Application of fibreglass mesh and render to external walls (double coat)

The work is done in two coats. The first coat of adhesive holds the mesh. The second covers it and levels the surface. Depending on the system, the total thickness is usually around 3 to 5 mm.

1. Check the substrate: the insulation boards must be firmly fixed and must not flex under pressure. Loose boards are re-bonded or extra anchors are added, and steps between boards are sanded flat.
2. Fit the profiles: corner beads with mesh, window connection profiles and the starter or plinth track. These give straight edges and protect the areas under the most stress.
3. Add diagonal patches: strips of mesh, roughly 30 × 20 cm, are bedded at 45° at the corners of windows and doors. They take up the stress that would otherwise cause diagonal cracks running out from the corners of openings.
4. Apply the first coat of adhesive: with a notched trowel, from top to bottom, in small sections, so the adhesive does not dry before the mesh goes in.
5. Embed the mesh: vertical strips with an overlap of at least 10 cm are pressed in with a trowel, working from the centre outwards, without folds or air pockets. The mesh must be fully covered and should sit closer to the outer surface, not directly on the insulation.
6. Let it dry and apply the second coat: a thin coat with a smooth trowel, in long strokes with a slight overlap between passes. At around 20 °C there is usually about a day between coats, longer in cold or damp weather. Some systems allow a "wet on wet" method. The manufacturer's technical data sheet decides.

The reinforcement layer must dry completely before the final render goes on. This usually takes several days, depending on the weather and the manufacturer's instructions. After that, the primer for the render is applied.

Façade mesh must be alkali-resistant. Cement-based adhesive is alkaline, and ordinary interior mesh slowly breaks down in it. You cannot see this from outside, but after a few years the layer has lost its reinforcement. For normal areas, mesh of roughly 145 to 160 g/m² is used. On the plinth and anywhere at risk of impact, a heavier armour mesh is added.

Adhesives differ mainly in what they are made of:

• Polymer-modified cement base coats are the most common, with good adhesion and some flexibility
• Cement adhesives are strong but less flexible and suit mineral substrates better
• Dispersion (ready-mixed paste) base coats are easy to apply and need no mixing with water, and some systems use them mainly for smaller areas

The key rule: mesh, adhesive, profiles and render should all come from one certified external wall insulation system (ETICS). The system is tested as a whole. If you mix components from different manufacturers, no one stands behind the whole build-up and the system warranty is usually lost.

The simplest case is a large, flat wall with no openings and good access. The following make the job slower and more expensive:

• Many windows, doors and corners, because each one needs profiles and patches
• A complex façade with bays, recesses or window surrounds
• An uneven or loose substrate that has to be repaired first
• Height and access, meaning scaffolding, moving it around and tight space next to a neighbour's property
• Poor weather, when work stops or the façade has to be covered with sheeting
• A small area, where setting up and cleaning take a large share of the time

If the reinforcement is part of insulating a whole house or block of flats, check with your local building authority beforehand. Ask which rules apply to your case, because external insulation can affect the look of the building, fire safety and energy requirements. In a block of flats, decisions about the façade are made by the building management or the owners' association.

A new façade nearly always looks fine at first. Problems usually appear after the first frosts and heavy rain. Typical mistakes include:

• Too little adhesive under the mesh, so the mesh does not bond in places and bulges form
• Small or missing overlaps, leaving no reinforcement at the joint, which is exactly where a crack appears
• No diagonal patches at window corners, leading to diagonal cracks
• Mesh pressed right onto the insulation, or the opposite, mesh showing through the surface
• Extra water added to the adhesive, which reduces strength and adhesion
• A second coat on a damp first coat, where trapped moisture later causes blisters
• Working in direct sun, rain or below +5 °C, when the adhesive cannot cure properly

Why does the weather matter so much? On a hot wall, the water in the adhesive evaporates before it can set, and the layer stays weak. In the cold, setting almost stops, and a night frost can damage it. That is why work is usually done at roughly +5 to +25 °C, ideally in the shade and without strong wind. Sheeting on the scaffold protects the fresh layer.

It is worth checking before the render goes on, because faults are much harder to fix afterwards. At handover, look for the following:

• The mesh is not visible anywhere and cannot be felt under the surface
• The surface is flat, and a straightedge of at least 2 m shows no deviations beyond what the manufacturer allows for the chosen render
• There are no air pockets, bulges, wavy mesh or trowel marks
• Every window and door corner has diagonal patches, ideally recorded in photos taken during the work
• Corners are straight and sharp, with corner beads fitted
• Window connection profiles are complete, undamaged and properly joined to the frames
• The plinth is reinforced with armour mesh
• The layer is dry, firm and free of cracks
• The contractor can tell you which ETICS system was used and hands over the technical data sheets for the materials