vertical cracks in new build

WARRENMTB · 11 Sep 2008

Hi all
I am seeking some advice as I am worried sick. I am in the process of having a house extension.The original garage was knocked down,footings dug and new ones built giving us new a bedroom and shower room above the new garage and extended kitchen. I am just about to give the builder the final payment,but have noticed cracks in the blocks in the garage from top to bottom.These cracks are 0.5-1mm wide,they go through the blocks and the mortar.There are two cracks about a metre a part.
Any advice would be much appreciated.
Regards
Warren

mattylad · 11 Sep 2008

1) Take a photo & post it here.
2) Dont make the final payment pending this being resolved to your satisfaction.

noseall · 11 Sep 2008

if you are on clay substrate then i would say it is not unusual, especially after the amount of rain we have had recently.

also aerated blocks such as celcon or toplite are prone to cracking. some may call it thermal movement, others may say a bit of ground heave due to excessive moisture.

not at all unusual though.

^woody^ · 11 Sep 2008

Just from what you describe - vertical continuous cracks about 1m apart, is indicative of shrinkage within aerated block due to the several factors, but mainly due to the mortar being too strong and not allowing normal expansion/contraction. Some more may develop, or it may remain as two

If so, these will be permanent, but will not widen after the new work has dried out. There will also be no issues structurally.

There may be other reasons, depending on where the cracks are and what is above or below them, but the main cause is likely to be as above, and not a significant concern.

But if your contract allows, you can withhold 1-2% of the whole job cost as retention, and if there are no probelms after 12 months, then you give this back to the builder.

noseall · 11 Sep 2008

i'm becoming less and less convinced woody, as to whether the strong mortar thing is as much an issue as once thought.

i am currently working on a building which has a celcon garage attached.
there are vertical cracks and the local ground conditions are free draining sandy gravel (hoorah

). the mortar betwixt is virtually sand.

i have seen it before where cracking has occurred and the mortar is very weak.

i'm puzzled. i think thermal damage will occur to these block whether strong or weak compo is used or not.

mikric · 13 Sep 2008

noseall said:
i'm becoming less and less convinced woody, as to whether the strong mortar thing is as much an issue as once thought.

i am currently working on a building which has a celcon garage attached.
there are vertical cracks and the local ground conditions are free draining sandy gravel (hoorah ). the mortar betwixt is virtually sand.

i have seen it before where cracking has occurred and the mortar is very weak.

i'm puzzled. i think thermal damage will occur to these block whether strong or weak compo is used or not.

I am with you on this one , had a structural engineer out to look at a really bad case of cracking in thermolites, he basically laughed and said it is a very common problem due to mainly soaking wet blocks ,weather conditions etc, in my experience I have seen this lots of times over the years and never heard of remedial works due to structural issues but only cosmetic.

If it happens in clinker blocks or brickwork then that would suggest a structural issue.

Shytalkz · 13 Sep 2008

The AAC-type blocks shrink as they dry out, especially if laid very wet, or they get very wet soon after laying. The strength of the mortar is not really a significant issue in the development of shrinkage cracks. The pattern of loading to the wall from above can have an impact in the position of these cracks. If it's vertical and relatively constant width cracking, then it will be due to shrinkage and not a foundation movement issue.

^woody^ · 14 Sep 2008

Yes, "too strong a mortar" is the standard answer and may apply for quite a few of these cracking situations, but the fact is that aerated blocks are very fussy in terms of the conditions in which they are laid.

Extremes of dryness or wetness affect these blocks much more than normal concrete blocks, and they have little tolerance to cope with drying out if conditions are not perfect.

Good practice normally means the blocks and bricks are kept dry in storage and protected from saturation when laid, protected in cold weather and precautions taken in hot weather.

Most of the time, we don't bother and clay and concrete products can cope. But aerated can't accommodate rapid shrinkage from drying out, so they suffer.

Give me a nice lightweight concrete block any day - and the compo actually stays on when the perp joint is buttered!

Shed Steve · 26 Oct 2014

I have cracks in some inner-leaf blockwork on an extension completed a few years ago, so searched here for views and comments of others. I found several threads on the topic.
Since it seems that these lightweight blocks are very prone to cracking, and as ^woody^ wrote, 'fussy in terms of the conditions in which they are laid', has their overall suitability for the uses they are put to - 'fitness for purpose' - ever been questioned?
I had wondered, have Building Control officers and the like ever questioned their use? I suspect however, that the blocks all have a string of 'qualifications' and approvals from the industry that adverse comment is not really possible!

tony1851 · 26 Oct 2014

zokko said:
I have cracks in some inner-leaf blockwork on an extension completed a few years ago, so searched here for views and comments of others. I found several threads on the topic.
Since it seems that these lightweight blocks are very prone to cracking, and as ^woody^ wrote, 'fussy in terms of the conditions in which they are laid', has their overall suitability for the uses they are put to - 'fitness for purpose' - ever been questioned?
I had wondered, have Building Control officers and the like ever questioned their use? I suspect however, that the blocks all have a string of 'qualifications' and approvals from the industry that adverse comment is not really possible!

The manufacturers recommend movement joints at 5 - 6m intervals to accomodate the inevitable shrinkage that aac blocks undergo.

As walls are now commonly plasterboarded internally (rather than traditional 2-coat work) these cracks often don't show through to the surface.

Many builders don't like them, and it does seem nowadays that thermal insulation is regarded as more important than structural integrity.

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vertical cracks in new build

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