How do I make good the plaster around new windows and doors?

Window and door replacement almost always damages the plaster around the opening, because the old frame was plastered up to and the new one is a different size. What is left is a ragged band round the opening with the reveal exposed, and making it good well comes down to three things: getting a straight edge at the frame, getting the reveal square, and treating the frame junction as a movement joint.

What you'll need

• Plaster stop bead or angle bead
• Bonding coat and finish plaster, or plasterboard and adhesive
• Scrim tape
• A straightedge and a square
• Flexible sealant for the frame joint
• Filling knife, trowel and hawk
• Cable detector

Step by step

1. Agree who is making good before the windows are fitted.Some installers include making good, some include only a basic fill, and some exclude it entirely. It is a common source of dispute, and the time to establish it is at the quotation stage.
2. Cut the damaged plaster back to a straight line.Score a line parallel to the frame in sound plaster and cut back to it. A straight edge is far easier to plaster up to and looks deliberate; a ragged edge shows through the finished work.
3. Check the reveal is square and consistent.Measure the reveal depth at several points. New frames are often set at a slightly different depth from the old, so the reveal may be deeper on one side or vary from top to bottom. Decide on the finished plane before you apply anything.
4. Fit beads at the arrises.An angle bead at the external corner of the reveal gives a straight arris that survives being knocked. A plaster stop bead where the plaster meets the frame gives a clean edge and a defined thickness, and stops plaster running onto the frame.
5. Bed the beads accurately.In dabs of bonding coat, checked with a level and a straightedge in both directions. The beads define the finished surface, so anything wrong here is wrong in the finished job.
6. Decide between plastering the reveal and boarding it.A shallow reveal is usually plastered — bonding coat then skim. A deep reveal, or one that has to be built out significantly, is often quicker and straighter boarded, with plasterboard bonded or fixed to the masonry and then skimmed.
7. Insulate the reveal if you are boarding it.The reveal is a cold bridge and it is where condensation and mould appear first. Insulated plasterboard, or a thin insulation layer behind the board, addresses that at almost no extra cost while the reveal is open.
8. Scrim every junction between new and old.Where the new plaster meets the existing wall, and around the bead edges. This is where the crack appears otherwise.
9. Plaster the reveal and the face together.Backing coat ruled to the beads, then finish plaster taken from the reveal, round the arris and out onto the face of the wall, feathered well past the repair.
10. Leave the frame junction to a flexible sealant.Do not fill the gap between plaster and frame with plaster or with rigid filler. Frames move with temperature and the building moves around them, so that joint needs a low-modulus sealant applied neatly after decoration or ready for painting.
11. Let it dry, mist coat, then decorate.New plaster around an opening dries quickly on the face and more slowly where it is thick. Wait for uniform colour before painting, and mist coat before any full coat.

Tips

• Take a photograph of the reveal before it is closed up, showing any cavity closer, insulation and the frame fixings. It is useful if a leak appears later.
• The reveal is the first place mould appears in a cold house, so if the wall is uninsulated this is a cheap opportunity to insulate a small but important area.
• Where a window board is being replaced too, fit it before plastering the reveals so the plaster can be finished down to it neatly.
• If the installer left expanding foam proud of the frame, cut it back flush before plastering. Plaster over foam has nothing to bond to and cracks along the line.

Common mistakes

• Filling the frame joint with plaster or hard filler — That joint moves seasonally. A rigid fill cracks, and the crack then runs along the whole frame and looks worse than the gap did.
• Plastering without beads — Getting a straight arris and a clean stop at the frame freehand is a skilled operation. Beads make it geometry instead of skill and cost very little.
• Not scrimming the joint to the existing wall — New plaster against old cracks along the join. Tape it.
• Plastering over proud expanding foam — Foam is compressible and has no bond. Cut it back to below the finished plaster line first.

Questions people ask