Dense blocks for a rendered external wall

[patp]

We will be starting on the brickwork and blockwork exterior soon on our bungalow. Part of the bungalow will be rendered and we are going to use a colour fast render. The bricklayer has asked us to check which blocks are required by the architect. The architect says = quote 

We hope you are well and should be using a dense block like a Hemelite Standard block (3.5N/mm2) (noted on the sections) or similar approved by the render manufacturer?  We always recommend a dense block aswell and so does most render manufacturers as they reduce the cracking in the render. unquote

 

Colourfast Renders have been recommended to us and they state quote  

The one coat render systems from Weber, Parex and K-Rend are designed primarily for going straight onto 7n concrete block and a minimum depth of 15mm.  However all of these products can be used over virtually any surface with the correct preparation. unquote

 

Which would be best?

 

 

[patp]

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[MarkyP]

aha, block confusion. You'd think blocks were blocks until you start looking.

 

I think you want to avoid aerated or low density blocks, these crack and in the case of aerated are very absorbant and I expect a bugger to render without copious priming. Aggregate blocks rated at 7n and in the order of 1500kg/m2 density are what I used. Confusingly, these are sometimes called lightweight blocks in the trade! I think this means they are lightweight relative to the super heavy cast concrete jobs used on foundation courses, etc. Personally I think a dense concrete block would be overkill for a wall, and your brickie wont thank you for choosing them becuase they are very heavy. Medium density aggregate blocks offer good key for render and are crack resistant, would be the typical choice for a blockwork outer skin in my opinion.

 

Low density aerated blocks are still available in compressive strengths suitable for high loads but seem to have less flexural strength and be rather brittle, hence the reputation for cracking under stress.

 

Note the block choice may affect wall u-value, esp if switching from aerated to aggregate. Not by much, but worth bearing in mind.

 

I used these but all builder's merchants stock an equivalent https://www.tarmac.com/media/959459/hemelitestandard_shortcover_2017__itab.pdf

 

nothing like an evening  spent reading block data sheets, have fun ?

 

 

[Mr Punter]

+1 another vote for medium weight aggregate blocks 7N

[LA3222]

I used 7n mediums for the blockwork on my house. 1450kg/m2. 

 

 

Edited November 10, 2020 by LA3222

[MarkyP]

i realise my density units are wrong! 1500kg/m3 is the typical range for medium density/aggregate