Working on older properties that are typically over 50 years, old means having to deal with old materials and construction methods, one of the most often encountered is having to repair damaged plaster,
Before the development of modern drywall and plasterboard, internal walls were often constructed using Lath and plaster techniques.
Lath and plaster construction consists of a wooden framing with vertical studs, the Laths are strips of wood that are fixed horizontally across the vertical studs, they are nailed or screwed in place and the plaster is applied on top of the wooden strips to provide a smooth surface.
This construction method was developed from the much older ‘wattle and daub’ process which used wooden strips or thin branches, woven between a wooden frame to create a solid surface, that a wet mixture like mud or plaster can be bonded to.
As the plaster ages it deteriorates and tends to lose its grip on the wall behind, it will then crack or fall away from the subbase.
Areas of plaster that have come away from the sub-base are known as ‘blown plaster’, it will sound hollow when tapped and often flex slightly when pressed.
In this guide we give you the steps to fix and repair damaged Lath and plaster walls and ceilings, you can learn more about the types of plaster used in construction in our Guide to Types of Plaster
Tools and Materials
There are a few tools and things you will need for the job:
Cut out The Damaged Plaster
Blown or cracked areas of Lath and plaster walls need to be repaired properly, rather than patched up using filler, as this will only delay the task for a short while.
Use a bolster and lump hammer to remove loose plaster, cut out the damaged area to expose the wooden laths beneath, cut the plaster back to a sound, strong bonded surface, brush the surface to remove dust and lose particles, repair any damaged laths.
I like to use a pencil to roughly mark out the area to be removed, by tapping the area with the handle end of a trowel and listening for the solid plaster and identifying the good surface, then I use a sharp utility knife and follow the line scoring the surface of the plaster.
Then using a wide bolster and a lump hammer knock away the old damaged plaster to expose the wooden structure below. Brush away any remaining dust and loose plaster.
Always wear appropriate protective equipment when carrying out demolition work-gloves, Eye protection and a face mask are the minimum protection required to help keep you safe.
Repairing Damaged or Broken Laths
More often than not the Laths supporting the plaster will be in good condition, but those that have rotted or become damaged need to be repaired or replaced.
Cut out sections of damaged Laths with a pad saw, use steel mesh or plasterboard to replace them, and fix back to strong sections or the studs with screws, treat any exposed laths with wood preserver to help prevent wood rot or boring bugs.
Damaged or rotten sections of wooden laths need to be completely cut out and replaced with new sections or modern alternatives like mesh or plasterboard
I use a Hitachi 18V multitool with a wood cutting blade to make quick work of this task, you can fit a dual material blade if you need to cut through old nails as well.
It is often easier to cut the damged section back to the studs on both sides then fill the gap back with plasterboard which can be fixed directly to the studs or onto battens fixed to the studs.
Mesh can be bent and molded into awkward or curved shapes then fixed to the laths using screws for smaller patch repairs.
If you cut the mesh using metal shears, ensure that you treat the cut edges with a rust inhibitor like red-oxide primer or Hammerite to prevent any corrosion from penetrating the surface.
Cutting the laths away will invariably mean that both sides of the wall will need to be repaired to check the condition of the rooms on both sides and allow for the work to be completed on both surfaces.
Skimming with Plaster
Once the sub base has been repaired and is sound, it can be replastered to give you a clean, smooth surface to decorate.
Dampen the laths with a wet sponge, avoid making any plasterboard wet as this will destroy the paper covering. Trowel the first layer of plaster over the whole area pressing the mix firmly into the laths. Build up layers until flush with the surrounding plaster, allow it to stiffen then smooth with a trowel and blend the edges with a damp sponge.
To repair the surface you can use a one-coat plaster and skim the plaster on by hand using a trowel, at first pressing the mixture so that it bonds firmly between the laths or the plasterboard
Then build up the layer until it is level with the original plaster.
Smooth and level the surface using a flat, straight edge or plastering darby, and fill in any low spots or hollows. Allow the mixture to stiffen then trowel the surface as you would for any other plastering project
You can learn more about applying and skimming plaster in our How to Plaster guide
To get a good bond, I prefer making the wooden laths slightly wet with a damp sponge, this prevents them from sucking too much moisture out of the plaster.
If you have used plasterboard as a new sub-base, make sure this does not get wet, otherwise, the paper surface will deteriorate and the surface will fail.
If the level of the original plaster is very deep, consider using a two-coat method for applying the plaster. Trowel on the first coat and press it into the surface to ensure a good bond onto the laths, then scratch key the surface and let it set.
Apply a second topcoat using a trowel and use a darby to level it with the surrounding surface, to blend the edges use a damp sponge to feather the new plaster over the old existing surface.
Repairing a Lath and Plaster Ceiling
Damaged ceilings are often seen in older houses, usually due to water damage from a burst pipe or a leaking roof, the brown patchy stains are a tell-tale sign of water damage,
As with repairing lath and plaster wall, the old, damaged plaster surface needs to be removed from the ceiling. Use a sharp utility knife to trim around the damage and then carefully pull the plaster away from the laths behind.
You can use a multi-function scraper to help lift the plaster away and I often use an 18V Multi-tool with an old, wide wood cutting blade to cut the plaster, taking care not to cut too deep and damage the wooden laths underneath.
Always wear eye and face protection when working overhead to help prevent any plaster from causing an accident. I find these Bole safety glasses very comfortable to wear
Cut the plaster back far enough to remove any loose or blown areas allowing you to work with a sound and solid surface.
Repair or replace any damaged laths as before using Plasterboard or wire mesh, cutting them back to the nearest joist.
Before you apply the first coat of plaster, make the wooden areas damp using a wet sponge then trowel on the first base coat, don’t build this up too thickly, scratch key the surface then allow it to set, and then apply a second thinner base coat.
You can scrape this back to leave you approximately 1/16” (2mm) ready for the top skim coat which can be blended with the surrounding area using a damp sponge as previously described.
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This article was written by: Richard Quinton – The DIY Help Desk Owner, Engineer & technical specialist.
Richard is one of the key partners in The DIY Help Desk team. He is a qualified Engineer, writer, and publisher, educated to Master’s level. He is a keen advocate of DIY and home improvements.
Richard enjoys helping others to learn new skills and reach their goals and believes that passing his knowledge and experience on through his writing is an effective way to positively impact the lifestyles and well-being of others on a larger scale.