Lathing external corners

[DavidB]

Hi

I am slowly removing plasterboard and fibreglass insulation and replacing with oak lath and wool insulation in my 18C cottage and with LBC approved. I have now come to the part where I need to lath around a dormer. How do I form nice strong, neat edges for plastering in lime. With gypsum I'd use a modern metal bead - has anyone done this and what was traditionally used?

Regards,
Dave

 

[plasticpigeon]

The external corners in my house had wooden beading, though this was on a solid wall, not lath. In posh houses the wooden beading had a quirk on the corner to form a nice round. The plaster will crack a little around the wood as the wood shrinks as it dries out, but it can be filled in with plaster of Paris.

 

[Flyfisher]

I don't think forming a straight edge for lime is really any different to forming one for gypsum. The modern solution would be metal beading/EML and the traditional one would be wood. I imagine both would work with either form of plasterwork. Indeed, I wouldn't be surprised if gypsum plaster pre-dated modern metal beading. We have plenty of wooden beads around the inside of windows with lime plastered walls. If you do use metal beading then I'd strongly recommend stainless rather than galvanised. We have some rendered outbuildings with galvanised metal beading and they are now rusting through and staining the painted render. Galvanising is ok in theory but any misuse, eg running a plastering trowel along the edge, can easily scratch through the surface leaving the underlying metal unprotected.

Personally, I quite like a rounded edge to plastered walls, for which no beading is required at all, plus it's fairly resistant to knocks.

 

[paulc]

The external corners on my walls have what appears to be an L section cast in portland cement and then fixed to the walls. The base coats of plaster would then finish flush with the cement, and the final finish coat over the whole lot. The walls, being of brick construction, have stood up quite well to some 90 years of families. However, the section above a bay window (which was lath over timber) had cracked in the middle and was at risk of coming down completely.

At risk of being ostracized, I used some plastic beading

 

[livias]

I am a fan of the rounded corner too. I make it with stainless steel mesh (horrible stuff rips your hands to pieces wear kevlar gloves) held in place with stainless screws and penny washers then plaster over that
Rounded corners look good and are very sturdy

 

[DavidB]

I am a fan of the rounded corner too. I make it with stainless steel mesh (horrible stuff rips your hands to pieces wear kevlar gloves) held in place with stainless screws and penny washers then plaster over that
Rounded corners look good and are very sturdy

Do you have a good online supplier for stainless steel mesh livias?

 

[1610ness]

In my 1820s cottage some external corners have wooden poles in the lime plaster to make the rounded edges, known locally as brush handle edges!

 

[livias]

The last lot I got was from Less2Fix £209 inc delivery for 10No 8'*2' sheets

 

[MatthewC]

In my 1820s cottage some external corners have wooden poles in the lime plaster to make the rounded edges, known locally as brush handle edges!

My house has at least 13 internal corners with a vertical wooden pole but it ain't no brush handle - it's P-shaped (almost) and the flat part is nailed to wooden blocks in the wall.

 

[Chris-Burgess]

Once your pricking up coat is on and carbonated, you can start floating for square edges you can tack a straight rule on and work to it once it is hard enough remove the rule and work to it from the other side. Get an external angle trowel and run it round the arris to keep it square. Or pencil round depending on your angle trowel. Don't use metal angle beads unless there stainless steel even then I wouldn't recommend it. Staff beading if you want the authentic look.

 

[DavidB]

Many thanks for the feedback and for the tips.

I have exposed rafter beams and the dormers are a modern addition (although dating back before 1930s according to photos I have) and unfortunately the dormers are not aligned with the rafters - in other words the gap between the last rafter and the edge of the dormer is not even on both sides of the dormer. Lastly the rafters are not all straight and the plastered surfaces will have some variation between the various rafters (i.e. it's not a nice flat sloping wall - it's all over the place).

So my thought process/question is a brush handle or staff beading is an exposed edge and this will look a bit busy (i.e. another material and at uneven positions from dormer to dormer. So my thinking is the stainless steel mesh would allow for a smoother transition and might be easier for me and my standard of plastering

But clearly the stainless steel is modern and our local CO would probably prefer an original / consistent approach and not a mix of modern and traditional - that said I could probably persuade them on the basis that the dormers are more modern and it's only a small strip of stainless steel wrapped around a sloping corner...

Maybe I could try mocking up a small corner section in offcuts and try a staff bead and see if what it might look like.

 

[Flyfisher]

Personally, I quite like a rounded edge to plastered walls, for which no beading is required at all, plus it's fairly resistant to knocks.

. . . and by a happy coincidence MikeG has just posted a description and photos of his recent external rendering where he has rounded the external corners. It's a matter of personal preference, of course, but now we can see what it looks like in the flesh, so to speak: https://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=16086&p=219549#p219533

 

[DavidB]

Thanks Flyfisher, yes I also saw those photos. I agree the soft rounded corners suits the lime material. I am also using thermalime (same as shown in those photos). But I have not used savolit boards - I am using oak lath.

I was not so bothered about the finish of the corner as such I was more concerned that I might need some kind of beading to protect the edge of an external corner around the dormer (I am doing the interior of the building) so by external corner I mean a corner that will get bumped into and hands run up and down it etc. Not external as in outside the house
I don't like the sound of a bead as it will be another piece of material alongside the rafters and plaster. One option would be to support the corners with stainless steel mesh. I guess another option would be to cut the lath carefully so they former an even edge and then just plaster onto that. I am not so worried about having a straight edge I was more worried about the plaster on the corner cracking when people knock into it and therefore thought it might need some extra support (hence why I liked the idea of the stainless steel mesh).

However, at the moment, and unless I argue otherwise. my CO has given permission for oak lath and lime plaster - not stainless steel mesh...

 

[DavidB]

This pic may help explain it. You can see the rafters sloping up the roof line and currently painted black. I have removed the plaster board and fibreglass insulation between them and replaced with lath and sheep wool insulation. I've now got to the dormer and stripped that out and as this picture shows I've put batons down the edge of the dormer which I was going to attach my laths to.

Going back to my original question do I simply lath up to the edge and then plaster over the top - or because this is an external corner and likely to get knocked should I do something different. One suggestion was staff beading or a broom handle. Another option was stainless steel mesh to provide extra support. The is a baton to attach the lath to, so there won't be sections of the lath unsupported at the corner. But this also means there is less space for the plaster to squeeze through and form nibs - so I just thought the edge off the corner might crack and be damaged... hence my question about how to lath an external corner...

 

[livias]

COs are normally fine about you using stainless steel mesh
Though in truth I would be tempted to just put the mesh up, get a base coat on quickly (which you will want to do anyway cos raw mesh is evil stuff) and who would ever know

 

[steveb]

I would fully expect a CO comparing a lime plastered wall and a lime plastered wall that had mesh buried inside it would get the same reaction, if that is the concern.

And even if they know, it's a) hidden and b)reversible and c) likely to improve the longevity of the construction. Why would it be an issue?

 

[DavidB]

COs are normally fine about you using stainless steel mesh
Though in truth I would be tempted to just put the mesh up, get a base coat on quickly (which you will want to do anyway cos raw mesh is evil stuff) and who would ever know

Thanks livias - one last question/clarification please.

So should I put my laths up but leave them shy of the corner by 5-10mm so they don't form a sharp edge and then put the stainless steel mesh on top and wrapped around the corner?

 

[livias]

Butt the stainless up to the laths not sat on top
Try and get a bit of tension in the mesh. Not much but dont let it be floppy. Use plenty of stainless screws and penny washers with extra washers behind if necessary to get a key