Strong light board for lime plastering sought

[MikeG]

Hi. You're all new to me, but I discover that I'm not new to you. Wobs led me here from another forum, and I notice that he has been running a thread on my renovation and extension of a 300 year old oak framed cottage on the Essex Suffolk border. http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=15011

I try to be a few steps ahead, and whilst I am scarfing in new feet on the principal posts, and replacing the sole plates, I've started thinking about the saggy old lath & plaster ceilings upstairs. What I would really like to do is screw a strong light board up underneath them, leaving the original ceiling untouched, but clamped firmly in place to the old oak raised ties of the original roof. Even though I'm an architect, I haven't come across anything strong and light enough to do the job, and which can be plastered directly. Wood wool boards are going to be too heavy: the structure really is.........erm.......rustic and minimalistic.

Does anyone have any suggestions?

 

[wobs]

Great see you here Mike!
Personally, I can't help, although I'm sure there are people here who could.

Would you want said boarding to be breathable?

 

[MikeG]

Hi Wobs. Nice little hidey place, this.

We have a whole house mechanical ventilation system, so the necessity for breathability in the external envelope is reduced. So, best answer is "it would be nice, but it isn't critical".

 

[MatthewC]

Hi Mike

Do you want to retain the old ceiling just because it is old, or because you don't want to go to the trouble of taking it down and re-doing from scratch? I took down two upstairs ones but [confession] used plasterboard on two downstairs ones [/confession], because the need to do them occurred so late in the process that I would have had to use the rooms above whilst it was still curing. The plasterboard has not been a problem (and of course breathability isn't really an issue for a ceiling between two floors).

Matthew

 

[MikeG]

Hi Matthew.

Above the lath and plaster ceiling is 500mm of insulation, put in place when I built the new roof over the top of the old one. It wouldn't be a lot of fun trying to push that back into place from below! As well as trying to retain as much of the original fabric of the building as practical, it is also easier (and much cleaner) to just put a new layer underneath the old to give it some strength. Plasterboard wouldn't be strong enough, and would possibly be too heavy for some of the ties I would be screwing to. They are quite fragile.

I could batten out below the ceiling, and plasterboard to that, but I'd rather find a suitable board to take lime plaster directly.

 

[JanJonWestWales]

hi, I have used Celenit boards on our ceilings and walls, from Ty Mawr.
I found them quite variale in their weight(so hardly light), depending on the amount of cement in the mix, some can be quite heavy
On the other hand, the variability makes some of them quite brittle, and difficult to handle long lengths.
But apart from that, they are great for liming onto, and retain the breathabaility

hope that helps
J&J

http://blaencwmbychan.blogspot.co.uk/

 

[livias]

I'm puzzled why you are particularly keen on light board. The lime plaster that you stick to it will be extremely heavy so I doubt if the weight of the board will make much difference to the situation

 

[Zebra]

I'd be surprised if you found anything lighter than woodwool board. I used 15mm woodwool for all my ceilings, even though if I remember correctly they said you should use 25mm. I did glue the joints together with gripfill though, so they wouldn't crack along the joins.

 

[MikeG]

I'm puzzled why you are particularly keen on light board. The lime plaster that you stick to it will be extremely heavy so I doubt if the weight of the board will make much difference to the situation

I want a light board because the structure I am attaching to is weak. Even if the plaster is relatively heavy, there is no point adding further unnecessary loading to a fragile structure.

 

[MikeG]

I'd be surprised if you found anything lighter than woodwool board. I used 15mm woodwool for all my ceilings, even though if I remember correctly they said you should use 25mm. I did glue the joints together with gripfill though, so they wouldn't crack along the joins.

Savolit* woodwool boards, 15mm thick (the thinnest they do), are 8.5 Kg/ sq.m. Ordinary 12.5mm plasterboard is 8.0 Kg/ sq.m. So the stuff is heavier than plasterboard. I assume this is because of the cementitious binders. I was really hoping to find something substantially lighter than this.

*https://www.savolit.co.uk/products/savolit

 

[MikeG]

I found these* wood wool boards from Naturepro, which at 20mm thick weigh only 3.2 Kg/ sq.m. Does anyone have any experience of them? Can I plaster directly to them?

*http://www.natureproinsulation.co.uk/woodfibre_rigid.htm

 

[livias]

To keep the structure light how about omitting the lime plaster completely? You already have a soundproofing layer so all you are looking for is a better visual finish I assume. You could get the 'lime look' with several coats of limewash and that woukd give you considerably more options on weight of board

 

[bennoa1]

Not sure why you want to add weight to a sagging ceiling. Either repair the sag or even better strengthen the ceiling from above with some cross battens. Maybe some of the lathe is broken causing the sag or the plaster has just come away from the lathe. Any pictures? We've used 15mm wood wool board but not on existing lathe, which I have used lime or lime hemp if doing from scratch.

