How a Garage Conversion Floor Should Be Damp-Proofed

When you turn a garage into a room, you inherit a floor that was never built to be lived on. Most garage slabs were laid with no damp proof membrane underneath them, because nobody expected the space to be anything more than somewhere to park a car and keep a lawnmower dry. A bit of moisture rising through the concrete simply evaporated, and nobody ever noticed.

Put a carpet, a radiator, a desk and a person in that room and it becomes a very different story. This is a job we finished in Hendon (NW4), where the customer wanted a home office. Damp was rising through the floor because the existing cement — laid by another firm around five years earlier — had gone down without any effective barrier beneath it. Here is exactly how we put it right — partly so you know what a proper job looks like, and partly so you can spot a bad one before it gets covered up and becomes your problem.

Why garage floors go damp when you convert them

A garage floor and a floor inside your house are built to completely different standards. Under a proper habitable floor there is a damp proof membrane — a sealed plastic barrier that stops moisture in the ground travelling up into the building. Under a typical garage slab, there very often isn't one at all, or if there is, it was never joined up to the damp proof course in the walls.

Ground moisture is relentless. It travels up through concrete by capillary action — the same way water climbs up a dry tea towel left in the sink. In an open, draughty, unheated garage that moisture reaches the surface and evaporates away, so the floor just looks a bit dark and cold and nobody thinks about it.

The damp was always there. Converting the garage doesn't cause it — it just stops it escaping.

The moment you seal that floor with vinyl, laminate or carpet, insulate the walls and start heating the room, the moisture has nowhere to go. It collects under the flooring and in the base of the walls. Within a year or two you get lifting floor coverings, black mould in the corners, peeling skirting, a permanent musty smell, and a room nobody wants to sit in. We get called to fix that far more often than we'd like — and by then the fix means pulling up everything that was just paid for.

What we found in Hendon

The customer wanted a home office, which meant this had to become a genuinely habitable room rather than a tidied-up garage. Our survey found three separate problems, and they all had to be solved together:

• Rising damp through the floor. The existing cement was only about five years old, but it had been laid with no effective barrier beneath it — which is why it had already failed.
• Open gaps at the wall-to-floor junction. A direct path for moisture — and, as it turned out, a direct path for something else too.
• Damp in the walls themselves. There was no point creating a perfect floor inside walls that were still wet.

Step 1: The walls come first

This is the part most people get in the wrong order. If you lay a beautiful new floor and then treat the walls, you have just paid for a floor that gets damaged and dirtied by the wall work. Worse, if you never treat the walls at all, damp simply arrives by a different route and you have solved nothing.

So the walls were done first. The old plaster was hacked off back to bare masonry, and a chemical damp proof course was injected into the brickwork. That means drilling a line of holes at set spacing along the wall and injecting a damp proof cream or fluid that soaks into the masonry and forms a continuous water-repellent barrier. It is effectively a new damp proof course, installed into a wall that never had a working one.

Then every void and gap around the base of the walls was filled solid with mortar. That does two jobs. The obvious one is closing off the moisture path. The less obvious one, and the reason we are fussy about it, is rodents. An open gap at the base of a wall is an invitation, and once rats get behind a new floor they will chew straight through insulation board to nest in it. You cannot see it happening and you cannot easily fix it afterwards.

Only once the walls were sealed and re-plastered did we touch the floor.

Step 2: The floor, layer by layer

A garage conversion floor is not one thing, it is a stack of layers, and each one has a job. Get any single layer wrong and the ones above it are wasted money. Here is exactly what went down in Hendon, from the ground up:

The membrane

This is the layer that actually stops the damp, so it is the one worth being obsessive about. A membrane only works if it is continuous — a barrier with a gap in it is not a barrier. Where two sheets meet, the minimum overlap is generally taken as 150mm. We lapped ours by a full metre, roughly seven times the minimum, and dressed the membrane up the walls so the floor and wall protection tie together instead of leaving a seam at the most vulnerable point in the room.

The insulation

Next came rigid PIR insulation board. A converted garage is usually the coldest room in the house — it has an outside wall where the door used to be and a floor sitting directly on the ground — so floor insulation is the difference between a room you use all year and a room you avoid from October onwards.

The detail that matters is that the boards are butted tight together with no gaps. Every gap between boards is a cold bridge: a path for heat to escape and, just as importantly, a cold spot where warm air condenses. Rushed insulation with gaps at the joints can create the very condensation problem you were trying to solve.

