block walls above ground

7 Oct 2007
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United Kingdom
I'm building a single storey extension with block walls. The external skin in 7n 100mm concrete blocks and the internal skin thermalite 100mm blocks. My question is, what mortar mixture is required for both skins ? I was going to use a 1:5 for both with a plasticiser but wondered whether they should be different because of the density of block ! Also are weep holes rquired at the bottom of the block work to allow any cavity moisture ?
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I would be inclined to use 4:1 or even stronger for dense concrete 7n blocks as you need the added body to cope with the extra weight, particularly if they are wet or if the weather turns bad.

Keep the stacked blocks covered at all times.
I think 4:1 is a bit too strong, so I'd say 5:1 for the outside and nothing stronger than 6:1 for the thermalites, or they will crack all over the place.
thanks noseall and woody. Any thoughts on the weep vents at the bottom of the cavity ?
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You would normally include cavity trays and weep vents above doors and windows and also at dpc detail should the need arise i.e. if you were breaching the cavity with, say, a gas membrane for instance.

But, if you are covering the masonry with a waterproof render than the chances of driven rain penetration is greatly lessened.

There is always the chance if interstitial condensation getting into the cavity but that's another story.

So to answer your question, no you will not need weep vents at dpc unless you are fitting an unusual dpc at this point. The 150mm sub-dpc trough will take care of any moisture.
Thanks noseall. The 150mm trough sub dpc is in place but no weep holes in the perp joints. am I right in thinking any moisture at this level will pass through the block walls into the sub soil outside? Above windows and doors are there 225mm weep vents designed for block work or just normal size ?
Yes, any moisture finding its way into the trough will soak away. Chances are none will end up in there anyhow, particularly if you fully fill the cavity with fibreglass cavity batts.

Just use regular brick sized weep vents, should the need ever arise.:cool:

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