replastering part of old lath ceiling

by paddy
(bristol)

hello,

i have had part of the ceiling collapse in my living room (directly below where people jump out of a mezzanine bed upstairs) . there was a crack there for a long time and it eventually gave way.. I want to repair onto the old lath.. the house is from the early 1900s so i am not sure what the materials used in the plastering were.. there is 2 coats, a cement or lime based render followed by a top coat.. both are very powdery and turn to dust quite easily. I am wondering do you know what these are and is it ok for me to use cement followed by gypsum beside this. I am not worried about creating the patterned finish which is on the ceiling... there is some of the plaster loose around where it has come away and it is hanging a bit.. should i pull it off until it is solid or can i attempt yo glue it back up there with pva?

i have done a good bit of cement and lime rendering but this is a bit of a new one for me.. any help you can give me would be great... cheers paddy

Answer

Hi Paddy,
Yes all the loose needs to be very carefully removed - a sharp knife can sometimes cut around to ensure no more plaster is disturbed.
The plaster is lime render with a top coat of lime and fine sand, to give a smoother finish.
If you have carried out some lime rendering before then you could always try this method, if you are wanting to restore as was, if not the plasterboard option is easier.

Lime render (Difficult option)

Check fixing of laths and repair if necessary.
Apply a first (lightly haired) coat (3:1) just to cover the laths and let this firm up over a day or so, making sure it is scratched to provide a key.
Then damp this down plus all edges of existing plaster and apply a second coat out to the thickness of the existing plaster and rule off flat with a straight edge. Float flat when firm. Allow to firm up over a day or so and check for shrinkage cracks. Skim with a very thin coat of lime and fine sand (mixed a bit stronger 3:2 or even 1:1) and apply this as a thin skim to the level of the existing) and trowel up and float as drying process occurs over the next few hours / day.

Plasterboard (Easier)

Fix plasterboard to joists in affected area (may be necessary to carefully trim a bit more plaster back).
Damp down all edges and apply bonding coat (making sure it is pushed into all gaps), then rule off flat with a straight edge. When it has started to firm up a bit scrim tape can be lightly pushed into the surface of plaster to prevent cracking on any problem areas.
As it firms up float or flatten with a trowel, then when set apply 2 coats of multi-finish and smooth out as it firms up.

Paul