What is a substrate?
A substrate is the surface a layer is applied to: the wall, floor or board under paint, adhesive, coating, plaster, tiles or sealant. Manufacturers use the word throughout their instructions, alongside “background” or “base”. On green roofs it has a second meaning: there the substrate is the growing medium the plants root into.
Whether a finish lasts depends at least as much on the substrate as on the product. Even the best coating will fail on a dusty, damp or crumbly surface.
In practice
- Assess the surface. It must be sound, clean, dry and free of grease and dust. Scratch it with a nail or screwdriver: anything sandy or crumbling must be removed or consolidated first.
- Test the suction. Flick some water on. If it soaks in at once, the surface is highly absorbent and needs a primer or sealer. If it beads, the surface is dense or glossy and needs a bonding primer or keying.
- Test old coatings. Press strong tape firmly onto the old paint and rip it off. If paint comes away, that layer must go. The standard method is the cross-cut test (ISO 2409).
- Measure moisture. New screeds, plaster and concrete must be dry enough. The permitted level is in the instructions for the product going on top.
- Match the primer to substrate and finish. Follow the maker of the top coat, who specifies which primer suits which surface.
- Remove anything loose. Flaking paint, wallpaper residue, old adhesive and efflorescence come off first.
Green roofs: when saturated, the substrate is the heaviest part of the build-up. An extensive sedum roof has a thin layer, an intensive roof garden a much deeper one. Always have a structural engineer check the roof can take the load.
Common mistake: painting or tiling a surface that looks dry but is not. Moisture in the substrate pushes the finish off later.
Related terms
- Adhesion
- Primer
- Load-bearing capacity
- Construction moisture
- Green roof
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.