How to plaster small strip at top of wall with timber wall plate after removal of coving.

[phillymclee]

Hi guys. I have recently replaced the first floor joists in our renovation project for a future loft conversion. I had to remove the old ceiling along with the old coving which took the plaster at the top of the walls with it. It is a 1930's property with cavity walls and lime plaster. I intend on fitting resilient bars to the underside of the new joists so looking at resilient bar details it looks like the wall plaster should continue up passed the ceiling plasterboard and the ceiling plasterboard sit inboard of the wall plaster with a gap filled with acoustic sealant. What is the best way to fill these gaps at the top of the wall? I understand that lime is better than gypsum for breathability (but then less important for cavity walls) and I will do my best to use lime plaster where I have any large patches that have come loose but I also understand that lime doesn't really stick to timber so was wondering if the best idea would be to use bonding plaster for the strips around the top of the wall where there are timber wall plates. I was thinking to additionally fix 100mm wide galvanised expanded metal lath to the 50mm wall plate and brickwork under, sealing with PVA and then bonding over in 10mm layers. I know that I could probably do the same with lime but would need stainless EML and fixings if I am not mistaken and it just starts adding to the cost so would only do that if it was absolutely necessary. Does my approach sound reasonable and won't end up cracking all over the place or inadvertently rotting the wall plates because they can't breathe haha. Also will this approach be fine for a bathroom? Many thanks in advance.

 

[Cubist]

I'm not sure what you mean when speaking of 'resilient bars', maybe you can help me with a better understanding?

However, its usual when plastering over timber and when attempting to close a cavity between plasters to use either a plastic mesh, or for cavities, crushed stainless steel netting, to provide an armature/skeleton into which the render coat plaster can be pushed before being finished with a final skim coat.

 

[phillymclee]

Hi Cubist thanks for the reply. Resilient bars are basically just very thin lengths of formed steel bars. They are fixed to the underside of the joists and then the plasterboard is fixed to them instead of the joists decoupling the plasterboard from the joists and reducing the impact noise from above, which I'm installing with the future loft conversion in mind. It's sort of irrelevant but just wanted to mention it as it means that the plaster on the walls will have to extend up past the underside of the plasterboard and probably cover the full height of the timber wall plate. I'm happy to use the expanded metal lath but just wanted to double check that using a gypsum plaster like Thistle Bonding Coat is not going to be an issue for a small portion of the wall i.e trapping moisture on the face of the wall plate and causing it to rot or that it won't end up cracking between the dissimilar plasters materials.

 

[phillymclee]

Or if there is a better method, which won't break the bank and easy enough for an amateur.

 

[Cubist]

Using a gypsum plaster for the small area concerned would not be an issue imo. However, if the walls below are lime plaster then you may want to use that to get a better bond at the joint and to skim the plasterboard for the ceiling.

 

[phillymclee]

I would prefer to use lime but there are a few things putting me off. 1. I'm not that clued up when it comes to picking the right lime product. The premixed bags from lime.org.uk seem to be what a few people are using in tutorials but It doesn't look like I can get them delivered to Dorset. Would a NHL 2 with sharp sand do the job for the base coat as I can can get that? 2. Also it looks like you have to use stainless mesh in Lime plaster as opposed to galvanised so doubles the cost of that. I have about 50m to do in total. 3. Even with the metal lath I thought gypsum bonding plaster adheres to wood better than lime so just thought it would be more solid but perhaps I'm over thinking it.

 

[Cubist]

The problem with plastering directly to wood is that the wood frequently expands and contracts and this movement results in the formation of cracks and delamination that appear over time. Frankly I agree, you are over-thinking it and in your shoes I would not be bothering with a lime plaster for this job if I could possibly avoid it.

Galvanised will do the same job and survive buried in plaster for 30+ years but there is a risk of rust stains bleeding through. SS will last longer but there will be no staining.

 

[Ashville87]

Or if there is a better method, which won't break the bank and easy enough for an amateur.

Is there enough room to fix plasterboard to the wall plate? Then if you’re going to use a gypsum plaster you can plaster and maybe finish the join with scrim tape and easifill?

 

[phillymclee]

Ok thanks for your advice Cubist. I think that's what I wanted to here to be honest.

 

[phillymclee]

That was another option I had Ashville. I could definitely fix a strip of plasterboard to the wall plate. Do you think that would be better than metal lath.

 

[phillymclee]

Would definitely be cheaper and less hassle.

 

[Cubist]

That was another option I had Ashville. I could definitely fix a strip of plasterboard to the wall plate. Do you think that would be better than metal lath.

Yes.

 

[phillymclee]

I just didn't know if there was a risk of any moisture in the bricks or wall plate turning the plasterboard to mush

 

[Ashville87]

That was another option I had Ashville. I could definitely fix a strip of plasterboard to the wall plate. Do you think that would be better than metal lath.

That’s the road I would probably go down. Tidy the existing edge of the plaster so it’s reasonably straight. I’d pack the wall plate out so the board finishes almost flush with existing plaster, joint tape between new and old and skim over with easifill, sand to a finish.

 

[phillymclee]

Thanks guys I think that's the way forward then.

 

[paulc]

Is there enough room to fix plasterboard to the wall plate? Then if you’re going to use a gypsum plaster you can plaster and maybe finish the join with scrim tape and easifill?

I've used a fair bit of plasterboard here (you can tar & feather me later). If you want to finish with lime plaster, a coating of something like FeBond Blue Grit gives something for the plaster to bite on to. Used it on my kitchen ceiling some seven years ago with a 5mm skim of lime plaster. No cracking to report, nor has the plasterboard turned to a mush (also used plastic corner beads where needed).
Gypsum will tolerate a small amount of damp from time to time. It is only when it is wet for a prolonged period does it turn to mush.