Lime putty mortars offer advantages over cement based mortars for external rendering, especially when decorated with a traditional limewash:
- their porosity allows the structure to “breathe”
- they can accommodate general movement better
- their self-healing nature reduces cracking problems
As with all lime based materials the best outcome requires patience and careful control of drying and suction, the reward being a good looking and long lasting render.
A typical render specification:
1. Ensure that appropriate scaffolding is in place and that the worksite is safe for workers and public.
2. Remove the existing render, except any existing sound lime mortars, taking care not to damage the structure. Look out for very thick patches of render that are effectively load bearing. It may be preferable to render on top rather than risk rebuilding an area.
3. Dub out any deep holes in the wall with a haired lime putty mortar, rebuilding defects with cob blocks, bricks or stone as appropriate.
4. Treat wooden lintels with preservative and counter batten with oak lath if rendering over them.
5. Apply one hand harled coat of Secil Consolidation Mortar to provide a key to the wall. This is recommended for all substrates. Leave this coat to dry for a day.
6. Apply sufficient coats of haired lime putty mortar with added pozzolan (Argical) to smooth the contours of the wall. With a suitable animal hair in the mortar coats can be applied up to 15 mm thick rather than the 8–10 mm thickness of unhaired mortar. The hair reduces any slumping whilst applying and shrinkage cracking whilst curing. Each backing is keyed with a scratch comb. Leave this coat to dry for approximately a week.
7. Apply a top coat of floated unhaired lime putty mortar with added pozzolan (Argical) to an average thickness of around 8-10 mm. Alternatively if you want a rough cast finish then an additional coat of hand harled lime putty mortar or Secil Consolidation Mortar can be applied.
Damping & curing:
It is very important to control suction from the wall by light spraying with water half an hour before applying each coat (especially cob and porous brick) and in warm weather it will be necessary to spray each coat afterwards. Whatever the season, protect each render coat during the curing process from all the elements such as hot drying wind, strong sun, rain and don’t apply in temperatures below 5 degrees centigrade or if there’s a risk of frost. A heavy cloth such as hessian sheeting will provide a suitable physical barrier and should be left in place as long as required.
Lime putty mortars gain added strength by carbonating over many months with atmospheric carbon dioxide. Whilst pure lime putty mortars are suitable inside or for sheltered locations, it’s recommended that for exposed elevations each coat of lime mortar has a pozzolan added. These are burnt clays that react with the lime to give harder, more frost resistant renders and historically were derived from volcanic ash, crushed bricks and other forms of burnt clay.
Alternatively some specifiers suggest using natural hydraulic lime based renders. The methodology is similar to the above guidelines but our advice is that these specifications require careful selection of appropriate sands, the actual strength of each NHL brand will differ from the minimum and hence there can problems with controlling shrinkage.
All coats need to be given time to dry before subsequent coats are applied. To test whether a coat is ‘green hard’ the surface should be resistant to a fingernail. Many factors will influence the timing, such as the season, exposure of wall and the thickness of the coat but it’s normal to expect a day for the harled coat to dry and harden and perhaps approximately a week for each of the thicker scratch and float coats.
It is important that lime renders are not totally sealed with an inappropriate paint. Limewash is the most cost effective paint to apply, offers the most traditional finish and will aid rainwater shedding . A minimum of four coats of limewash incorporating a little linseed oil should be painted on the final coat of render.
Alternatively a silicate paint is suitable. These cost more than limewash but have a longer life expectancy, require fewer coats and can be more water repellent whilst still offering breathability.
Lime mortars are easy to use and can readily be applied by the enthusiastic amateur given a little tuition and guidance. Although, the work needs to be carried out correctly, the beauty of the traditional building is that it rarely looks right when everything is straight and perfect, so the DIY owner can begin work comfortable in the knowledge that a certain amount of ‘character’ would not look out of place.
For the example specification per square metre:
1) Harled coat of Secil Consolidation Mortar @ 3 mm~ 5 kg/m²
2) Scratch coat of haired lime putty mortar @ 15 mm ~ 30 kg/m² (+ an appropriate amount of pozzolan [Argical])
3) Float coat of unhaired lime putty mortar @ 10 mm ~ 20 kg/m² (+ an appropriate amount of pozzolan [Argical])
If you’re applying a hand harled, rough cast finish you will need the same amount as the harled coat.
Time of year:
Please note that great care should be taken not to be applied too late in the year or too soon in spring or else frost damage may occur. It is important to prevent frost crystals forming within the mortar soon after application. The ultimate hardening process takes up to a month for each millimetre of thickness. Therefore it may take 20 months before mortar has carbonated to a depth of 20 mm.
Please contact us for more guidance and information.
Safety: Limes are caustic.Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels
Our advice and information are given in good faith. It’s important that users satisfy themselves that they’ve chosen an appropriate product and have a suitably skilled workforce.