Render onto plywood ?

The only way that i know of using plywood as a background is to fix expanded matal lath or sheet across it even with PVA on plywood the render will never stick

Wickes used to sell a roll-on render for plywood. It's the same stuff that is used on Portakabins.

rond here (suffolk) we do a lot of repairs to 'plaster' cottages.

The way its done is to take the ply, then put a breathable membrane over it (cheapsktes use plastic bin liners, but its not recommended) and nail vertical laths to that. 2x1 usually.

Then with all the windows and other frames in place, expanded metal lath is nailed on. horizontal lath is nalied across the bottom and a drip bead lath nailed to that.

Then the whole thing is rendered with mortar - sharp sand and bulders sand together give a nice rough finish, and this can equally be applied over the brickwork. If the house is not ply'ed then lime mortar may be used to accomodate some shrinkage.

This way you have an air gab between the ply and the render, down which water can run, but the ply is protected by the breathable membrane, in the event of gross failures in the render.

To protect doors and window frames drip boards are used - horizontal 'porches' above the openings, best covered in lead.

Someimes these are used halfway down a wall to throw any water running down the wall clear of the storey below.

any... with the right preparation.

I used a variation on the theme suggested TNP:

First hang tilers underfelt over the ply as a waterproof membrane (some people used builders paper), just tacked up with bent over galvanised nails. Then I ripped some 1" wide strips of 1/4" ply and nailed them in vetical strips to match the stud locations (i.e. 400mm centeres). Then nail / staple expanded metal lath sheet (expermet etc.) over that. Finally rendered with a 1:2:3 mix of cement, sharp, and builders sand with some waterproofer admix.

The ply strips hold the metal lath away from the wall a little to allow it to integrate into the render better. This will help reduce the chance of cracking and maintain a better fix. At the base (I was rendering down to tiles on a roof) I used 6" lead flashing with a galvanised drip bead on top of the first couple of inches of that.

As above but used small mesh chicken wire. No problems over the last 20 years.

Thanks for the responses folks. It seems simple enough - I suppose you could use the mesh idea to render over practically anything. Cheers, Simon.

Yes. In fact bent mesh stapled over to the brickwork would be a fine way to stop teh joints between timber and brick cracking..

Ive seen in hotter climes, quite artistic interior arches made of essentially frames, chicken wire and render..

Yup, just used the same trick to render the ply side walls for a new shower, so if the tile grout does let any water through, it gets to meet waterproofed render next!

That is a great idea...may use that to make my own shower tray!

If you fit the tray first (i.e. flush against the ply) and then render, the render will ende up sat on the tray. This helps ensure the run off from the walls gets directed into the tray rather than being able to get between it and the wall should the sealant/grout fail.

You misread me. I need to make a spial tray (and enclosure) and was thinking of ply at te nase and plasteroard/std walls. I was gont o use fibreglass to form the tray over the ply, but was worried about the ability of that to take tiles.

But stapling down chicken wire over a plastic sheet and rendering it might make a rough cast finsih that didn't need tiling at all...tho I probably will.

Ah ok, was not reading litteraly enough! With mortar you should be able to get a smooth enough surface, although it will still look better tiled I would expect.

I suppose if you want a custom buit in situ tray you could also cast your own with a mould. That way you would be able to get a nice capping coat in the mould first for a better surface.