Rendering near you

The question to settle before anything is mixed is what the wall is made of. A solid wall built in soft brick or rubble stone and bedded in lime was designed to absorb some water and give it back to the air. Put a dense cement render over that and the wall loses its route out. Water still gets in, through a crack, a failed gutter, an open head or the top of the wall, and then stays, showing up as damp indoors and eventually pushing the render off in sheets because the bond has failed from behind.

A permeable lime render does the opposite: it is weaker than the masonry, it moves a little without cracking through, and it lets the wall dry outward. That is why it is the usual specification on older and historic buildings, and why the finish generally weathers rather than failing all at once. On a modern cavity wall with a proper cavity and a damp-proof course, cement-based and thin-coat systems are entirely appropriate, and the breathability argument matters much less.

Details decide the rest. Render needs to stop clear of the ground so it does not bridge the damp-proof course, which the Planning Portal describes as a layer installed to prevent rising or penetrating damp by preventing moisture entering the walls. It needs a bell cast or a drip at its base and over openings so water is thrown clear rather than run back onto the wall, and it needs beads and stop ends at reveals and edges. Most render failures that look like material failures are actually edge details that were never formed.

The first job is mapping, not mixing. A competent renderer taps the whole elevation to find the blown areas, looks for the reason the coat failed, and checks whether the wall is wet from something else entirely, such as a defective gutter, a bridged damp-proof course or a failed parapet. Rendering over an undiagnosed damp problem is the most expensive mistake available on this job, because the new coat hides it for a couple of winters and then fails in the same places.

Patch repair suits a sound wall with a local failure. The blown area is cut back to firmly bonded material with a clean edge, the substrate is prepared and keyed, and the patch is built up in coats to the same thickness and finish as the original. Matching a textured finish is the hard part: pebbledash and roughcast are matched by aggregate size, colour and the way the material is thrown, and an honest contractor will tell you a patch in the middle of a front elevation may always be visible, which is why patching is often done on rear and side walls and a whole elevation is renewed on the front.

A full strip is the answer where the coat has failed generally, where an inappropriate cement render is trapping damp in an old wall, or where the substrate needs attention across the elevation. The old material is hacked off, the wall is repaired and allowed to dry, and the new system is built up in coats with beads, bell casts and stop ends formed properly. Curing then governs the programme, since lime and cement renders both need protecting from direct sun, wind, frost and driving rain while they gain strength. Hacking off is also the dusty part of the job: HSE names silica as a major constituent of construction materials such as bricks, tiles, concrete and mortar and warns that regularly breathing construction dust can cause lung cancer, asthma, chronic obstructive pulmonary disease and silicosis, so the work should be controlled at source rather than simply endured.

Repair is generally free of planning control. The Planning Portal's position on external walls is that you do not need to apply for planning permission for repairs, maintenance or minor improvements, and that cladding may be carried out without applying for planning permission provided the materials are of a similar appearance to those used in the construction of the house. Re-rendering a rendered house therefore rarely raises a planning question; rendering a brick house that has never been rendered can, because the appearance of the building changes.

Designated land removes that latitude. The same guidance is that if you live in a Conservation Area, a National Park, an Area of Outstanding Natural Beauty or the Broads, you will need to apply for planning permission before cladding the outside of your house with stone, artificial stone, pebble dash, render, timber, plastic or tiles, with pebbledash and render named explicitly. Listed buildings need consent regardless: the Planning Portal's position is that a listed building needs listed building consent for any significant works whether internal or external, and national planning practice guidance is that works affecting a listed building's special architectural or historic interest require consent and that carrying them out without it is a criminal offence.

Building regulations arrive with the scale of the work. The building regulations guidance for external walls is that where 25 per cent or more of an external wall is re-rendered, re-clad, re-plastered or re-lined internally, or where 25 per cent or more of the external leaf of a wall is rebuilt, the regulations would normally apply and the thermal insulation would normally have to be improved, with Approved Document L the England guidance on the energy performance of new and existing buildings, published in a volume for dwellings and a volume for other buildings. In Scotland, the list of work not requiring a building warrant includes replacing flooring, lining, covering or rendering with the same general type to a standard no worse than at present, but specifically excludes the application of an external wall cladding system consisting of thermal insulating material. Northern Ireland works to its own technical booklets.

Reading a crack saves money. Fine crazing spread evenly over a surface is usually shrinkage in the finish and is a cosmetic matter. A crack that follows a straight line, steps diagonally, appears over an opening or repeats at the same point each year is structural or thermal movement being expressed through a rigid coat, and filling it without addressing the movement simply reopens it next season. Cracks at the junction of two different materials, or where an extension meets the original house, are movement joints that were never formed.

Blown render is a bond failure, and it does not reattach. Water that has got in behind the coat, through a crack, an unprotected top edge or a bridged damp-proof course, breaks the bond from the back, and frost then lifts it further. This is why a wall can look sound and sound hollow, and why tapping an elevation tells you more about the state of a render than looking at it does.

The complaint that surprises people most is damp appearing after a wall was rendered. On a solid wall, an impermeable coat and an impermeable masonry paint stop the wall drying outwards, so moisture that gets in by any route accumulates and shows up inside. Approved Document C is the England guidance on the resistance to moisture of floors, walls and roofs with examples of damp proofing and ventilation provisions, and any specification worth accepting should be able to say how the wall is expected to keep dry and how it is expected to dry out.