Installation of External APU Bead with Mesh

The name comes from the German word Anputzleiste, meaning a bead you render up to. In the UK you may also hear it called a window connection bead or reveal bead. It has several parts. A self-adhesive strip fixes it to the frame. A plastic profile sits on that strip. A glass-fibre mesh is embedded in the base coat, and there is often a tear-off protective lip with a film that keeps the window clean while the render goes on.

Why go to all this trouble? A window frame expands and contracts with temperature differently from render. If render were taken straight onto the frame, it would crack at the joint with the first frosts. The APU bead absorbs that movement and also seals the joint against driving rain.

It is fitted around windows, balcony doors, entrance doors and other fixed elements where render meets a frame. External building corners and reveal edges need a different profile: a corner bead with mesh. The two are often mixed up in practice, but they do different jobs.

The bead goes on after the insulation boards are bonded and, if needed, mechanically fixed, but before the full reinforcing layer. The insulation system manufacturer's instructions always take priority. In broad terms the job looks like this:

• Preparing the frame. The protective film is removed from the frame where the bead will stick, and the frame is cleaned of dust and grease. It must be dry. The adhesive strip will not hold on a dirty or damp frame.
• Preparing the insulation. The insulation next to the frame should be flat and cut tight. Gaps are filled with expanding foam or slivers of insulation, not with adhesive.
• Cutting the bead. The bead is cut to the exact length of each side of the opening. At the corners the pieces either butt up neatly or overlap, as the manufacturer specifies, so that no hole is left.
• Sticking it on. The backing tape is peeled off and the bead is pressed onto the frame along its full length, with no waves and no air pockets.
• Embedding the mesh. Base coat is spread on the insulation and the bead's mesh is pressed into it with a trowel, working outwards from the profile. The mesh must not stick out or wrinkle anywhere.
• Joining the facade reinforcement. The bead's mesh is overlapped by the full-area reinforcing mesh, usually by about 10 cm. Diagonal strips of mesh are also added at the corners of openings to prevent diagonal cracks.
• Finishing. Once the base coat has cured, primer and the final render follow. Only then is the protective lip with its film snapped off.

APU beads differ mainly in the thickness of render build-up they are made for. The profile has to match the combined thickness of base coat and finish in your system. If it is too shallow, the render overhangs it. If it is too deep, you are left with a visible step by the frame.

A simple profile is enough for ordinary windows. For large glazed areas, sliding doors or dark frames that heat up more in the sun, a more flexible bead is chosen, one that takes up movement in several directions. It makes sense to buy the bead from the same system as the base coat and mesh. The system is tested as a whole, and mixing products from different suppliers can cost you the warranty.

The job is simplest with new windows that are clean, with straight frames and insulation that has been cut accurately. Things that add time and cost include:

• Many small windows and openings, which means more corners per metre of bead
• Old or uneven frames that need cleaning and straightening first
• Retrofitting to a finished facade, where the render by the frame has to be cut out and then patched and colour-matched
• Working from scaffolding or a platform on upper floors
• Poor weather that holds up the programme

If you are insulating a block of flats, the decision lies with the building management or the owners' association. For a house, check with your local building authority before insulating whether your plans need approval, especially if the work changes the look of the facade or the building is in a conservation area. Do not simply assume nothing is required.

Most mistakes do not show straight away but only after the first winter. These are the ones seen most often in practice:

• Bead stuck to a dirty or damp frame. The strip peels off over time and a gap opens up.
• Wrong profile depth. The render either overhangs or leaves a step at the frame.
• Short mesh overlap. A few centimetres is not enough, and a crack appears at the joint.
• Mesh pressed into base coat that is already setting. It does not bond and later separates. If the base coat has skinned over, apply a fresh layer.
• The flexible part of the bead covered with rigid base coat. The bead can no longer move, and its whole purpose is lost.
• Bead on only part of the perimeter. For example, left off the bottom by the sill. Every spot without a profile is a future crack.
• Working in unsuitable weather. Manufacturers usually specify a minimum of roughly 5 °C, no direct sun on a hot wall and no rain while the coats cure.

If you are repairing the facade around a single ground-floor window that you can reach comfortably from the ground, you can do this yourself. On upper floors and for a full insulation job, leave the work to a contractor.

1. Find out which insulation system is on the facade and buy bead, base coat and mesh from the same system.
2. Remove the film from the frame, then clean, degrease and let the frame dry.
3. Check that the insulation next to the frame is flat and has no gaps.
4. Measure each side of the opening and cut the bead to the exact length.
5. Peel off the backing tape and press the bead firmly onto the frame along its full length.
6. Spread base coat on the insulation and press the bead's mesh into it.
7. Overlap the bead's mesh with reinforcing mesh by about 10 cm and smooth it in.
8. Add diagonal strips of mesh at the corners of the opening.
9. Once cured, apply primer and render.
10. Snap off the bead's protective lip with its film.

It pays to inspect the beads before the final render goes on, while any faults can still be put right quickly.

• The bead runs around the whole opening, including the bottom
• The profile is stuck to the frame along its full length and does not lift anywhere
• The corners are closed with no gaps
• The mesh is fully embedded in base coat and does not stick out
• The overlap onto the reinforcing mesh is sufficient
• The flexible part of the bead is not covered with base coat
• The render meets the profile smoothly without a step
• The window frame is clean and undamaged after the protective lip is removed