Hi, I'm nearing the end of my "wall" questions ... If render is applied over a plywood substrate, using mesh etc, it is a requirement to ventilate using an air gap between the plywood and the render. What techniques are used to ensure an adequate air flow in the gap ? I can imagine grills under cowling, and grills at the bottom behind and above a drip bead etc. What are the standard techniques for doing this ? Simon.
I can find no reference in the building regs to it.
Basically as I said, what seems to happen is as follows.
At the top, the gap is more or less open and under the eaves and vents into the roof space. Soffit boards may cover it, but in general they don't, often being fixed after rendering anyway.
At the base, the drip bead is normally spaced a little away to add a curve to the render base, and a gap left.
The first line of defence against driving rain is the render: And that gets through would trickle down the gap and in time possibly rot the battens holding the mesh up, but in general there is moderate airflow through the gap - enough to prevent a build up of water and of course you will be using pressure treated wood for the battens won't you?
Building paper over the ply is the second line of defence, and the exterior grade ply is the third.
The battens that mount the mesh can be either vertical or horizontal, but its better if they are not horizontal and contiguous, as that can allow wet spots to form if rain does get in.
General practice is to whack them up very roughly, and let the natural angles and gaps take care of the odd scarp of water.
The final issue is to use foil backed plasterboard to reduce warm moist air getting to the cold timbers and ply from the inside. You cannot totally provide moisture proofing, as sockets and plugs and pipes will breach the general 'moisture barrier' but it does help a lot.
Having had cause to rummage around behind my own internal walls to relocate pipes and wires, it seems to be totally adequate, but water running down inside stud walls eventually rots the stud bases and sole plates, so its important to control the problems as far as possible.
By fare and away the greatest danger is a breach in the render - such as is provided by windows and door frames, and on gable ends, by the barge boards? Think that's what they are called. Anyway on the house I pulled down, water used to drip down inside the walls and through the inside of the windows from rotten barge boards.
I've got little lead covered drip boards now over every window, and although the cills are taking a beating, that's the only bit that gets the full driving rain force.
Use of larger drip boards is also useful to throw rain clear of any openings below.
No. In fact there is no need for a gap at all. The expanded metal lath is typicaly fixed directly to the ply (possibly with some form of membrane between). The render goes on, in, and through the lath to make contact with either the ply or the membrane. Sometimes when spacers are used under some of the lath, this just serves to allow the lath to be integrated further into the render, hence adding reinformcent to the render.
Have a look at:-
John Rumm wrote
John, RE the second picture, how is the render done around the windows ? I notice that the scratch coat is on, and the windows are in. Are the windows put in before or after the scratch coat ? If the windows are effectively "rendered in", it must be tricky if the windows need replacing in the future. Cheers, Simon. Simon.
Yup the windows went in first, then the corner beads were fixed in place, and finally the render went on. So the windows are in effect rendered in (the render covers a 1/2" strip all round the sides and top of the frame).
Rendered in on one side, and plastered in on the other! (and screwed to the wall just for good measure)
That would probably explain the usual hash the replacement window companies make of windows in rendered walls! Having said that you could dismantle the frame in situ, pack out the edges, and install a very slightly smaller window in its place without needing to do too much damge to the render.
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