Applying lime render / plaster.
It would be impossible to cover all aspects of lime render application in a few pages. So below we have given a fairly comprehensive overview of a 3 coat system which can be used both inside and out and on a range of different substrates.
This guide is aimed at experienced plasterers who have limited or no knowledge of working with lime renders. A basic knowledge of mixing and applying render coatings is assumed.
Preparation
It is really important to carry out the correct preparation when working with lime.
Make sure the surface to be plastered is cleaned thoroughly and free of all dust and debris.
The surface also needs to have a good ‘key’, smooth surfaces will need to be made rough to provide a good surface for the first coat of lime render to adhere to.
Is the surface too dry? Lime mortar applied to a dry wall which contains little or no moisture, will dry quickly, shrink and fail. Lime needs to go through a curing process which takes place while there is still moisture available. Use a garden sprayer to damp the wall down, if the wall dries immediately, spray it again. Repeat the process until the wall is damp but not saturated.
Filling large holes Since lime mortar is more expensive than stone it is a good idea to fill larger voids with a combination of stones and mortar. Throw the mortar into the voids to create a good bond and pack with stones. Add more mortar if necessary.
Aftercare
Exterior - Pay attention to the weather. Strong sun, wind, frost and rain will all affect the performance of lime render.
Try to protect work from direct or wind-driven rain, and where necessary use hessian sheets to stop the work drying out too quickly from wind or strong sun. Hang the hessian sheets 9-12 inches away from the mortar. In warm weather, wet the hessian to create a humid microclimate in the space, this will help to maintain the required conditions for the lime to cure.
It is also very important to avoid frosty conditions during the render’s early set, particularly within the first 14 days following completion.
Interior - If working inside, it is best not to introduce heat into the room until the lime has cured. Let it dry naturally, spraying lightly with water one or twice a day. If the temperature in the room is likely to drop below freezing, a low level of heat can be introduced.
Application
Base Coat (Bonding Coat) 9-12mm
The bonding coat is designed to create a good bond between the substrate and the render. The mortar mix should be wetter than for subsequent render coats and if possible contain some more gritty material. The traditional method of application of the base coat is by ‘Harling’ (throwing) it onto the wall with a trowel forcing mortar into cracks and imperfections to create a tight bond. Harling is the preferred method for very porous surfaces like cob or soft brick, or impervious surfaces such as granite or engineering brick. Alternatively you can lay it on with a trowel, making sure to press it firmly into the surface to create a good bond. You will find that lime mortars require greater pressure to apply than cement based mortars.
- Don’t worry about levelling the base coat, lay it on and leave it.
- Protect the render if necessary and spray with water daily or more frequently in sunny / windy weather conditions.
- Once the mortar starts to set, scratch the surface of the bonding coat using a diamond pattern. A traditional 3 pronged lath scratcher is a simple tool to make. (See making a 3 pronged scratcher, below)
IMPORTANT – ensure the render has set sufficiently before applying the next coat (This could take up to a week). You should be able to scratch the render with a finger-nail but not dent it easily with a knuckle. If subsequent coats are applied too soon, cracking can occur as the individual layers shrink back independently.
Levelling Coat(s) 9-12mm
As the name implies, the levelling coat is used to build up the depth of the render so that there is sufficient material with which to create a level surface.
- Apply the render with a trowel to a maximum thickness of 12mm. If a 12mm coat is not thick enough to create a level surface, add an extra coat(s) as required. Scratch the surface of any underlying coats with the diamond pattern as before.
- Once the required thickness has been achieved, scrape back the render with a straight edge to create a level surface.
- When the render has set sufficiently, rub back with a float and then lightly score with a devilled float (a float with a nail protruding around 2mm). It is now ready to take the finishing coat.
- As before protect the mortar as it is setting with hessian and water spray if necessary.
Finishing Coat 5-7mm
The mortar for the finishing coat will usually be lime rich and will include a finer sand than previous coats.
- Apply a 5-7mm coat of the render. Can be thinner for interior finishing coats.
- After application leave the render for as long as is practically possible before finishing. Working on lime mortars too soon will result in free lime being pulled to the surface (Case Hardening), which will affect the property of the mortar and could lead to failure.
Making a 3 pronged Lath Scratcher
A simple scratcher can be made using 3 short lengths of lath. Cut the ends of the laths at an angle to create a point. Holding the laths in your hand, fan them out so that the points are a couple of inches apart from each other. Use to scratch a diamond pattern in lime render base-coats.
Further Reading
Using Natural Hydraulic Lime (NHL). Includes a guide to mix ratios.
Lime product FAQ’s
Liability Waiver
The information provided in this guide is intended for general use for operators with limited experience of traditional renders. The advice offered here is given for guidance only and will assume that best practice will be used in its execution. No claims for liability can be considered on its intent.