Guide

Cracked floors

A crack in a floor is a symptom. Before you cover it with anything, it is worth knowing what caused it, because a coating hides a crack but does not stop the movement behind it. This guide is honest about where microcement helps, and about when the right next step is not a product at all.

Why floors crack

• —Shrinkage. Concrete and screed shrink as they dry. Fine, random hairline cracks in the first months are common, and usually stop once the slab has dried out.
• —Temperature. Slabs expand and contract with heat. Large floors without movement joints, and floors in the sun, crack along the lines where that stress collects.
• —Settlement. If the ground under a slab compacts, washes out or was poorly prepared, part of the floor drops and cracks.
• —Too much load. A slab that is too thin, or not reinforced for what stands on it, cracks under the weight.
• —Water. A leaking pipe or water under the slab softens the ground beneath it, and sections can sink or lift.
• —Structure. A building that is moving — foundations, beams, columns — shows it as cracks in the floors and walls.
• —Tiles. A crack running across tiles often follows a crack in the slab underneath. Tiles that crack and lift together usually mean the bed they were laid on has failed.

Two kinds of crack

For any coating, what matters is whether the crack is still moving.

How to tell whether a crack is moving

• —Mark both ends of the crack with a pencil line and the date, and measure its width at one or two points.
• —Check again after a few weeks, and after a change of season if you can. A crack that has grown past your marks is moving.
• —Look for a step. If one side of the crack is higher than the other, the slab has moved.
• —Look around it. Doors or windows that have started to stick, cracks in the walls above, or a floor that has begun to slope all point to movement in the structure.
• —A crack monitor fixed across the crack shows movement more precisely than pencil marks.

What microcement can cover, and what it cannot

Microcement is 1–2 mm thick. It gives an old floor a new, continuous surface, and it can hide the repaired hairline cracks and grout lines that made the floor look tired. It is not a structural repair: it does not hold a slab together, fill a void under it, or stop a building moving.

Can cover

• —Fine, stable shrinkage cracks, once they are filled and the floor is prepared.
• —Old, stable cracks in a tiled floor whose tiles are still firmly bonded.

Cannot fix

• —Cracks that are still opening, growing, or stepping.
• —Cracks caused by settlement, water under the slab or structural movement.
• —Hollow, loose or lifting tiles over a failed bed.
• —A floor that has sunk or tilted.

Terraesta does not sell a crack filler or a repair mortar. A stable crack should be opened up, cleaned and filled with a suitable repair product by your contractor before Surface Primer goes on.

Where Fibermesh helps

Fibermesh is a fiberglass reinforcement mesh embedded in the base coat of microcement. It gives the thin layers strength while keeping them flexible, and it is strongly recommended for every floor and for large areas. Over an old floor with repaired hairline cracks and grout lines, it helps the new surface stay stable.

What Fibermesh cannot do is hold a moving crack closed. The mesh reinforces the coating, not the slab. If the crack below is still moving, the surface above it will crack again, mesh or no mesh.

When to call a structural engineer instead

Do not buy a coating — from us or from anyone — until a structural engineer has looked at the floor, if you see any of these:

• —A crack wider than a few millimetres, or one that is getting wider.
• —One side of a crack higher than the other.
• —Cracks that run from the floor into the walls, or diagonal cracks in walls, especially from the corners of doors and windows.
• —Doors and windows that have started to stick or no longer close.
• —A floor that has sunk, tilted, or sounds hollow over a large area.
• —Water coming up through a crack, or a crack that appeared after a leak or a flood.
• —Cracks near columns or beams, or where a heavy load stands.

An engineer can tell you what is causing the movement and how to stop it. Once it has stopped and the floor has been repaired, it can be a sound base again — and that is the point at which microcement makes sense.

Once the floor is sound

The system is the same as for any floor: Surface Primer, Fibermesh, two coats of Microcement Extreme, then a top coat. Replace your floor without demolition goes through it in order, and the product calculator works out how much you need.