Preparation is most of the job. The wall is checked for soundness and suction, failed material is cut out, coatings that would block adhesion are removed, everything fixed to the wall comes off, beads are set to establish the thickness and the drip details, and the substrate is primed or stabilised to suit what it is made of. Render applied over skipped preparation fails at the bond, not at the surface.
- Suction is the thing most often untested and most often fatal. A thirsty wall pulls water out of the basecoat and destroys the bond.
- Every meter box, flue, bracket, alarm bell and outside tap has to come off and go back at the new wall thickness.
- Beads are not trim. They set the thickness, protect the arrises and throw water clear at the base.
- Airbricks must be sleeved through, never rendered over — they are ventilating the floor void for a reason.
Why preparation is the job
Render fails at the bond far more often than it fails as a material. Modern render systems are well made; the common failures are debonding, cracking at a badly detailed edge and water getting in behind. All three are decided before any render is mixed.
This is also the part of the work that is invisible once the job is finished, which is precisely why it is the part that gets compressed when someone is working to a thin price. You cannot tell by looking at a finished wall whether the substrate was tested for suction or whether the beads were set properly. You find out in about three winters.
Step one: establish what the wall is
Solid brick, cavity brick, concrete block, no-fines concrete, stone, or a previous render over any of them. Each takes a different preparation and in some cases a different system entirely.
A solid wall needs a vapour-permeable build-up so it can still dry through its surface. A concrete panel wall may need a mechanically fixed mesh rather than relying on adhesion alone. A wall that has already been rendered twice may be at the thickness limit at the window reveals.
This is also when the damp-proof course is located and the ground levels checked. If the render is going to have to stop above a DPC that is currently buried under a raised path, lowering that path is part of the preparation, not an optional extra.
Step two: remove what cannot stay
- Debonded render — the whole elevation is tapped and every hollow area cut back to a sound, undercut edge. This is where the scope of a job is really decided.
- Masonry paint and textured coatings — many will not accept render, and some are actively impermeable. Whether they come off entirely or can be bonded over depends on what they are and how well attached they are.
- Organic growth — algae, moss and lichen are removed and the surface treated, because render will not bond reliably to a living surface.
- Loose and friable material — perished mortar, spalled brick faces and dusty surfaces are cut out and made good first.
- Old fixings — redundant brackets, rawl plugs, nails and cable clips all come out. Anything ferrous left in place will eventually rust and stain through a new finish.
Step three: test the suction
This is the step most often skipped and the one most likely to cause an expensive failure.
Masonry absorbs water at wildly different rates. A dense engineering brick barely takes any; an old soft red brick or an aerated block drinks it. Render contains water that the cement needs in order to hydrate and gain strength. If the wall pulls that water out too quickly, the basecoat never properly cures at the interface and the bond is weak from day one — even though the surface looks perfect.
The test is simple: flick water onto the wall in several places and watch. If it vanishes almost instantly, suction is high. If it beads and runs, suction is low and the problem is the opposite — poor adhesion to a sealed or water-repellent surface.
High suction is controlled with a primer or by damping the wall down before application. Low suction, or a mixed wall where some areas drink and others do not, is controlled with a bonding primer or a stipple coat. Mixed suction across one elevation is common where brickwork has been patched, and it is a genuine cause of patchy colour in the finished render.
Step four: everything fixed to the wall comes off
Rendering adds thickness — anywhere from around 8mm for a thin-coat system to 20mm or more for monocouche, and 100mm or beyond for insulated render. Everything attached to the wall has to be removed and refitted to suit.
| Item | Why it matters | Who deals with it |
|---|---|---|
| Gas and electricity meter boxes | Must be reset flush at the new thickness, not buried | Usually the renderer; the supplier for the meter itself |
| Boiler flue terminals | Need extending to project correctly past the new face | Gas Safe registered engineer, not the renderer |
| Soil and waste pipes | Refixed on longer brackets with a clear gap behind | Renderer or plumber depending on the pipework |
| Outside taps | Extended through; a buried tap is a leak nobody can reach | Plumber |
| Alarm bells, lights, cameras | Removed, cable sleeved, refitted on packers | Renderer, with the electrician for anything wired |
| Satellite dishes and aerials | Removed and refitted; brackets usually need extending | Renderer or an aerial fitter |
| Airbricks and vents | Sleeved through, never covered | Renderer — and this one is not negotiable |
The boiler flue is the item worth flagging early. It needs a Gas Safe registered engineer, it cannot be done by the rendering team, and if it is left until the scaffold is up it will hold the whole job.
