Insulating and levelling a garage floor usually means laying a damp-proof membrane over the existing concrete slab, lapping it with the damp-proof course in the walls, adding rigid insulation, then finishing with a screed or a timber floor to bring the level up towards the rest of the house. LABC guidance gives a floor U-value target of 0.18 W/m²K. We check the slab, the step to the house and the headroom first, because they decide which build-up suits your garage.
Why does a garage floor need so much work?
Garage floors are usually uninsulated concrete laid straight onto hardcore. They were built for a car, not for people sitting or sleeping on them, so they are cold and can draw moisture from the ground. They are often lower than the house floor, and some slope towards the door so water can drain out. None of that is acceptable for a habitable room, which is why the floor is one of the first items when converting a garage in Liverpool.
Done well, you should not be able to tell the new floor from the rest of the house. Done badly, it shows up as a cold room, damp skirting boards, mould along the base of the walls and a trip hazard at the doorway.
What goes into a converted garage floor?
There are two common approaches. Both start with the existing slab being cleaned, checked and, where needed, levelled out.
| Screed floor | Timber (floating or battened) floor | |
|---|---|---|
| Build-up | Damp-proof membrane, rigid insulation, separating layer, sand and cement or flowing screed | Damp-proof membrane, rigid insulation, moisture-resistant chipboard or a battened deck |
| Feel | Solid, like a modern ground floor | Slightly softer underfoot |
| Drying time | Needs time to dry before flooring goes down | Can be finished quickly |
| Underfloor heating | Well suited to wet systems laid in the screed | Possible with suitable systems |
| Depth | Usually needs more depth overall | Can be slimmer, useful where headroom is tight |
The right choice depends on the size of the step to the house, how much headroom you have, how the room will be used and whether you want underfloor heating. A gym with heavy kit, for example, needs a floor with good compressive strength, which we cover on our garage to home gym page.
Linking the membrane to the damp-proof course
This is the single most important detail. LABC guidance says the new floor membrane must lap with the damp-proof course in the existing walls. If it stops short, moisture can bypass it and rise into the base of the walls, which is the classic damp line seen in poorly converted garages. Many garage walls were built with a DPC, but some single-skin walls have one at a level that does not suit the new floor, and the infill wall in the old door opening needs its own DPC at the right height. We plan these levels together so the membrane, the DPCs and the new floor all work as one barrier.
The infill and walls are covered on our garage door infill and garage wall insulation pages. The floor is not finished until those junctions are right.
How much insulation does the floor need?
LABC's garage conversion guidance gives a U-value of 0.18 W/m²K for floors as part of Part L. In practice that usually means a substantial layer of rigid insulation board, plus an upstand of insulation around the edge so heat does not escape at the junction with the walls. The exact thickness depends on the product and on the floor's size and shape, and building control confirms what is required for the job. Edge insulation is easy to forget and is one reason some converted rooms have cold strips along the skirting.
Dealing with the step to the house
Garage floors are often set down from the house floor, particularly in integral garages, because guidance has long asked for a fall or step between a garage and the house to stop fuel vapour or spills reaching the living space. After conversion the room is part of the house, so the reason for the step has gone. The insulation and screed or deck take up much of the height, and in many garages the floor can be brought level or close to it.
Where it cannot be made fully level without losing too much headroom, we plan one small, well-placed step rather than a sloping floor. We also check the height of the escape window, if one is needed, from the finished floor level, since raising the floor changes it.
Things we check before designing the floor
- Condition of the slabCracks, sinking corners or a broken edge at the door opening can point to settlement that needs looking at first.
- Air bricks to the houseIf the house has a suspended timber ground floor, air bricks that once vented into the garage must keep venting to fresh air, usually through ducts or sleeves under the new floor.
- Drains and gulliesA floor gully, manhole or drain run under the garage has to be dealt with properly, and any new or altered drainage falls under Part H.
- Inspection pitsOlder garages sometimes have a pit, which is normally filled and compacted in a way agreed with building control.
- ServicesPipes for heating, water or waste that will run in or under the new floor need planning before it is laid.
- HeadroomEvery layer raises the floor. We measure the height left after the floor and roof are upgraded.
Breaking up or digging out a concrete garage floor, or altering drains, is not a DIY job. It can affect foundations, drainage and the damp-proofing of the whole house, and it is inspected by building control.
Underfloor heating in a garage floor
A converted garage is often the furthest room from the boiler and has more external wall than most rooms, so heating needs thought. Underfloor heating frees up wall space and suits a screed floor well. Wet systems can be connected to the existing heating if the boiler has capacity; electric systems are another option. Any gas work must be done by a Gas Safe registered engineer and electrical work by a registered electrician. See plumbing and heating for conversions for more.
Floors across Liverpool and Merseyside
The garage floors we see vary with the age of the house. Integral garages on the 1980s and 1990s estates around Croxteth Park and Widnes often have a clear step down from the hall. Older side garages on inter-war semis in places like West Derby can have thinner slabs and edges that need more attention. Whatever the house, the floor is planned as part of a full garage conversion, alongside the walls, infill and roof.
Want a warm, level floor in your converted garage?
Tell us about the garage and the step to the house, and we will call you back to talk through the floor options. Call 07777 659786 or use the form.
Frequently asked questions
Why is my garage floor lower than the house?
Many garages, especially integral ones, were built with the floor set down from the house. Guidance for garages attached to houses has long asked for a fall or step so that fuel vapour or spills from a car do not reach the house. The size of the step varies from house to house, so we measure it on the visit rather than assume.
Can the converted floor be level with the rest of the house?
Often it can be brought level or very close, because the insulation and screed or timber deck take up much of the difference. Whether it ends up exactly level depends on the size of the step, the build-up needed to meet the insulation target and the headroom available. Where it cannot be fully level, we plan a single small step or threshold in the safest place.
How much insulation does a garage conversion floor need?
LABC's garage conversion guidance gives a U-value of 0.18 W/m²K for floors. The thickness of insulation needed to reach it depends on the product used and the size and shape of the floor. Building control confirms what applies to your job, and the specification is shown on the drawings.
Can I have underfloor heating in a converted garage?
Yes, in many cases. Wet underfloor heating can be laid in a screed and connected to the house system if the boiler has capacity, or electric mats can be used under some floor finishes. It needs planning into the floor build-up from the start. Our plumbing and heating page covers the options.
What happens to a drain or inspection pit in the garage floor?
Some garages have a floor gully, a drain cover or, in older garages, an inspection pit. A manhole that must stay accessible is usually kept with a sealed, recessed cover. A pit is normally filled and compacted in a way agreed with building control. Any change to drainage falls under Part H, so we raise it at the visit.
What goes wrong with badly converted garage floors?
The most common faults are damp at the base of the walls where the floor membrane was never linked to the damp-proof course, a floor with too little insulation that feels cold underfoot, and a step down into the room that was never dealt with. Our garage conversion problems guide explains how these are found and put right.