The edge strip — the detail nobody photographs

Around the whole perimeter of the room, against every wall, we fitted a foam edge strip. The screed is laid up to it and is never allowed to touch the walls. That makes it what is known as a floating screed, and it does two important things.

First, screed moves. It shrinks as it cures and expands and contracts with temperature. A screed locked hard against four walls has nowhere to go, so it cracks — usually diagonally out of the corners. Second, a screed touching cold external masonry conducts heat straight out of the room. The strip breaks both of those connections. It costs very little and skipping it is one of the most common shortcuts we see.

The mesh — and the one thing worth knowing

Steel reinforcement mesh went in next, and here is the honest bit: the mesh was not necessary. A floor like this would have been perfectly acceptable without it. We put it in because it makes the screed far more resistant to cracking over the years, and we would rather not be back.

But if you look at one photograph in this whole article, make it this one. Notice that the mesh is sitting up on small spacers, lifted clear of the layer beneath it. That is the entire point of reinforcement. Mesh has to sit within the body of the screed to do anything at all. Mesh dropped flat on the membrane and screeded over sits below the stresses it is supposed to resist and is, structurally, decoration.

If you visit site once during your conversion, visit while the mesh is down and the screed is not. Everything that matters is still visible.

The screed

Finally the screed itself — sharp sand and cement, mixed semi-dry, laid and levelled to the line we set around the room. It is heavy, unglamorous work and it is the stage where experience shows, because a screed that is not properly level or properly compacted will tell on you the moment a floor covering goes down on top of it.

Step 3: Curing — the bit everyone rushes

The floor was finished in hot weather, which is a genuine hazard. Screed does not "dry" so much as chemically cure, and it needs moisture present while it does. Screed that dries too quickly in the heat ends up weak and crazed with fine cracks, no matter how well it was laid.

So for seven days we watered it, keeping the surface damp and slowing the drying right down. Then it was left for twenty-one days before the room was used or anything was laid on top.

Most of the cracked floors we get called out to were not laid badly. They were dried badly.

This is the stage where corners get cut, because it is the stage where nothing appears to be happening and everybody wants their room. If a builder tells you the floor will be ready to cover in a couple of days, that on its own tells you what you need to know.

Would it pass building control?

Worth being precise here. Converting a garage into a habitable room — an office, a bedroom, a gym — is a material change of use, and it does come under the Building Regulations. That covers resistance to moisture from the ground, thermal insulation, ventilation, electrics and structure. It is not optional, and it matters later: when you come to sell, a solicitor will ask for the paperwork on a converted garage.

The floor described in this article was built to a standard that would satisfy a building control inspection — a continuous damp proof membrane linked to the wall treatment, insulation to the floor, and a properly cured screed. We would not have built it differently if an inspector had been standing in the doorway.

How to spot a bad garage conversion floor

Almost everything that determines whether this floor lasts is invisible once the screed goes down. That is precisely why shortcuts are so common here — nobody can see them, and the failure arrives long after the builder has been paid. Here is what to ask about, and when:

Should you do it yourself?

An honest answer: parts of it, maybe. Laying a membrane and cutting insulation board is careful, methodical work rather than skilled work, and a competent DIYer can do a decent job of it with time and patience.

The screed is where it gets difficult. Mixing a semi-dry screed to the right consistency and laying it level and properly compacted across a whole room is genuinely a trade skill, and it is unforgiving — you get one attempt, and you find out whether you succeeded when the flooring goes down on top of it. Chemical DPC injection is also worth handing over, because inconsistent spacing leaves gaps in the barrier that are impossible to detect until damp comes through.

The bigger point is that this is a system, not a list of tasks. The membrane, the insulation, the edge strip, the mesh and the curing only work as a set. Get one layer wrong and the rest cannot compensate — and the feedback arrives eighteen months later, under a finished floor, in a room you have already furnished. If you want to read more on where that line sits, we wrote about it in DIY or hire a handyman.

Garage conversions in North West London

We carry out garage conversions across Harrow, Uxbridge and North West, West and Central London, including Hendon and the surrounding NW postcodes. Damp-proofing, floor build-up, insulation, closing up the door, electrics, plastering and finishing — one team, one written quote, and the same standard behind the plaster as in front of it.

If your garage is cold, damp or simply wasted space, we will survey it properly, tell you honestly what it needs, and price it clearly. And if it needs less than you think, we will tell you that too.