Step five: airbricks, and why they matter
An airbrick ventilates the void under a suspended timber floor. That airflow is what keeps the joists dry. Render over it and you have not just blocked a hole — you have stopped the ventilation to the structural timber under the floor.
The result is slow and expensive: damp, then rot in the joist ends, then a floor that has to come up. It can take years to appear, by which time nobody connects it to the rendering job.
Every airbrick and vent is located before work starts and sleeved through the new render with a proprietary extension, so the airflow is maintained at the new thickness. If someone does not mention airbricks when they survey your wall, ask about them.
Step six: beads
Beads are the skeleton of a render job and they are a reliable indicator of whether the work was done properly.
Stop beads give a clean, protected edge where the render finishes — at a window reveal, at the top of a wall, at a boundary. Without them, a render edge chips and lets water in behind.
Corner (angle) beads keep external corners straight and resist knocks. Rendered arrises without beads look passable for a year and then chip.
Bellcast or drip beads sit at the base and at the head of openings and throw water clear of the face below. This is the detail whose absence causes damp at low level along an entire elevation, and it is the one most frequently missing on older render.
Movement joints are set out on large elevations so thermal expansion is accommodated at a planned line instead of a random crack.
Beads also set the thickness. They are fixed and levelled first, and the render is worked to them, which is how an even thickness is achieved across a wall that is not flat.
Step seven: protection and timing
Scaffold goes up with sheeting where the weather needs managing. Fresh render has to be protected from driving rain, which washes fines out and streaks the colour, and from direct hot sun, which dries it too fast and causes crazing.
Cement-based materials should not be applied when the substrate is frozen or when temperatures are heading below about 5°C. Frost in uncured render destroys the bond, and the damage is not visible until later.
Windows, doors, paving and planting are masked and protected. Render splashes are much easier to prevent than to remove, particularly from glass and from block paving.
What preparation does to the price
Preparation is the most variable part of a rendering quote, because it is the part that depends on what is actually found. For context, these are the rendering figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Rendering | £40–£70 per m² | System & access dependent |
| Three-bed semi | £2,500–£5,000 | Full elevation render |
| Detached house | £9,000–£12,000 | Larger area & scaffolding |
A sound, clean wall with few fixings sits at the lower end. A wall needing render removal, brick repair, level changes at the base and a lot of refitting sits higher. We tap the walls and survey the fixings before quoting, so the figure holds.
We tap the elevation, test suction, locate the airbricks and list every fixing before pricing. No surprises once the scaffold is up.
Frequently asked questions
Does render need a primer?
It depends entirely on the suction of the wall. A high-suction substrate needs a primer or damping down so it does not pull the water out of the basecoat before it cures. A low-suction or previously coated surface needs a bonding primer to give the render something to grip. Testing which you have takes a minute and skipping it is a common cause of debonding.
Can you render over painted brickwork?
Sometimes, but it has to be assessed rather than assumed. A sound, well-keyed mineral paint can often be bonded over with the right primer. Thick plastic masonry paints and textured coatings usually have to come off, both because render will not reliably bond to them and because they are impermeable, which matters a great deal on a solid wall.
Do airbricks have to be kept when rendering?
Yes, always. They ventilate the void under a suspended timber floor and rendering over them leads to damp and eventually rot in the joist ends. Proprietary sleeves extend them through the new render thickness. This is one of the clearest tests of whether a renderer is doing the job properly.
How long does preparation take?
On a straightforward wall, a day or two of the overall programme. On a property needing render removal, brick repair, coating removal and a lot of fixings dealt with, it can be the largest single part of the job. It is also where unexpected work appears, which is why we price it after tapping the walls rather than from a photograph.
What happens to my boiler flue?
It has to be extended so it still projects correctly past the new wall face, and that is work for a Gas Safe registered engineer rather than the rendering team. It is worth arranging early: discovering it once the scaffold is up and the wall is prepared is a common cause of a job stalling for a week.
Can I prepare the walls myself to save money?
You can usefully clear access, move planting, take down your own satellite dish and remove non-structural fixings. What is not worth doing yourself is cutting out render, dealing with coatings or anything involving gas, electricity or working at height. If the preparation is wrong the renderer still owns the failure, so most will want to do it themselves.
Preparation is the part you cannot see afterwards
We survey the substrate, test the suction and list every fixing and airbrick before pricing, so the quote reflects the wall rather than an average one. Free survey, fixed written quote.