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● Internal damp patches and staining
● Renders that have bridged a damp-proof course (DPC)
Damp walls
Modern cement-based renders and ‘plastic’ paints can trap moisture so walls feel cold and are a frequent cause of damp within old buildings, sometimes resulting in structural failure.
● Look for and rectify associated causes of damp such as defective rainwater goods.
● Consider removing inappropriate render.
● Hack off a trial area to ascertain the building’s structural condition.
● Beware of the danger of heavy sheets of render falling away.
● If removal is difficult or damaging to the wall, consider removing only loose sections or areas where damp is most problematic.
● Remove renders that bridge a DPC, causing moisture to rise up the wall.
Structural issues
When evaluating the condition of render, wider structural issues should always be considered. Over time, cement-based renders may have trapped water with the result that the wall behind has decayed or been washed away to such an extent that the render is supporting the building’s structure.
● Always be wary where walls are cracked, lean or bulge.
● When removing cementitious render, only remove small sections at a time.
● Be ready to prop the building structure.
● Take particular care with cob walls. These are particularly prone to collapse as the cob is easily washed away.
● Examine timber-frame structures for decay as each section of render is removed.
● Consult a structural engineer if in any doubt.
Repairs and re-rendering
Wherever possible, original rendering should be retained and any damaged areas patched by carefully building up thin coats that match the traditional materials used. Re-rendering should be undertaken using lime with no added cement.
● Understand why the original render has failed.
● Ensure any underlying damp or structural problems have been rectified.
● Allow the wall to dry before applying render.
● Consult specialist lime suppliers to find the most appropriate materials.
● Remember that rendering is a skilled job so usually best left to professionals.
● Beware of using lime-based materials when frost is imminent as they may fail.
● Keep new work damp with a mist spray to avoid it drying out too quickly.
● Protect new render from direct sun and drying winds with damp hessian.
Limewash
Where a lime render is used, limewash is normally the most appropriate finish as this will allow the wall to breathe. Applying limewash is relatively easy, although potentially messy, and is ideally done on a mild drizzly day when there is no direct sunlight that will dry the wall too quickly.
● Remember that limewash is not durable on non-porous surfaces like cement renders.
● Protect surrounding areas.
● Ensure the render is damped down with water.
● Apply limewash with a brush in thin coats, working it into crevices.
Energy efficiency
If re-rendering, it’s worth considering using an insulating lime render to cut heat loss through the wall, but it’s important to ensure overall breathability. Numerous products are available with insulating additives that include foamed glass beads, hemp and cork. Insulating renders can usually be applied to a maximum thickness of 25mm in one application, but additional layers may be added to build up the desired thickness and improve thermal efficiency as required.
Useful contacts
Chalk down Lime – traditional materials.
Tel: 01580 830092; chalkdownlime.co.uk ecolime – supplier of lime building products. Tel: 01653 648566; ecolime.co.uk
Lime Centre – natural lime materials. Tel: 01962 713636; thelimecentre.co.uk
Lime Green – lime specialist. Tel: 01952 728611; lime-green.co.uk Anglia Lime Company – lime product supplier. Tel: 01787 313974; anglialime.com
mike wye & Associates – natural building specialist. Tel: 01409 281644; mikewye.co.uk Scottish Lime Centre Trust – advice and resources. Tel: 01383 872722; scotlime.org Society for the Protection of Ancient buildings
(SPAB) – information and advice. Tel: 020 7377 1644; spab.org.uk
Ty-mawr – sustainable building materials.
Tel: 01874 611350; lime.org.uk
womersleys – advice and materials. Tel: 01924 400651; womersleys.co.uk