Microcement - M·04
Pools & exteriors.
Swimming pools, terraces and façade features in frost-stable, UV-stable exterior-grade systems. A different specification from the one that goes indoors and not interchangeable with it.
Outside, a finish has to survive three things an interior never faces: ultraviolet light, freeze-thaw cycling and substrate movement across a twenty-degree temperature swing between a July afternoon and a January night. Exterior microcement is formulated for all three, with a more elastic seal and a higher polymer content than anything we would put in a bathroom. Using an interior system outdoors is a guaranteed failure within two winters and it happens often enough that we are regularly called to look at it.
Pools
A microcement pool is a genuinely different object from a tiled one. There is no grid, no grout and no dark joint lines under the water, so the colour of the shell is the colour of the water - which is why the deep greys and greens have become so popular. It can go over an existing tiled or rendered shell, which makes it a very effective way to modernise a dated pool without excavation.
The waterproofing rule is the same as indoors and even less negotiable: the shell is sealed and tested before any finish is applied and the microcement is a continuous, sealed, chemically-resistant surface on top of it. Pool water is chlorinated or salted and both are aggressive, so the seal specification is chosen for the water treatment in use, not by default.
Terraces and pool surrounds
The obvious pairing with a seamless interior floor: the same colour and material running from the kitchen through the sliding doors and out onto the terrace, with no threshold. Externally it is always specified with an anti-slip additive and laid to falls so water leaves rather than sits - standing water and freezing temperatures are what destroy exterior finishes.
Façades and features
Over sound render or blockwork, exterior microcement gives a monolithic, seamless elevation with none of the joint lines of board cladding. It is well suited to garden walls, outbuildings, planters and pool houses. On a full house elevation we would want to talk about vapour behaviour and the wall build-up first - some walls want to breathe and a polymer-rich exterior system is not the right answer everywhere. On a solid or listed wall we would specify a lime system instead and say why.
Weather and programme
Application is temperature and humidity dependent. We do not apply below about five degrees, in direct strong sun on a hot day, or with rain forecast inside the cure window. That makes exterior work seasonal in the UK and it is worth booking well ahead rather than hoping for a fortnight in October.
Questions we get asked.
Can microcement be used outside in the UK climate?
Exterior-grade systems can and they are formulated for it - UV-stable pigments, freeze-thaw resistance and a more elastic seal. Interior microcement used outdoors will fail within about two winters and we are called to look at that often enough to say it plainly. The two systems are not interchangeable.
Can you apply microcement to an existing swimming pool?
Yes, over a sound tiled or rendered shell, which makes it an effective way to modernise a dated pool without excavation. The shell is sealed and tested first and the seal specification is chosen for the water treatment in use, because chlorinated and salt-treated water are chemically different problems.
Is an exterior microcement terrace slippery?
Not as specified. Anti-slip additive in the final seal is standard on all our exterior work rather than optional and the surface is laid to falls so water leaves rather than sits. Standing water plus freezing temperatures is what destroys exterior finishes.
When can exterior work be carried out?
It is weather-dependent. We do not apply below roughly five degrees, in strong direct sun on a hot day, or with rain forecast inside the cure window. That makes exterior work seasonal in the UK and worth booking well ahead rather than hoping for a dry fortnight in October.