 

[MikeG]

I've just spoken with the guys at NBT, about Pavatex Diffutherm woodfibre boards*. At 20mm thick these weigh 3.2KG/ sq.m, so under half the weight of 12.5mm plasterboard or the Ty Mawr woodwool boards, and they take lime plaster directly, albeit with the use of a fine mesh embedded in the plaster. I think it may be problem-solved.

*http://www.natural-building.co.uk/products/pavatex-woodfibre

 

[MikeG]

To keep the structure light how about omitting the lime plaster completely? You already have a soundproofing layer so all you are looking for is a better visual finish I assume. You could get the 'lime look' with several coats of limewash and that woukd give you considerably more options on weight of board

Hmm......interesting idea. I guess that once the edges are dealt with there is little difference between a lime plaster and a lime wash. You're right, the principle concern is the aesthetics, but also that lumps of the old plaster don't actually detach themselves and fall to the floor. There are also service penetrations to deal with.

 

[MikeG]

Not sure why you want to add weight to a sagging ceiling. Either repair the sag or even better strengthen the ceiling from above with some cross battens. Maybe some of the lathe is broken causing the sag or the plaster has just come away from the lathe. Any pictures? We've used 15mm wood wool board but not on existing lathe, which I have used lime or lime hemp if doing from scratch.

This is my attempt at curing the sag. I can no longer access the void above the ceiling, so pulling something up below it is the only option. Here are some photos:

The last one shows the paint. I think, without being in any way certain, that this is a modern emulsion painted over a limewash. Any suggestions as to how to deal with this generally would be appreciated.

Anyway, just at the moment I have more pressing matters:

 

[Andys]

Blimey. That's quite an extensive renovation to your ground floor...

We have lath and plaster ceilings upstairs with some sagginess where a few joist ends need reinforcing, but nothing quite a challenging as yours. Our emulsion-on-limewash started to peel in a few places and the best approach to removing it is a bored person to peel off the satisfying bits then to attack it with a sharpened scraper (with rounded edges to avoid gouges). A few coats of claypaint finishes it off quite well as it has quite a high-build quality to smooth over the ravaged surface.

If it was me, I'd want to stabilise the existing ceiling, even if that meant bracing it from underneath or chopping a few access holes to get at the timbers above it. I'd hate the thought of an un-monitor-able disintegrating structure locked away behind a nice new surface....

 

[Andys]

oo, oo, dunno what your plaster is made of but ours is very thin and chalky - this looks like it would be good stuff for patching as it might come out more flexible than a sand-based plaster
http://www.anglialime.com/products/wet-mix-plasters-angliamix-mortars-renders/haired-chalk

 

[MikeG]

.........If it was me, I'd want to stabilise the existing ceiling, even if that meant bracing it from underneath or chopping a few access holes to get at the timbers above it. I'd hate the thought of an un-monitor-able disintegrating structure locked away behind a nice new surface....

I have built a new roof over the old, leaving the existing structure intact underneath it, with the ceiling attached. The raised ties are fine: dry, not woodwormy at all, and the fixings are good. The laths are just a bit loose and saggy in places, and in places the plaster is not that tight to the laths, hence wanting to squeeze the old ceiling up with a new one from below, sandwiching the old into place and taking the load off the laths. There is no way that access would be of any use. As I said previously, there is now half a metre of insulation up there, which leaves a small void below the rafters and ridge:

Blimey. That's quite an extensive renovation to your ground floor...

Upstairs is also going to be pretty extensive, with all the render coming off, the frame repaired as necessary, and new windows located in different locations on the front elevation. However, it is basically structurally sound from the girding beam level upwards, and indeed, pretty sound from about 9 inches up from the sole plate. The sole plate is shot to bits all round, and the plinth was wrecked by sand/ cement render (as was the frame), and a flood. It doesn't help that the floor inside was lowered in the 70's to below ground level outside, and that the house sits in a depression in the ground, meaning that the plinth has been permanently damp for about 40 years.

 

[MikeG]

oo, oo, dunno what your plaster is made of but ours is very thin and chalky - this looks like it would be good stuff for patching as it might come out more flexible than a sand-based plaster
http://www.anglialime.com/products/wet-mix-plasters-angliamix-mortars-renders/haired-chalk

There is a version of that which has chopped fibreglass strands in place of hair, but is otherwise the same. It is this that we intend to use here.

On the subject of chalk in place of sand in lime mixes, the plinth mortar is really interesting, being chalky, flexible and weak at the top levels, but close to the ground, and in the ground, it is distinctly different, with obvious sand, and much stronger. I can re-use many of the bricks from the upper levels of the plinth (and chimney), but not from the bottom 3 or 4 courses where the lime mortar sticks like crazy. It's even a different colour (yellow compared with creamy white at the upper levels).