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Rendering on wooden studding between bay windows.

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toolsntat

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Is there an online resource for the correct modern specification of doing this?
Ply, vapour barrier,mesh, bell bead then render etc ?
Cheers Andy
 

MikeG.

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Not that I know of. It is rather more complex than that, because of the endless variety of wall types to which any new bit of rendering is attached. It sounds like you have a specific issue. I'll help if I can.......
 

toolsntat

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Thanks for your time Mike, it's Cls studwork/ply flush to the window frame face
Water getting in behind barrier and into render
Trying to reinstate in the correct proven manner and not just copying what's failed.
Try as I might, I just cannot find any specs anywhere for any methods?
Cheers Andy
 

MikeG.

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Ply on the outside of softwood studs, with a vapour barrier over that? Are there any battens between the back of the render and the face of the ply/ membrane? And is the water getting in at the junction of the window to the render, or is it elsewhere? A photo would help.
 

toolsntat

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Existing is (or was) cls stud with xtratherm, 18mm ply, vapour barrier, expanded mesh fixed through barrier then rendered.
New quality ply ready to fit but is the rest above OK?
No battens involved or airgaps.
Cheers Andy
 

MikeG.

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No, that's not OK. I suspect that may be the reason for any failures you are noticing.

It is essential that there is an airgap above the breather membrane. There will be micro-cracking of the render which means the back of it will be damp during rainfall. This dampness has no way of drying off if there is no air movement behind the render. The ply is on the wrong side of the frame, too. It should be on the inside. I'll have a little bet that there is no vapour barrier on the inside of this wall, so you would have the perfect combination of internal and external design errors causing interstitial condensation.

If it were me, I'd strip both the internal and external faces of the frame, assess whether sheathing board (ply or OSB) was actually necessary at all and if it was, fix it to the inside, not the outside. I would then fix a new breather membrane to the outside, directly to the frame, then counterbattens an inch thick over all of that. I would then fix a building paper to the outside of the battens to prevent the render pushing through the wire and entirely filling the void you have just carefully created. The EML ("wire") is then nailed through the paper to the battens before render is applied. For a top-notch job, there should be an insect-meshed air gap designed in somewhere (usually under a fascia board) to allow fresh air in behind the render, and the DPC at the bottom should be carefully designed to take any water that pools at the bottom away from the frame. Internally, I would fit foil-backed plasterboard, before skimming and decorating in the usual way. Of course, you should check that the insulation is 100% whilst it is exposed, and if it fits with the interior walls, you might consider fixing 25mm of Celotex or similar across the inner face of the studs before fitting your plasterboard.

I would also caution you to expect cracks at the junction of new render with old. These should be very obvious within a year or so.
 

toolsntat

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Wonderful insight Mike, may I just ask if there is a recommended spacing for the vertical battens?
I'm thinking 150mm centres.
Using extra battens for a better result is not an issue this end.

Cheers Andy
 

MikeG.

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400mm usually, Andy. This is a case where fewere battens would be better, because it increases the airflow. 600 would be OK, but you'd need to be sure your render was strong enough, so 400 is a good compromise. However, the location (and thus spacing) of the battens is determined by the location of the studs, because they are nailed directly over the studs. Otherwise, if they were just fixed top and bottom and at any noggins, they'd bounce around horribly when you tried to nail the EML into place, and the render would have an increased chance of cracking.
 

MikeG.

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Oh, and BTW, you can use breather membrane in place of building paper as a backer for the EML & render, to save going out and buying a separate product. If you've got building paper, use that, as it is obviously a much cheaper material than breather membrane. Counterbatten tight to the sides, top and bottom of the windows, too, and seal with silicon at that level (rather than on the outside at render level).
 

RobinBHM

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Could rendaboard be used instead of the eml and render?

Ive done a few jobs timberframe jobs with rendaboard. From memory though I think the final coat was a thin coat render system like k-rend.
 

MikeG.

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Yes. There are a number of render carrier boards, some specific to certain renders. However, this isn't something you typically see in builder's merchants, and I'm guessing this is only a very small area we're talking about, so readily available materials would seem the obvious way to go. If it were a whole house, or even the whole side of a house, then that's a different story.
 

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