External Renders

Render goes on in layers, and each layer needs its own drying time. A typical job runs like this:

1. Assess the wall: how solid it is, any cracks, loose old render, how flat it is and how damp the masonry is.
2. Put up scaffolding and cover windows, sills, paving and anything else that must stay clean.
3. Clean the surface of dust, old paint, salts, algae and loose material. Repair damaged areas or remove render that no longer holds.
4. On insulated walls, build the reinforcing layer: base coat adhesive, glass fibre mesh pressed into it, then a smooth finish.
5. Apply a primer that evens out how much the wall absorbs and helps the render grip. It must be the primer intended for that specific type of render.
6. Apply the top coat evenly, by hand with a float or by spray machine, and create the texture.
7. Protect the fresh render from rain, direct sun and wind until it has dried.

Why so many steps? Render is only as good as the surface underneath it. If the primer is skipped or the render goes onto a damp wall, the fault often only shows after a year or two, when it is much harder to claim under warranty.

Silicone render

It is flexible, absorbs little water, and dirt struggles to stick because rain washes it off. It copes well with changes in temperature, so it is often used on new builds and on external wall insulation systems (often called ETICS or EWI). It also suits houses next to busy roads or in exposed, windy spots.

Silicate render

Its binder is based on potassium water glass, and it lets water vapour pass through easily. This means the wall can release moisture to the outside. That is why it is used on older houses, brick and stone walls and other mineral surfaces. It is less forgiving to apply: the manufacturer's instructions need to be followed closely, otherwise patchy areas appear easily.

Acrylic render

It comes in a wide range of colours and is easy to apply. It lets less vapour through, so it does not belong on damp masonry or on breathable mineral wall build-ups, where it would trap moisture. It works well on dry, flat surfaces, for example when refurbishing concrete panel blocks of flats.

Mineral and lime render

These are based on lime or cement, they do not burn and they breathe well. They are the usual choice for old houses, listed or historic buildings and stone walls. This group also includes lime and lime-cement finishing coats, which give a fine, traditional-looking surface. Mineral renders need an experienced plasterer and enough time for each layer to cure. They are often finished with a masonry paint afterwards.

Thin coat or thick coat?

Thin coat renders are light and flexible, so they are almost always used on insulation systems. Traditional thick coat renders suit old and historic buildings, where uneven masonry needs levelling out and a solid, hand-crafted look should be kept.

Textured finishes, such as a scraped or grained finish, hide small unevenness and dirt better, which is why they are popular on family homes. A smooth finish looks modern, but it shows every mark and is harder to keep looking good.

Colour is not only a matter of taste. Dark shades heat up more in the sun, so the facade expands and contracts more and the render ages faster. On insulated walls, manufacturers often restrict dark colours, so check that the system allows the shade you have chosen.

The least work is a flat, solid, dry wall on a new build or a newly insulated wall. The job takes more effort when:

• The house has many windows, bays, cornices and decorative details
• The old render is peeling and has to be hacked off and the masonry repaired
• The walls are damp or salt-affected and the cause of the damp has to be dealt with first
• The building is tall and needs extensive scaffolding
• The chosen texture is fine and demanding to achieve
• The weather is unsettled and work keeps stopping

If you live in a flat, the facade of the building is decided by the building management or the owners' association, not by individual owners. Whether a change to the facade or external insulation needs approval depends on local building regulations, so check with your local building authority before work starts. Older or listed buildings and conservation areas often have extra rules. If the scaffolding will stand on a pavement or public land, you will usually also need permission from the local council.

• Rendering onto a damp, dusty or crumbling surface
• Skipping the primer or using the wrong one
• Working in direct sun, in wind or at low temperatures
• A coat that is too thin or uneven
• Doing one wall in sections with breaks, which leaves visible joins
• Mixing materials from different systems, for example acrylic render on damp mineral masonry
• Poorly finished details at sills, window reveals and where the wall meets the roof

In practice, problems are often caused not by the material itself but by a procedure that was not followed. That is why it is wise to use primer, base coat and render from one manufacturer and within one system.

When you take over the work, look at the facade in daylight and also from the side, when the light falls at an angle. That is when you see the most.

• Colour and texture are even, with no patches or visible joins
• There are no cracks, blisters or peeling areas
• Tapping the wall nowhere gives a hollow sound
• Corners, reveals and edges are straight and clean
• Joints to sills, windows and flashings are properly sealed
• Windows, paving and surroundings are cleaned of render splashes
• You have a record of the materials used and ideally photos of each layer
• The agreed warranty is stated in writing