New Cavity wall: does it need trays and weeps?

[Recci]

I have noticed on the internet and in books a cavity wall with double skin blockwork wall doesn’t have cavity trays and weepholes just is rendered. Why is that?

 

[jeds]

You should fit trays and weeps over any openings; windows, doors etc, also over any areas where the external wall becomes an internal wall below; i.e. over knock-throughs etc. I would advise special care with trays over knock-throughs to make sure the ends are capped or turned up and, if you make the tray from dpc, don't include any laps - use one continuous length.

 

[Recci]

Outer wall is blockwork would you render around the weep holes?

 

[jeds]

Use render weeps - they have a spout which extends to the outer surface of the render.

https://www.travisperkins.co.uk/brickwork-ventilation/manthorpe-buff-peep-weep-vent/p/917320?awc=16300_1680788316_9d90f05ee094802a12931456595d2bed

 

[stuart45]

NHBC guidance is that you don't need weeps if the render stops at the toe of the lintel, and doesn't go under the soffit.

 

[Recci]

Is the becase the render is waterproof and the cavity will extend pass the allow the water out? How does the water escape?

 

[stuart45]

The theory is that a rendered wall will allow much less water through than face brickwork, although small cracks will allow some through. Any that gets into the cavity will gradually escape at the toe of the lintel.
This is the guidance from the NHBC, but you can put weeps in if you want.

 

[Notch7]

If you are building new, it would be silly not to have cavity trays.

Ive seen quite a few rendered walls that have leaked, I wouldn’t trust them to be waterproof.

 

[Recci]

If you are building new, it would be silly not to have cavity trays.

Ive seen quite a few rendered walls that have leaked, I wouldn’t trust them to be waterproof.

I will have cavity tray just omit the weep holes

 

[jeds]

Building regulations, on the other hand, is less specific and requires 2 weep holes per opening - as does LABC warranty. I certify dozens of new build houses and the only time any have ever caused issues is with water ingress over openings. Personally I would go with best practice.