Cracking Render on Timber Loft

Steve P

New Member
I would really appreciate some advice on this. In August 2016 we had a loft conversation done and the dormers were rendered white to match the house. It was a very hot week. When I painted the dormer a week or so later there were already hairline crack on both ormers which the builder filled and said it would be fine. I had my reservations, however after a long build we just wanted the job finished. He promised to rectify any future issues.


The construction method was:_


Timber joists, boarded, waterproof breathable membrane, expanded metal, scratch coat and top coat. Sandtex on top to finish.


The render is now cracking on both sides and the rear.


It would seem the whole lot needs to be taken off and redone at a significant cost. My question is should we go again with sand and cement based render or should we use one of the acrylic renders like K Rend. The obvious issue is ongoing maintenance as it requires scaffolding to gain access for any future repairs.

Can upload pictues if that would help.
 
It should have been done in three coats, the first coat a pricking up coat should be 3 to 6 mil thick, the second coat scratch coat should be 10 to 14 mil with a top coat over this which should be thinner than the scratch coat.
This comes from bre building research establishment good building guide. Having said this sand and cement renders over metal lathe on wood do crack one theory is because there is no suction in the first coat as the water runs down taking cement with it, it becomes a weaker mix than the following coat which is the wrong way round each coat should be slightly weaker than the last
 
I would really appreciate some advice on this. In August 2016 we had a loft conversation done and the dormers were rendered white to match the house. It was a very hot week. When I painted the dormer a week or so later there were already hairline crack on both ormers which the builder filled and said it would be fine. I had my reservations, however after a long build we just wanted the job finished. He promised to rectify any future issues.


The construction method was:_


Timber joists, boarded, waterproof breathable membrane, expanded metal, scratch coat and top coat. Sandtex on top to finish.


The render is now cracking on both sides and the rear.


It would seem the whole lot needs to be taken off and redone at a significant cost. My question is should we go again with sand and cement based render or should we use one of the acrylic renders like K Rend. The obvious issue is ongoing maintenance as it requires scaffolding to gain access for any future repairs.

Can upload pictures if that would help.

Hello Steve

Sorry to hear about your problem....

Timber frame structures will always settle, this is where the weight of the roof and building being supported by the timber studs makes them embed themselves into the sole plates (the vertical timber studs do not shrink, then get forced into the horizontal timbers above and below them), and the timber partly dries out making some extra movement.
The same is true with dorma roofs, but maybe not to such a great extent as a lower level structural timber frame wall....
Timber frames are usually fully settled after 18 to 24 months after competition of the build (you can qualify the above statement with the likes of TRADA and the STA)

Sand and cement render is a very brittle and hard finish, it can also be a difficult product due to variations on mixes and strengths, and the conditions when installed and while drying (you state that it was a hot day when drying) - so when applied to a EML on a structure that may move it is at risk of cracking... While this is a very old and common construction method, it was always understood to be a risk and subject to future maintenance

Modern construction methods now offer the use of a boarded render system using modern thin coat render finishes - these product are more flexible and less likely to crack when the structure moves...

I am the Knauf Aquapanel Exterior Technical manager (Knauf Aquapanel Exterior is a 100% cement board, and one of the market leaders as a directly applied render carrier board), and I would be happy to review and recommend a replacement solution. My contact details can be found on my profile (click where my name is top left and go to my profile)

I am sure that if you post up some images, and where you are based, there will be some local experienced guys who will be able to comment and assist

Good Luck!
Richard Lord
Knauf
 
Thank you for your replies. Reassuring to know that the movement should be over now. If I am understanding this correctly it's seems to be a combination of hot weather when rendering done, movement and the wrong technique in terms of sand and cement.

Thank you for your input. I have found a local firm who have been around for over 50 years (son took over from Father) who only do rendering and pebbledashing and guarantee their work for 15 years. The original builder has agreed to cover most of the bill.

Thanks again.
 
I would really appreciate some advice on this. In August 2016 we had a loft conversation done and the dormers were rendered white to match the house. It was a very hot week. When I painted the dormer a week or so later there were already hairline crack on both ormers which the builder filled and said it would be fine. I had my reservations, however after a long build we just wanted the job finished. He promised to rectify any future issues.


The construction method was:_


Timber joists, boarded, waterproof breathable membrane, expanded metal, scratch coat and top coat. Sandtex on top to finish.


The render is now cracking on both sides and the rear.


It would seem the whole lot needs to be taken off and redone at a significant cost. My question is should we go again with sand and cement based render or should we use one of the acrylic renders like K Rend. The obvious issue is ongoing maintenance as it requires scaffolding to gain access for any future repairs.

Can upload pictues if that would help.
What you have is the way it was done years ago.
Nowadays it should be;
Timbers covered in OSB
Then a breathable membrane
Followed by timber battens
Another layer of breathable membrane
EML or Riblath
Followed by three coats of render, two can be acceptable on Riblath.
How the mesh is fixed can make a lot of difference. Each sheet should be joined to the next with tie wires, not just nailed to the battens.
 
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