Where an external leaf lintel is to be fitted, and therefore no brickwork is to be removed from the internal leaf. How is a cavity tray normally fitted to the internal leaf? Is it omitted in this scenario?

Chased in to a joint

 

[dishman]

And I am assuming that this should always be done (installing a cavity tray) in that scenario.

 

[John D v2.0]

You can get lintels the right shape to support the dpc. The tray is just to catch falling water and direct it out, not to be a weather tight flashing

 

[wessex101]

It is probably worth establishing WHY the QS has specified the cavity tray. Is the new wall being built off a solid wall? or has the existing kitchen wall got full fill cavity insulation and upper level is partial fill? or is it just over door and window openings?

 

[John D v2.0]

You can't have a cavity tray without weep holes. How will the water get out?

Render isn't impermeable so a small amount could escape over time. If render were impermeable, presumably water wouldn't get in so it would only be condensation anyway.
Even Encon, who have an incentive to sell more weep vents, say there's no need in rendered walls:
https://www.encon.co.uk/sites/default/files/products/downloads/rytweep_datasheet.pdf

 

[wessex101]

Down here we can take quite a pounding from the weather and a rendered cavity wall can still get driving rain penetrating to the cavity.

I would not want to risk omitting the weep holes, otherwise, even with stop ends, the water is likely to spill off the ends of the cavity tray and you can get damp patches either side of the opening. Who bothers with stop ends or turns up the end of the tray these days?

 

[Notch7]

It is probably worth establishing WHY the QS has specified the cavity tray. Is the new wall being built off a solid wall? or has the existing kitchen wall got full fill cavity insulation and upper level is partial fill? or is it just over door and window openings?

Yes, I was wondering in what context a cavity tray would be required.

I suppose the new construction could be finishing in parapet wall.

In any case is 600mm dpc ok? I would think surveyors would spec up a proprietary cavity tray rather than dpc.

 

[wessex101]

You would only use an expensive proprietary cavity tray if you were cutting a tray into an existing wall such as an extension roof abutment or had a very complicated detail such as an internal steel/concrete frame. A good quality roll of DPC is perfectly good for building in.

 

[^woody^]

I would think surveyors would spec up a proprietary cavity tray rather than dpc.

They would not.

DPC is a cavity tray when installed as one.