A winter guide to Lime Renders

In an ideal world, working with lime mortars and renders would take place in conditions not too hot, too cold, too wet or too dry.

Unfortunately the British climate and the programme driven modern building industry doesn’t allow for this and work sometimes has to be carried out at times of year and in conditions that are far less than ideal.

No amount of well intentioned advise is as sensible as putting the work off until the weather poses less of a threat, this short guide is intended to help in situations where winter work is totally unavoidable.

Proper thought and planning should be given to the detailing on scaffolding and this should be carefully budgeted for in the contract cost.

The scaffolding should be capable of supporting extra covers and project sufficiently above the top lift to offer protection of the work from above the roof-line, however well detailed the outer protection is, water running down inside will create areas of saturation from splash-back along the lines of the scaffold boards.

A good double sheeting of Monoflex or similar will protect against wind driven rain and the effect of wind chill.

Do not underestimate the effect of wind chill, an 8.7 Mph wind will knock a working temperature of + 5ºC down to -1ºC, at the other extreme a 31 Mph wind will take +5ºC down to -12ºC. 

Avoid saturation, saturation is the point at which the pore structure of the render or mortar is full of water. A drop in temperature to freezing point will see that water expand by 9%, that expansion will break apart the surrounding mortar or render. Special attention must be paid to the correct installation of working rainwater goods. 10mm of rain falling on a 25m2 roof will discharge a quarter of a tonne of water.

A render with an over-worked, closed-in surface is more likely to suffer from frost damage than a more open textured surface. Freezing is a gradual process occurring from the surface of the render and working inwards. The extra passes of a trowel bring water and fine particles to the surface, for this reason we would recommend that the minimum amount of work is done to an external render once it is flat.

Thought should also be given to the storage of materials on site, obviously any undercover/indoor space should be used. Dry materials should be kept off the ground. Wet materials, such as sand should be kept on clean, hard surfaces and covered with weighted down tarpaulins to protect from rain and frost.

Factor in the use of water on site for mixing, cleaning etc. and ensure that if water needs to be stored on site that it is well insulated, do not use water containing any ice. Never add anything to keep water ice free.

Clean down all equipment at the end of the day, do not leave any water standing in mixers etc.

Ensure that all pipes are suitably lagged.

If background heating is to be introduced, propane gas will provide higher humidity as well as a higher level of carbon dioxide. Obvious attention should be paid to the placing of the heaters, not only from a safety/fire risk point of view, but to avoid over heating spots of the render or mortar. Good general circulation of warm air should be the goal.

At Best of Lime all of our Dry Readymix products have an element of air entrainment already included. The use of air entrainment has two main benefits, it reduces the amount of water required to achieve workability, it also increases the air content of the mortar or render to around 14% effectively reducing capillarity and giving the space required for the expansion of the frozen water.

This information is for general guidance only and is not specific to any particular site or building, for more specific advise, please contact Best of Lime Ltd