Concrete cracks. This is not a statement of pessimism but of physics. The shrinkage stresses that develop as concrete cures, the thermal expansion and contraction that occurs with temperature cycling, and the structural deflections that result from loading and settlement all generate tensile forces that concrete — a material with excellent compressive strength but limited tensile capacity — cannot fully resist. Cracks form, and they continue to move with the ongoing thermal and structural cycles of the building.
A cementitious floor coating applied directly over a cracked or crack-prone substrate will, without reinforcement, reflect those cracks through to the finished surface. This phenomenon — known as reflective cracking — is the most common failure mode in unreinforced cementitious floor systems, and it is addressed through the incorporation of alkali-resistant fibreglass mesh in the coating build-up.
How Mesh Reinforcement Works
The operating principle of fibreglass mesh reinforcement is straightforward: the mesh distributes concentrated tensile forces across a wider area, reducing the stress intensity at any single point below the threshold at which the coating matrix will fracture.
When a crack in the substrate attempts to propagate through the coating, it encounters the mesh layer. The fibreglass strands bridging the crack absorb the tensile force and redistribute it laterally along the mesh network. Instead of a single crack propagating to the surface, the energy is dissipated across many strands over a wide area, and the surface remains intact.
The mesh does not prevent the substrate crack from existing or from continuing to move. It prevents the crack from becoming visible at the finished surface — which, from both an aesthetic and a functional standpoint, is the outcome that matters.
Material Selection
The mesh used in cementitious coating systems must be specifically rated for alkaline environments. Standard fibreglass mesh — the type used in EIFS (External Insulation and Finish Systems) and plastering applications — will degrade rapidly when embedded in a fresh cementitious matrix, because the alkaline pore water of the cement attacks the glass fibres. Alkali-resistant (AR) fibreglass, manufactured with a minimum of sixteen percent zirconia content in the glass composition, resists this attack and maintains its tensile strength throughout the service life of the coating.
The mesh is characterised by its weight (typically 60 to 160 grams per square metre), its aperture size (4mm to 10mm), and its tensile strength (typically 1,500 to 2,500 N per 50mm strip). For cementitious floor coatings applied at thicknesses of two to four millimetres, a mesh weight of 75 to 100 g/m² with a 5mm aperture provides an effective balance of reinforcement capacity and embedment compatibility.
The mesh is invisible in the finished floor. Its contribution is silent but structural — like the reinforcing steel in a concrete beam, it works without being seen.
Installation Protocol
The mesh is embedded during the primer coat application — the first wet layer applied to the prepared substrate. The primer is applied at a heavier-than-normal coverage rate, the mesh is laid into the wet primer, and additional primer is applied over the mesh to fully saturate and encapsulate the fibreglass strands. The mesh must be free of wrinkles, bubbles, and dry spots; any area where the mesh is not fully embedded in the primer will create a weak point in the reinforcement system.
At joints between adjacent mesh sheets, an overlap of at least fifty millimetres is required to ensure continuity of the reinforcement across the full floor area. The overlap joints should be staggered rather than aligned, to avoid creating a continuous plane of double-thickness mesh that could telegraph through the thin finished surface.
When Mesh Is Required
Mesh reinforcement is standard practice in all floor applications of cementitious coatings — not because every substrate will crack, but because the consequences of reflective cracking are commercially unacceptable and the cost of mesh is trivial relative to the total system cost.
On wall applications, mesh is generally omitted. Vertical surfaces do not bear the mechanical loads that can initiate cracking, and the substrates for wall coatings — typically plaster or fibre-cement board — are less prone to the movement patterns that generate reflective cracks in concrete floor slabs.
The decision to include mesh is not a judgment about substrate quality — it is a risk management measure that acknowledges the reality of how buildings behave over time. Substrates that appear stable and crack-free at the time of coating application may develop cracks months or years later, as the building settles, as temperature cycles accumulate, and as the structure's equilibrium shifts. The mesh anticipates this eventuality and ensures that the floor surface remains intact regardless of what happens beneath it.