How to repair a Victorian lath and plaster ceiling
1. The cracked ceiling before any work.
- The client advised the crack was caused by a leaking pipe in the ceiling above which had since been repaired.
- There was a slight bow to the ceiling around the crack and from inspection the plaster was coming away from the laths in places.
- From a little light tapping with my fingers and a few stitch drill holes I located all the ceiling joists and marked them in pencil.
- To provide some temporary support and to stop the plaster from collapsing I put a few screws into the ceiling joists either side of the crack.
- Be careful not to overtighten or overcompensate so the screws don’t pull through the plaster or pull the dip in the ceiling too tight which could cause further cracks, this part needs a bit of judgement.
2. Bracing the ceiling with stainless steel buttons and raking out the full extent of the broken plaster to see what repair is needed.
- After providing temporary support the cracked plaster can be carefully raked out using a chisel or stanley knife to see the extent of the damage, which is normally much worse than it appears from the superficial crack.
- This crack had been exacerbated by the previous installation of some ceiling spotlights which had cut away the laths around each spotlight and looked as though the ‘key nibs’ of plaster were broken off too. The plaster around the spotlight fell away with some light poking around with a chisel and left a large void before I was able to cut back to reach sound plaster.
- The broken laths were replaced around the light fitting & screwed into the adjacent joists
- To provide the long term reinforcement stainless steel ‘Plaster Buttons’ are used.
- I find it best to recess the buttons into the plaster slightly by using a circular hole saw drill attachment with diameter slightly larger than the button. Cut away a small disc of finishing plaster to approx 10mm depth so when the buttons are screwed into the joists they sit slightly below the surface.
- Depending on the extent of the support needed put two or three buttons each side of the crack using 60mm drywall screws into the marked joists.
3. Providing some fibreglass mesh backing for the subsequent bonding coats.
- To provide some backing for the bonding coats I carefully cut to shape, overlapped and screwed fibreglass mesh to the laths and joists. As the laths are at risk of splitting I pre drilled them and used small screws and a few washers to hold the mesh in place.
- Before the first coat of bonding I brushed away any loose plaster from the edges of the crack and treated the exposed edges by brushing on a couple of coats of neat SBR.
4. First & second coats of bonding
- Whilst the SBR is still tacky the first coat of bonding (British Gypsum) can be applied. I mixed this to a fairly thick consistency and also added a handful of polypropylene fibres to get the bonding as sticky as possible.
- Using a small putty knife I work the bonding up into the mesh and between the laths as much as possible, pressing the bonding up against the SBR treated crack edges. This coat is all about adhesion and a good key with the mesh, the laths and the edges of the crack so don’t worry about how it looks at this stage.
- Start to fill the plaster button holes with some bonding as well.
- This coat needs a few hours to dry before a second coat is applied.
- No fibres need to be added for the second coat and the mix can be made a little smoother to get a flat finish. The bonding can be levelled off with the edge of a trowel or timber batten worked over the surface in a sawing motion.
- I was taught to make each coat of subsequent plaster slightly weaker than the one before as this prevents cracking and helps the layers bond together. I.e make the base layer the strongest and top layer the weakest.
5. The first & second coats of finishing plaster (Tetrion interior filler)
- The bonding needs to dry overnight before the finishing plaster can be applied (I use Tetrion interior filler)
- Before applying the finishing plaster I brush SBR over the whole surface and let it start to go tacky.
- The first layer of finishing plaster can be trowelled over the bonded area and feathered out either side another 250mm approx.
- Once the first layer is dry the second layer of finishing plaster can be trowelled over the area feathering out maybe 500mm either side so any undulations are smoothed out.
6. Final coat of finishing plaster.
- The last coat should provide the final finishing touch to the repair, blending the patched edges into existing so as to make the repair invisible, smooth over the plaster button holes one last time.
7. Sand paint and finish to complete the repair.
- Let the finish plaster dry overnight.
- The following day, give the area a light sand with 180 grit sanding pad, brush away any dust, wipe with a tack cloth and give the whole ceiling 2 coats of emulsion paint.
- The repair is complete.