Plaster onto wooden lintel?

womble69 · 25 Mar 2010

Tidying up an upstairs room, I've found to my surprise that what I thought was a concrete lintel with a couple of courses of bricks above is in fact a series of wooden beams one above the other.

It's sound enough, but the problem is the plaster, which came off when I removed a curtain rail (4 inch masonry nails, since you ask).

The original plaster coat was obviously put on by someone who was good. It was solid and very thin indeed.

So now how to replace it? Base and skim ( maximum of a few millimetres only)? One coat plaster?

Above all, how to make it stick to the timber?

roughcaster · 25 Mar 2010

Old wooden lintels were quite often hacked over with an axe, to roughen up the surface for the plaster to key onto. I plastered onto one a few months ago. Because it was an old, bone dry wooden lintel,,, rather than try to soak it before plastering over it, i covered it with polythene (nailed on), and then tacked on some quality galvanised EML, (wire lath).
The polythene stopped the old dry timber lintel sucking the moisture out of the new plaster, causing cracks,,,, and the plaster itself gripped onto the wire lath. You could do it the same way, but let the wire lath overlap onto the brickwork,, 2 or 3 inches or so, all around the lintel.
Depends how much depth you have to play with on the lintel,, although you could always cut back the old plaster, all around the lintel on, giving you a larger area to work with, and that'll help to blend it in, and it'll give you more cover over the lintel. Nails you use for this type of work should be small galvanised,, clout nails,,, around the 25mm ish in length.

Micilin · 25 Mar 2010

womble69 said:
Tidying up an upstairs room, I've found to my surprise that what I thought was a concrete lintel with a couple of courses of bricks above is in fact a series of wooden beams one above the other.

It's sound enough, but the problem is the plaster, which came off when I removed a curtain rail (4 inch masonry nails, since you ask).

The original plaster coat was obviously put on by someone who was good. It was solid and very thin indeed.

So now how to replace it? Base and skim ( maximum of a few millimetres only)? One coat plaster?

Above all, how to make it stick to the timber?

First of all, is it possible to cover the lintel with a finished timber to save you plastering it.

If not, here is a suggestion, if there is no room for eml mesh or plasterboard. I'm sure you will get others

If it is a very thin coat, you will have to hack the plaster off the timber and around it. Say 50 -60mm around it

make a load of small indentations on the timber, say with the claw of your hammer and clean off

Get a 100mm fibatape (this is where the rendering mes would be ideal cut to the right size)

You can pva the timber and surrounding hacked plaster, and stick the fibatape to it then nail or staple the fibatape also, making sure all the deges and joints are lapped. The pva will seal up the timber but also stick the tape

Let all of that set and next day, PVA again and skim.

If you can use a bit of 9.5 board on the underside of the lintel (if there is one and not just a face) do so if it does not foul vents etc.

womble69 · 26 Mar 2010

Thanks for that.

There is virtually no depth to play with at all. I would guess that the way to go is to use the thinnest possible steel mesh, nailed on and then coated over.

diydazzler · 26 Mar 2010

I'm just about to have this done.
Over 1 window I was able to use plasterboard but the other,main window, I don't have the depth.
My plasterer is suggesting a few layers of scrim over the timber lintel.
Don't know wether this is going to work but he is an experienced plaster...he's 64 !

roy c · 26 Mar 2010

I personally don't think this would be good enough over a long period of time
I would go for one of the methods the lads have suggested, the scrim method would fail because he would still have to plaster straight on to the wooden lintol.

wozza56 · 27 Mar 2010

Either go with the mesh idea or you could try painting the beam with unibond and a sand mix which should give a course finish when dried and then skim with a one coat plaster

Thats what i would do anyway

Richard C · 27 Mar 2010

The only chance you have is to thoroughly seal the surface with an SBR based waterproof sealer not PVA, that won’t stop the timber absorbing moisture from the plaster. What happens is the timber expands as it absorbs moisture, plaster sets & dries, then moisture evaporates from the timber causing it to shrink back again, plaster isn’t flexible so it cracks, usually along the join line.

The problem with timber is that it will also expand & contact at a different rate to the plaster surrounding it so what ever you fix to it, will move & a thin skim coat will always crack. A metal lath under lime render will give you the best chance but difficult if you haven’t got the depth but even the lath will still move if it’s too securely fixed to the timber. Have you thought about using a flexible filler rather than plaster but with no depth, even scrim tape won't help; every single trick I've tried has failed in the longer term, unfortunately, I think you’re onto a looser.

diydazzler · 27 Mar 2010

If it does crack at some point,won't they be hairline and probably hidden by wallpaper anyway ?

joe-90 · 27 Mar 2010

Just use an acrylic spray to seal the wood then an easy sanding car body filler and a sandpaper block.

external · 28 Mar 2010

op just trust your spread ,hes 64 an prolly better than the entire forum,your paying him for his knowledge,just let him get on ,you may be talking to milkmen on here,dont try and secound guess the tradesman

Richard C · 28 Mar 2010

external said:
op just trust your spread ,hes 64 an prolly better than the entire forum,your paying him for his knowledge,just let him get on ,you may be talking to milkmen on here,dont try and secound guess the tradesman

Well I can see you certainly know how to endear yourself to the experienced plasterers on this forum.

joe-90 · 28 Mar 2010

I'll have three pints and a yoghurt please Richard.

diydazzler · 28 Mar 2010

hey maybe you could use the yoghurt on the lintel as it is a living organism it will expand and contract with the timber

Richard C · 28 Mar 2010

joe-90 said:
I'll have three pints and a yoghurt please Richard.

Fruit or plain?

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Plaster onto wooden lintel?

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