Cracks in mortar and blocks under windows
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Cracks in mortar and blocks under windows

by Bro88 » Mon Apr 19, 2021 8:19 am


I have a few concerns about some cracks on my self build that I am half way through.

This is my first project, I might add!

I have found a few hairline cracks under the window reveals and wonder if I should be freaking out?

Here is the story:

the site is 8m x 7m

I dug the foundations myself! I dug down around 2-4 metres in places into fresh/virgin soil in our sloped garden.

The concrete in the trench is a minimum of 375mm deep by 600 wide. It’s probably a bit deeper and wider in places. I made sure that the digger bucket excavated with the teeth slightly upward, so as never to drag the bottom of the trench. So all soil in the trenches is nice and compact.

Everything has been done to the structural engineers calcs.

the building control officer has inspected the soil and trenches prior pouring the concrete and blocking up the foundations. He then also returned to see DPC.

he was happy through the process.

The concrete was poured in august last year. I then began to lay the courses of blocks in the evenings, at weekends and in some spare days off work.

Below ground the walls are 100 wide block outer leaf, with 140 block inner, with a concrete filled 60mm cavity cavity up to around 200mm below DPC! Above DPC the walls are a mixture of bricks and n7 dense blocks in the outer leaf with 3.6 aerated inner leaf.

The build is a 1 & 1/2 story build, with a room in the roof basically.

To keep the rooms free of any load bearing supports and low ceiling joists, the engineer specified a RSJ ridge beam to support the rafters. The rafters are 200x47 rafters at 600 centres. The rafters also sit in a wall plate on both side walls.

The RSJ sits on the end wall, on a padstone, above a Heavy Duty Lintel in the wall, and upon a 100x100x5 box steel uprights elsewhere.

Progress has been steady, in summer, winter and spring. We have had rain on and off!

I was able to progress a lot more recently due to being at home during the lockdown in January to April.

I eventually lifted the RSJ ridge be into place in February/March. I have since then been adding roof rafters, building dormer windows and rolling out membrane and nailing in battens!

Here is my issue....

in the last two weeks, I have noticed some cracks! They are in the end wall, under the windows. This is the wall that supports the RSJ. The cracks mainly follow the mortar, but has split a n7 dense block in half too! Again, all hairline! All issues are in the centre of the window reveals on the first floor, about 2 courses above DPC!

There are no issues, as far as I can see, in any other areas of the wall. There are no cracks beside the windows or above them, It’s just under the windows. I’m guessing that there is less pressure on these areas or that they are the weaker section?

The cracks do not go down near the DPC (yet) and are not visible in the foundations.

My main question is, would anyone expect me to see some cracking under the windows, due to the fact that these block corses were laid back in September and october last year, and have only just recently started to take to load of the roof from the RSJ above, which must be adding loads to new sections of the walls.

Could it be a little bit of uneven settlement, with the load bearing bits settling a deeper and heavier than the unloaded sections? Is so, should this stabilise once the roof is finished and the tiles are all on etc?

I can’t see any issues with the foundations, but should I be getting the digger fired up and get down to the foundations and have a better look at them?

Would bad weather have kept these blocks and mortar moist, moist enough to make them softer than other areas, due to the fact that the rain can soak down through these exposed window reveals?

Also the ground has not had any drainage installed yet, and was like a swamp for a month or so! Could the moist ground be adding to some minor settlement?

The cracks are only hairline, and you really need to look hard to see them! But now I know they are there, I can’t un see them! Unfortunately, I have no idea how long they have been there, I only spotted them recently! They could have been there for ages!

I re pointed one crack about two weeks ago and it hasn’t returned. (Touch wood).

Should I just keep calm and carry on?

Im pretty confident that I’ve done a good job, but a little bit of doubt is creeping in!

Any help would would be really appreciated!

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Re: Cracks in mortar and blocks under windows

by stoneyboy » Thu Apr 22, 2021 10:07 pm

Hi bro88
If you have used AAC blocks for the inner skin this is the most likely reason for the cracking especially if you have not incorporated horizontal reinforcement.
Regards S

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