Cracking on new front extension wall

16 Jan 2018
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United Kingdom
Hi all,

Cracks appeared about 5 months ago on the external side of a new front wall of my 18 month old extension. The wall also incorporates a uPVC window as shown below (picture was taken just below extension completion):


The extension is actually a garage conversion as well for a single detached garage that had a pathway between the house and the garage and the wall that was the right hand side of the garage started roughly where the right side of the uPVC is.

The extension was fully signed off by building control who got a thorough look at how the foundations which the cavity wall was going to be placed on and I also checked with building control about this method and they said it is absolutely fine and more than capable of taking the load. Here are a couple of photos that show how it was done:


Above you can only see one of two of the steel reinforced pre-stressed concrete lintels that form the foundations of the cavity wall. Both the lintels were embedded into pre-existing structures that formed the left and right hand walls of the garage.

The picture below shows a couple of steel reinforced pre-stressed concrete lintels that formed the rest of the foundation for the wall that would fill the gap between the house and what used to be the garage:


The crack that has appeared and starts a short distance from the top left of the window and almost immediately goes to the right where the left side of the window is (under 2nd brick) as shown in the picture below:


In the above picture for the top brick that has cracks either side and underneath it the crack to the left of the brick is just less 1mm wide and the crack on the right is 2mm wide. To the right of the 2nd brick down the crack is about 1mm. From then on all the other cracks are hairline.

I know that walls cracks will take path of least resistance in majority of cases and in this case the purpose of the picture below the path of least resistance goes down the left side of the window as I have noticed the wall has slightly pulled away from the uPVC attached to it and the silicone seal has been stretched.


The picture below shows where the left side of the window sill where the hairline crack continues down for a bit:


Where you see the vertical crack stop in the mortar the hairline crack continues to the right as shown below and the crack then goes down and through a brick and then continues through the mortar:


The yellow circled area shows the crack going through a brick and the thin red line is where a hairline crack goes to the right and then down at the next brick join where it finishes. The brick with the crack is the only brick with a crack right through it. It have noticed a smaller crack (just about visible in the picture) in the top of the brick to the top left of the yellow circled area which goes down about a quarter of the way.

The width of the cracks have not changed at all from top to bottom and have not progressed since I first noticed what was going on at beginning of September 2017. I think it first appeared roughly about a month or two before I noticed the cracks and myself and my wife remember hearing a noticeable loud crack noise about a month before and we probably think that is related.

There are no cracks inside and the plasterboard was attached to the internal wall used dot and dab method. The window frame inside shows no signs of pulling away from the wall unlike outside as previously mentioned where the silicone has been stretched.

I do remember soon after the extension was built there was quite a bit of white efflorescence which I managed to scrub some of it off and the rest soon disappeared.

I hope I have explained things well enough but just wondered if anyone can offer any insight as to what is going on. I will probably get a structural engineer to take a look but I am slightly concerned about subsidence but I can’t find anything on Google that is close or might explain what is going on with my cracks and what might have caused them! Movement appears to be horizontal and I have checked the rest of the front wall as well as the left hand side of the extension and there is no sign of anything out of alignment or other cracks.

There are no mains water or drains that go underneath the extension

Any advice would be much appreciated and will do my best to answer any further questions that are fired my way :)
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Looks typical of thermal cracking (shrinkage), and the crack sound is typical of it too. If so its not an issue, and best filled with a clear mastic.
Thanks Woody for your reply :) With regards to the cracked brick it must have been some force to go through a brick rather than the mortar. Didn't think shrinkage would exert such a strong pulling force?
What would make you think it isn't subsidence? Appreciate you have limited means to asses and like you say it is typical but just curious as to your thinking!
If something else was going on would I have noticed during the 5 months the cracks getting wider ?
Thanks Woody for your reply :) With regards to the cracked brick it must have been some force to go through a brick rather than the mortar. Didn't think shrinkage would exert such a strong pulling force?
Brickwork can expand in the heat of the day and contract at night when it cools. That's normally when it cracks. If the mortar is stronger than the bricks the cracks can go through the bricks instead of the joints.
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Thanks Stuart for that. We have heard no further cracking since then so probably a one off I hope and also hope cracking doesn't progress further.
Would the builder be responsible for the repairs at no charge? Any answers given will be accepted as just advice with no come back if he doesn't want to pay!
No, it's common and a natural thing, and not related to a builder's negligence.

I recall a particular spring around 2010 when a really hot dry spell followed a really wet spell and I went to several different properties miles apart within days with this exact thing - loud crack and all. Some of these were built 40 and 50 years previously. Walls facing the sun, with an absorbent brick, and some rain and a bit of sun can create the perfect conditions, no matter what the build quality.
Thanks again Woody...would have been nice to get it done for free but not much the builder can do to take into account the English weather!
How long do you reckon we could leave things as they are?
Sorry for all the questions!
Thermal cracking tends to be once only and does not get any worse. Thats a small panel so would not be expected to contract much in any case. Monitor it over four seasons to confirm.

If the cracks do widen (and not close) after monitoring, then that might indicate ongoing movement which would need investigating further. But as it is you can leave it for now.
I would have said there is not enough brickwork to have any kind of meaningful thermal expansion. I just cannot get my head around Woody's thermal "shrinkage" theory that he keeps repeating, to the best of my knowledge bricks don't shrink when they get warm, they expand. Also moisture/thermal movement in clay bricks tends to give a crack with a fairly consistent width for the full height of the crack, whereas the photos seem to show the crack is wider at the top and tapers out just below the window.

I would be more suspicious of that strange foundation detail. Most of the weight of the front wall and roof are now concentrated on that narrow brick pier that was the right hand side of the old garage door. Was the original foundation under that pier strong enough to take that concentrated load?
So if you have a south facing wall wouldn't this be very frequent, as Woody seems to be describing very typical English climate!

I am not saying you are wrong Woody, just interested.
@wessex101 - The foundation isn't strange according to building control though apparently. Building control explained to me that those types of lintels in my case can easily support that type of setup when embedded into existing footings where the right side wall of the garage used to be and on the left side of course. I am not at all knowledgeable in building matters (as most people probably aren't) and so that is why I have to trust the people involved. Just out of interest, if the foundation were affected would I not have seen other areas crack including inside as well as out?

Granted, concrete foundations are more common I am sure but instead it was decided the lintels were sufficient for an ex-garage door opening and I wasn't having a 2nd floor.
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