Cracks in new block work

[cdh]

We're having an extension built using thernalite blocks on the inner leaf.

The builder has reached 1st floor level and we have just noticed a crack about 1mm wide running down from underneath the middle of the downstairs window opening. It runs through the blocks and the mortar joints. There aren't any cracks on the outside brick work.

Is this something that we should be concerned about?

Thanks

 

[^woody^]

Its shrinkage, and it is common.

Causes can be too strong mortar mix, wet blocks, sudden dry weather. The wall dries out quickly and contracts

You'll have to keep your eye on it, as I have seen these get to 10mm by the time the shell is up, or it may stay as it is, or some more may appear in other places along the wall.

It only requires remedial wok if it gets a lot wider

 

[cdh]

Thanks Woody!

Once the shell's finished do you know approximately how long I should leave the block work to settle/dry out before any more cracks are likely to appear so that I can plaster?

Also, is it OK to plaster straight onto the thermalite blocks as I think someone has mentioned on this forum that you have to use dot & dab ( but can't find it again now)!

 

[^woody^]

See what your spec says - some walls need dot and dab to get the u-value down, to meet the required b/regs insulation value

For plastering thermalites, you just want to dampen them down first

It all depends on the weather, and how the blocks were stored, or if soaked prior to or after laying.

They can literally crack at any time and even in a few years time

Ideally you want to have an even drying out over a few weeks, but you don't have to wait this long to plaster as this can be included in the overall drying out time.

Its when you get a sudden spell of warm dry weather, when problems can occur, as drying is accelerated.

But if the issue is too strong a mortar, then the blocks can crack at any time