shrinkage cracks?

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Two walls exhibit vertical cracks on the inner leaf between windows on ground and first floor.
These are both mid-wall and do not extend above the top window or below the bottom window and there is no cracking outside. I hope they are just due to differential shrinkage. Once I'd removed the plaster, the crack in the 27' long wall was about 2mm wide and in the 12' short wall was about 1mm wide. As you can see, I have raked out the weak mortar, chipped away the broken edges of the blocks and replaced it with some 4:1 or 5:1.
 
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probably the good old Sussex Clay moving a bit ;) . Are you in a semi- I bet your neighbours love you :)
 
probably the good old Sussex Clay moving a bit ;) . Are you in a semi- I bet your neighbours love you :)
It's built on chalk downland and my plot is excavated over a metre. There is only a few inches of soil before chalk bedrock. I'm a mile inland and so am less likely to be sitting over a cave than houses on the coast.

The outer skin on the end wall is built with very weak OPC mortar but, even so, is not showing any sign of cracks. Removing the mortar between inner blocks only takes a light tap and quick rub - I've seen paving slabs with more stubborn mortar! I first noticed the crack in the bathroom when the house was only a couple of years old.
 
I had just the same on the new part of my last house - crack inside - sound outside . Never caused any problems , but it was a bit of a nuisance . Put that down to being on clay , albeit with proper founds. May be same as yours with weak pug on blockwork - those celcon blocks seem to draw the water out like mad
 
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Never caused any problems
Thanks, that's reassuring. Now I've repaired at least half the crack lengths to the full depth, I hope the house is now stable and they will stay fixed.

I only wish I'd had this site 30 years ago! ;)
 
Overlap the repair with metal lath at least 200mm either side of the crack before you re-render/plaster.
 
How long is the wall before you hit a return or internal wall?
The end wall is 27' long and straight with only Paramount partitions internally. It also has joists built in running side-to-side. The crack runs from the top of a window, through a joist socket up to the bottom of another window.

The front wall is 12' long and clad rather than double skin.
 
You can get away with about 7m of straight blockwork without a movement joint for moisture/temperature so your 8.2m is beyond that.. chances are the crack has appeared where the wall is weakest vertically..
There might be a movement joint behind the partition.. in which case ignore me :)
 
There might be a movement joint behind the partition.. in which case ignore me :)
Thanks Static, no I shan't ignore you 'cos I took down the partition and there is just plaster. You can see it in the 2nd photo.

A great place to have a movement joint would be in the service duct by the soil pipe, I opened that up last week but did not see anything like an expansion joint.
 
You could always add one there.. normally its about 5-6m between joints on a block wall to allow for some thermal movement..
 
Robb, Let me try and answer your question. The crack is not due to differential shrinkage. This only occurs when you bond two different materials together.
The crack is not due to heave or settlement. You would have had a crack by the side of window and a crack in external face work skin.
I strongly suspect that the wall has made its own expansion joint at the point of least resistance.
The long wall is a touch over 8 metres. Assume flank wall with no openings. We often run them 15 metre long, no openings no expansion joints.
Block manufacturers are very cagey with regard to stating expansion joins for internal walls. They usually quote{expansion joints not normally required for domestic dwellings}. The only regulation is for dense concrete blocks and they must have expansion joint every 6m.
As previously stated a piece of mesh over crack will not come amiss, but will more than likely crack again. If so rake out 3 to 4mm wide, fill with flexible decorators caulk. Light sand over, touch up emulsion.
Do not even think about trying to cut an expansion joint, you will cause yourself serious problems as you will not have adequate ties between outer and inner skin at that point.
Rest assured, you have no serious problem.
old un. guidance only.
 
As previously stated a piece of mesh over crack will not come amiss, but will more than likely crack again.
Such cracks in older properties are common; they are usually located under windows similar to yours, at the top corners of windows/doors at the ends of old style concrete lintels but can sometimes run the full height of the wall terminating under a joist; here are a couple of pics. showing cracks undergoing repair;
View media item 19585View media item 19586
I repair cracks like this all the time as part of the renovation work I do; I strip it right back (300mm either side), rake out & repair the crack & then fix a continuous strip of metal lath overlapping by at least 200mm either side of the crack, re-render to around 3-5mm below the surrounding wall using a sand/cement/lime render & fill out flush with Bonding; I then double tape all around the joins & skim the whole wall. You can never guarantee it won’t reappear, especially if things are still moving around but I’ve had an extremely high success rate using this method & have only been advised of a couple reappearing.

If it reappears, caulk may help but it sinks back & can’t really be sanded, you will most likely always see the repair.
 

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