How to use lime putty

There is a building product out there which has some truly remarkable characteristics, so I thought it only fair that I should tell you about it, and offer some hints on how to use it successfully. When used in renders it can self heal tiny cracks that may form. It allows the building to breath so avoiding damp. It is also a ‘sacrificial’ layer. After it has degraded (as all exterior coatings do eventually) it can be removed and replaced without damage to the underlying structure. When used in brick laying it is flexible enough to move with the building thus avoiding cracks in the mortar. It is also porous so moisture can escape through it rather than the brick, and so preserve the face of the brick. Truly an incredible product.

“What is it? When did it arrive on the market? Can I get it at B&Q?” I hear you cry. ‘It’ is of course lime putty. Alas, with all our eggs firmly in the basket of modernism we have neglected the lessons from the past. This cutting edge product is in fact thousands of years old, but today fewer people know how to use it than ever before. You will struggle to find it in the builders merchants, and the boys and girls at the DIY sheds will probably point you in the direction of the Toys ‘r’ Us if you ask them for it. Ever the fan of the underdog, The Building Experience is championing this venerable muck and we would encourage builders and DIYers, especially those with old properties, to show some pluck and try something new… even if it is very old. Confused? Read on…

What is it?

If you want a chemistry lesson you are in the wrong place, but the basics are as follows. Burn some Quicklime, chuck a load of water on top of it (a process called slaking), let it stew for a few months and then sell it as matured slaked lime putty. As the end user we skip the burning and slaking and buy it in 25kg tubs ready for mixing. When you take the lid off you have a pure white lump with the consistency of cheese.

I have not come across another area of the building trade so hazy and ill-defined as the use of lime putty. After extensive research on the web, in books and with companies specializing in restoration products there is still no discernible consensus on the best way to use lime putty. The notes that follow are a synthesis of advice given and our own experiences of what worked for us. It is by no means beyond reproach but it was learnt the hard way.

Mixing

• Lime putty is NOT the same as cement, do not treat it as such.
• Lime putty sets by carbonation. It chemically reacts with carbon dioxide in the air and slowly changes form to calcium carbonate. Consequently if there is no air, there is no carbonation and so the mix will not set. It will last indefinitely in the tubs (the layer of water is there to stop the lime contacting with the air) and any sand/putty mix will also last some time if kept covered.
•  A mix of 3/3.5:1 (Sharp Sand:Lime) is suitable for most applications. The putty is not easy to shovel so it is best to mix with two identical containers, one for sand and one for the putty to give correct ratios.
• The putty is very stiff to start but once you turn it over with the sands a few times it begins to loosen up. If you wish use a plasterers whisk and drill to knock the putty up first. This saves a lot of shovelling. The wetness of the sand also affects the ease of the mix and the final consistency.
• Once you have knocked up the amount you need either heap it in a pile and cover it with plastic sheets or put it back in empty tubs with a layer of water on top. Go home, and when you come back the next day be prepared for something magical. Yesterdays stiff lumpy mess will have softened into a perfect creamy mix. At this point you can mix cement, plaster or a pozoloid depending on your requirements. This will dry the mix so you will need to add a small amount of water to compensate. Once you have knocked the mix up again you are ready for action.

Application

• Techniques for plastering have been covered in other hints and tips but there are a few points to bear in mind.
• Lime putty was, and is, mainly for old buildings with a rustic look. Do not expect, or attempt a perfect finish. A coarser, uneven finish actually gives a greater surface area for moisture to escape from the walls of the building. More importantly the putty is unsuited to being overworked so keep trowelling to a minimum.
• The mix is best applied with a hurling or gauging trowel rather than a float. It can be thrown onto the wall in a ‘wristy’ action, smoothed once and left alone to set. Then the wall can be levelled with a wooden float as the mix hardens but this should be minimal and executed with a light touch.
• Direct sunlight will dry the mix too quickly, and rain will not allow it to set at all. The answer is Hessian sacking laid in front of the render to protect it from the elements and allow the slow carbonation process to take place in its own time. It may sound like an effort but it is well worth it.

Wear decent gloves when working with lime putty. Dry hands are an occupational hazard when working with cement but lime is even worse so cover those pinkies or prepare yourself for a world of pain. The rest is a question of your individual skill on the trowel. Allow plenty of time, apply two or even three coats on subsequent days and enjoy working with a fantastic building product.