Installation of PVC Corner Beads

A wall corner takes most of the knocks: shopping bags, bicycles, lawnmowers, ladders. Render or base coat on its own would soon chip off a sharp edge. The bead stiffens the edge and protects it.

The second job is less visible. Two surfaces meet at a corner, and stress builds up in the render there, for example when warm days alternate with frost. A bead with mesh spreads that stress into the surrounding area, so the corner does not crack.

The third reason is appearance. It is very hard to form a long, straight, crisp edge by hand. The bead defines the edge, and the plasterer uses it as a guide.

PVC does not rust and copes well with moisture. That is why it is used on facades and in insulation systems more often than galvanised steel beads, which can leave rust stains over time.

• PVC corner bead with glass fibre mesh has a strip of mesh attached to both wings. It is the usual choice for ETICS (external thermal insulation composite systems). The mesh joins up with the mesh over the whole facade, so the corner becomes part of the reinforcing layer.
• PVC corner bead without mesh has perforated wings only. It suits internal plastering and corners that do not take much wear. It is not suitable for external insulation because it does not reinforce enough.
• Adjustable angle bead is used on corners that are not at a right angle, such as splayed reveals or bay windows.
• Drip edge bead goes on lower edges, for example the head of a window opening or the underside of a balcony. Water drips off it instead of running back along the wall.
• Movement joint profiles are not fitted on every corner. They belong where different materials meet or where the design calls for them, so that movement in the structure does not pass into the render.

With external insulation there is a simple rule: use the profiles listed in the system manufacturer's technical documentation. The system is tested as a whole, and randomly mixed components can weaken it.

The method depends on whether the bead goes into an insulation system or into conventional render or plaster. The principle stays the same: a flat, sound background, accurate fixing and enough time for the materials to cure.

On external wall insulation (ETICS)

1. Make sure the insulation boards are bonded, mechanically fixed and rasped flush at the corner. There should be no steps or gaps along the edge.
2. Apply a layer of adhesive and base coat to the corner, as wide as the wings of the bead.
3. Press the bead into the base coat so the nose of the profile follows the line of the corner exactly.
4. Check each length with a spirit level or straightedge, and on tall corners with a plumb line as well.
5. Trowel the bead's mesh into the base coat with no air pockets or creases.
6. Lap the main facade mesh over the bead's mesh by the amount the system manufacturer specifies, usually around 10 cm.
7. Skim the whole area and let it cure before applying primer and the finishing render.

With conventional render or plaster

On masonry, the bead is usually set on dabs of render or gypsum plaster, lined up and left to set. Only then is the corner rendered. Here the bead acts as a screed guide for ruling off the render on both walls. The depth of the dabs behind the bead should match the render thickness, otherwise you get a bulge or a dip at the corner.

Joining lengths

If a corner is longer than one bead, the lengths are butted together with no gap and no overlap. The mesh at the joint should overlap so the reinforcement is continuous.

How demanding the work is, and so how much it ends up costing, depends mainly on these factors:

• How complex the facade is. A house with a simple footprint has a few long corners. Lots of windows, recessed balconies, bays and reveals mean many short lengths, cuts and joints.
• Height and access. At ground floor level a stepladder or small tower is enough. A taller building needs full-height scaffolding, and work on it is slower.
• Condition of the background. In renovation work it may turn out that the old render on a corner is hollow or damp. It has to be removed and repaired first.
• Type of bead. Mesh beads and drip edge profiles need more precision than a simple internal bead.
• Weather. Cold, rain or strong sun interrupt the work and change how the materials dry.
• Combined work. When beads are fitted as part of a full insulation job, the contractor already has scaffolding and materials on site and the work follows on smoothly.

If you live in a block of flats, the facade is usually a shared part of the building. Changes to it are decided by the owners' association or the building management. Before insulating a whole building, check with your local building authority what the local building regulations require in your case.

• Bead fixed to a dusty or weak background. The base coat does not bond and the bead comes away over time. A hairline crack appears along the edge and lets water in. Absorbent backgrounds such as aerated concrete block should be primed first.
• Crooked corner. A few millimetres out over a long corner is plainly visible once rendered, especially in raking light. Putting it right means breaking it out.
• Broken mesh reinforcement. If the bead's mesh does not tie into the main mesh, a weak spot is left. Cracks usually show there after the first winter.
• Missing diagonal mesh patches at openings. Stress concentrates at the corners of windows and doors. Without diagonal strips of mesh, the typical slanting cracks run out from the corner of the opening.
• Gaps and overlaps at joints. A gap cracks, an overlap leaves a step.
• Rushing. Further coats on uncured base coat can shift the bead. Rain or frost soon after fitting disrupts curing. Manufacturers usually give a working range of roughly +5 °C to +25 °C and recommend protecting fresh surfaces from rain and direct sun.

It pays to inspect the corners before the finishing render goes on, while faults are still easy to fix. Go through these points:

• The corner edge is straight over its full height, checked with a straightedge or plumb line
• Looking along the wall at an angle, there are no waves, bulges or hollows
• The bead does not sound hollow when tapped and does not move when pressed
• Joints between lengths are tight, with no gaps or steps
• The mesh is fully embedded in the base coat and does not stick out or ripple anywhere
• The main mesh overlaps the bead's mesh by a sufficient amount
• Diagonal mesh patches are in place at the corners of windows and doors
• Lower edges above openings have a drip edge profile where the design calls for it