Rendering onto timber frame

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robj

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Hi
This is my first posting on this site. So firstly let me say hello, and introduce my self.
I must point out that I am not a plasterer, but I hope to be a customer of one shortly.
We live in the south west and are having a timber frame extension built, and I liked the idea of rendering onto mesh rather then a block and render finish.
The spec calls for horizontal and vertical battens fixed to the osb, then stainless steel mesh and then a 20mm sand and cement render.
Now there are two things I cant get my head around.
Firstly all that I have read on the net about the subject mentions a minimum 25mm air gap behind the mesh for ventilation, but if the battens are fixed horizontally and vertically how can air flow freely.
The second question I have is am I right in thinking that a layer of breather paper gets layed between the mesh and the battens. I think this is to keep the render from falling into the air gap. I asked the chap who has priced the rendering for me about this and he says no.
He has priced for ordinary SS mesh not rib lath.
Since I have been looking on this site I have realised that there are other options to SS mesh, but the plans have been signed off by the building inspector so I don't know if we now have to stick to the spec as approved.
Hope all this makes sense. I had good look at other posts about timber frame but none covered my specific questions.
 
firstly, does your spec ask foe a 25mm air gap?
second question, yes building paper on top of the batterns, yes eml would work on this system, eml is cheaper then rib lath.
if your house is positioned about 8 miles from the sea you should be able to use galvanized and not stainless as there is a large price difference.
 
You should have put this on the rendering thread.

As for an air gap you leave gaps in the battens for air to flow around but remember to put some fly mesh on the edges that are exposed to stop creep crawly,s getting in.

Breathable felt on the battens then wire so the material does not fall into the lath cavity. Expanded metal lathing is more of a render carrier but should still do the job. Rib lath would be the better performer but at a price.

Now for putting 20mm of sand and cement onto a timber frame in this day and age well good luck! Sand and cement will set hard and brittle. You may get cracks in the future and this will be down to the person that specified it and not the spread that put it on.

There are lighter more modern polymer based renders that has a bit of give but even these I would fully mesh with the manufactures mesh. What finish are you going to have? When you say block and render do you mean an ashlar joint finish?

I would consider another talk with the inspector and consider using cement particle boards and a modern polymer based render system.

SAS Europe is down your way and they will advise you on what is best for timber frame. Use their materials throughout including mesh and beads and they will give you a 10 year warranty but you have to log the job with them to get a warranty.
 
I have only ever done 2 jobs like this with sand and cement and both were bungalows both had laths fitted first but not crossed. Both were for friends locally and i told them at the time they may crack in the future both were done the same year about 5 years ago. last year one started to show slight air line cracks on the south elevation the other has not as yet. But to be fair as they were for friends and they wanted to keep costs down against my advice they wanted to mix for me etc so it could be the case that one of them was better at mixing than the other ?
 
Hi Rob

What has been spec is a suitable method of sand and cement rendering onto a TF, but as the guys say above, TF shrink and S&C is hard and rigid... so may well crack.

As Rigsby says above, there are more modern ways of rendering onto TF's with cement boards and then thin coat renders.... see the attached link for details of the Knauf Aquapanel Exterior Cement Board..... Knauf Aquapanel Exterior

If you need the S&C to match the exisiting building, you can apply S&C to the outside of the basecoat and mesh.....

whatever you choose to do, make sure you use a movement joint between the exisiting building and new - this will mean that the TF can move and not cause problems to the exisiting building!

Get in touch with me if you want further advise on the Knauf Aquapanel Exterior - my details are on my profile (click my forum name on the top left of this post and follow the view profile part)

Good Luck

Richard
 
Use aquapanel instead off mesh, you can change the spec easily enough, building control won't have a problem as long as it conforms to building regs
 
Thanks for all your replies.
Aquapanel does look like a good option. The area to be rendered is quite small about 27m2. the only problem is that I am not sure that local jobbing plasterers in our area will be familiar with the finish systems so it may be hard to find someone.
I would quite like to fit the board my self. Not only to save some money but also to have some personal input in the project.
 
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