Lime Strawbale Plaster/Render

Details

Please note:

Lime plasters should be applied correctly. It is highly recommended to use a plasterer experienced in the use of lime plasters or at the very leat, you should get some practical experience e.g. on a course, as it is not possible to cover every point in detail here.

Preparing the mix

• The plaster needs to be 'knocked up' - a process of chopping, beating and turning which will release some water. Only the minimum amount of water should be added if required to make a workable mix - it should be as stiff as possible.
• a mortar mill or forced action mixer is the idea way to 'knock up' the lime plaster but this is not always possible. If a bell mixer is used, then it should be left turning for long enough to achieve a suitable consistency without adding water (20-30 minutes).
• if water is  required, it should be added CAUTIOUSLY.

Preparing the surface

• Al loose straw should be trimemd back to a consolidated surface using a hedge trimmer or a strimmer.
• Dub-out the walls with Strawbale Plaster where necessary before applying the first coat of base coat plaster to create an even surface (each coat of lime plaster can then be applied at the same even thickness). This should be left to harden before applying the first base coat.

Application

• It is usual to apply three coats of plaster/render, although this will be dependent upon the desired weather protection and also the desired aesthetic for the building. The render can be spray applied using certain equipment (Note: please inform us what you are thinking so that we can advise on suitability) and it is often quicker and more effective to do so as the render gets forced into all of the voids in the surface of the straw. Spray guns can be hired from Ty-Mawr so please contact us for details. 
• A thin 'scat' or harled coat can be thrown on using a harling trowel or coal shovel. The strawbale mix is mixed with water to the required consistency for throwing on the wall. This application method will help the plaster key into the wall and provide suction for the first trowel applied coat.
• Dampen down the scat coat. Then aply scratch coat with a steel trowel evenly to a depth of 9-12mm. When firmed up, scratch the plaster (to about 3-4mm depth) diagonally using a wire or lath scratcher. This coat takes up any shrinkage and may crack. You need only worry if the plastr becomes detached from the background.
• Apply the next (floating) coat at a depth of 9-12mm - as above but instead of scratching, you need to float the plaster whilst it is still soft enough to take the indent of your thumb. This will compact the plaster to avoid shrinkage cracks as well as flattening the wall. it should be carried out using either a straight grained wodden float or a plastic float. Floating is hard physical work, sometimes a little water sprayed onto the surface can help the process. If only two coats are required then there is no need to key it with a devil float.
• After the floating coat has been floated and the plaster has firmed up, use a devil float to form the key for the finish coat. This is done by rubbing the surface of the float coat in small, circular motions to achieve a depth of not less than 3-4mm.
• the surface of the floating coat should be scraped down with the side of the trowel and then brudhed to remove loose materials whilst still green (holoding moisture).
• A final finish coat of 6-9mm to be applied with a steel trowel. Once it has firmed up, this can be floated using a with a sponge or a wodden float, depending on the desired aesthetic.

Protection of the work

• Allow each coat to 'go off' before applying the next one. As a guide, it takes 1-2 weeks for the first coat, 1 week for the second, and a few days for the finish coat. This will vary considerably depending on weather conditions and substrate. Each coat should be hard enough to resist indentation from a knuckle, but soft enough to scratch with a fingernail.
• protect new work as conditions dictate e.g. for hot, dry or windy weather, damp hessian and plastic sheeting may be required to prevent rapid drying. Gently spraying may be necessary if areas of plaster/render are dying to quickly. Note often the top needs to be treated differently to the bottom of the plaster/render as moisute moves down and different elevations may need different levels of protection.
• Do NOT use humidifiers and heater to speed up the set. Good and even ventilation is key. Accelerated drying will preven carbonation and may cause the paster/render to fail. Gentle heat may be used cautiously in cold, damp buildings.
• protect new lime plaster or render from frost - external work, whereever possible, should not be carried out when temperatures fall below 5 degrees Celsius before carbonation has taken place. If work must continue externally, although not advisable, the scaffold should be fully enclosed and well-circulated heating supplied.
• protect from rain - heavy rain canwash the lime out, or at least, draw it to the surface before the render has carbonated. A new render should be protected until the surface carbonation has taken place.

Storage

• store air tight and frost-free
• plaster with natural hair must be used within four weeks otherwise more hair will need to be added.
• premixed plasters/renders will become hard over time. Although still usable, they will they will take considerable effort to 'knock up'. For easier results use as soon as possible.

Aftercare

• Your finished lime plaster will care for your building for years to come as well as helping to control the humidity of the internal environment. It gives a beautiful finish that no modern plaster can replicate. We highly recommend that you finish it with a breathable natural paint. Your choice will depend on the level of durability, vapour control, and desired aesthetic required.
• Call us on 01874 611350 for advice.

Additional Information

• Further instructions on plastering and rendering with fat lime plaster onto different substrates are available from your local suppliers. We run regular courses for contractors, home-owners and specifiers which can save much time on-site as well as ensuring a successful project.

It will care for your building for years to come as well as helping to control the humidity of the internal environment. It gives a beautiful finish that no modern plaster can replicate.