What to Do if Your Plaster Cracks Up

Cracks and holes not only look unsightly, but are signs of deter oration in the fabric of the home-am should be quickly put right. Plastering techniques are a matter of some practice, with the careful and systematic application of the methods involved. Choice of plaster materials is also important in relation to the type of surface, in order to achieve good results.

Plaster cracks are usually caused by slight settlement or traffic vibration, a heavy knock or, sometimes, by damp penetration or even impurities in the original plaster mix. There are two types of crack: check cracking, due to shrinkage of plaster which is usually slight and of minor importance, and map cracking, which is deeper and wider and may be due to settlement of a building or possibly caused by timber shrinkage.

Plaster types

Plaster is applied basically to two types of surface-those to which some type of key is attached, such as wooden laths or expanded metal, and solid surfaces, such as brick, building blocks or concrete.

On solid surfaces two coats of rendering, known as the floating coat and the setting or finishing coat, are usually applied. When almost hard, the floating coat is keyed by scratching the surface; this ensures a good grip for the finishing coat.

When plaster is applied to lathing an extra coat, known as a backing coat (or pricking-up coat, when applied to metal lath) may be necessary.

Plaster may be bought ready-mixed in bags; only the addition of water is required to make it workable-or traditional mixtures of sand, lime and Portland cement or plaster of Paris may be prepared. .

The type of plaster you decide to use will largely depend upon the existing surface you are making good. If it is a sort finish, such as lime-hair plaster, found in many old houses, then a proprietary ready-mix, soft plaster, called a retarded hemi-hydrate, will best suit the work.

If, however, the surface is hard and grey in colour then a slow-setting hard plaster over a backing coat of cement : sand : lime is preferable.

There are different types of soft plaster, for various background surfaces: solid, with normal absorption, a grade for high absorption, and grades containing anti-rust agents and bonding fibres for applying to metal lath, wood-wool slabs and expanded polystyrene. There is a bonding coat for hard, low-absorption surfaces, a grade for heavier surfaces, and a setting coat, suitable for all backing coats.

The harder, anhydrous plasters, which are grey, pink or white in colour, should not be applied over a soft, hemi-hydrate backing coat or to patch soft plaster; either will produce cracking.

A suitable backing coat for anhydrous plasters is white hydrated lime (obtainable in 25kg and 50kg bags), Portland cement (obtainable in 50kg bags), and good-quality fine building sand in the proportions 1:1:6 by volume.

For filling cracks and small abrasions, use one of the cellulose-based fillers. Cellulose fillers have the advantage over the older gypsum-based fillers that they do not shrink.

Setting times

Anhydrous plasters have a gradual and continuous setting time of two to three hours; towards the latter end of this time they may be softened with water and any irregularities polished out with a steel finishing trowel.

Retarded hemi-hydrates have a ‘final-set’ time of about 1-1/2 hours. Only during the first 60 to 70 minutes can the plaster be worked, and it is not possible to soften it once the hardening process starts, although a little water, as a lubricant for the trowel, may be used during the final polish.

You may find, due to the short setting time, that hemi-hydrate plasters are difficult to work over large areas; on the other hand, they are ready mixed, lighter and cleaner to work with. It is worthwhile persevering as, once the technique is mastered, they are in fact easier to .use than anhydrous plasters.

If you do use ready-mixed plaster only mix up the quantity you need and do not ‘over-mix’-that is do not add more plaster to water than recommended.


The way in which plaster is mixed is critical to achieving a good finish. Whenever possible, follow manufacturers’ instructions exactly. When mixing up sanded mixes, make sure that the gauge box (or measuring container) is kept clean, and that the quantities are exact. Turn the dry materials over thoroughly until an even colour is produced.

Finishing plasters can most easily be mixed in a bucket. Fill a clean bucket half-full with clean water and sprinkle the plaster into this until plaster settles on the surface, then stir briskly until a creamy mixture is obtained.

Should you wish to use lime plaster, rather than a lightweight mix to repair an existing surface, first make a lime putty, by mixing hydrated lime to water until a creamy mix is achieved. Let this stand for 24 hours, then tip some on to the spotboard in the form of a ring.

Pour some water into the ring and into this pour plaster of Paris (casting plaster) until all the water is soaked up. Mix thoroughly with a gauging trowel. Always use clean water and tools, otherwise the plaster may be weakened.

Estimating quantities

For general gap filling with cellulose fillers, a large carton will usually be more than enough for a room. For estimating the coverage of soft plaster allow, dependent upon background, the following:

Backing coats:

Normal absorption : 11mm thick 6.5m2 to 7.4m2 per 50kg.

High absorption : 11mm thick 6.5m2 to 7.5m2 per 50kg

Metal lath (metal) : 8mm thick 6.5m2 to 7.8m2 per 50kg

Metal lath (wood wool, polystyrene) : 11mm thick 3.3m2 to 5.0m2 per 50kg

Bonding grade : Hard surfaces 11mm thick 5.3m2 to 8.3m2 per 50kg

Heavier surfaces : 8mm thick 7.0m2 to 7.5m2 per 50kg

Finish : 2mm thick 20.5m2 to 24.7m2 per 50kg

When estimating a sanded floating-coat, mix, calculate according to the quantity of sand needed. Roughly 380mm3 of sand mixed with lime and cement to proportion 1:1:6 by volume will cover a 30m2 wall. The 380mm3 of sand should be mixed with 100kg cement and 50kg lime. Pink cement 3mm thick will cover ll-5m2 per 50kg. Grey cement 3mm thick will cover 13m2 per 50kg.

Include about 10 per cent for wastage on all calculations.

Tools for the job


• Putty knife or old screwdriver. Used for scraping loose plaster from cracks.

• Bolster, cold chisel and club hammer for cutting out patches and trimming old plaster.

• Paint brush for damping backgrounds.


• Two buckets, preferably rubber or polythene, for mixing up plaster and holding water; it is important to clean these out thoroughly after each mix.

• Spot board – This can be a piece of ply, preferably marine, about 270mm2, to contain plaster. Plaster can also be placed in a light metal bath.

• Gauging trowel available in various sizes, but one of 15mm length will be most convenient; it is used for measuring (gauging) small quantities of materials and working small areas of plaster.


• Laying-on trowel has a steel blade of about 280mm x 120mm, reinforced by a steel or alloy tong attached to a handle. The banana shape is easier to hold as straight-turned handles are liable to slip.

• Skimming (or hand) float; made of wood measuring about 300mm by 115mm.

• Angle trowel (or twitcher). These can be for internal or external angles. The latter is more for convenience than necessity.

• Small tools; various shapes and sizes, the most common of which is the leaf and square. They are usually made of steel.

• Scratcher; used for keying backing coats. It can be made from a piece of ply through which nails are driven.

• Devil float; has a similar use to the scratcher, but is better on large areas as it smooths burrs as it. Keys.

• Feather-edge rule. A straight piece of wood with one edge chamferred to 6mm thick. Made from 125mm x 25mm timber about 15m long, it is used for working down backing coats and working angles.

• Filling knife; used for filling cracks. It should have a thin, springy blade.

• Hawk. Made of metal or wood, it has a working surface of about 300mm2 and is for holding plaster up to the working area. Other general tools you need are a saw, a spirit level, a plumb line, a measuring rule, a claw hammer and, for mixing large quantities of cement, lime or plaster, a shovel.

10. November 2011 by admin
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