Garage conversion floor construction: levels, damp and insulation

The right garage conversion floor build-up depends on the existing slab, moisture risk, available height and the required thermal performance. A typical solution may involve a sound base, damp protection, insulation, a suitable surface layer and a threshold detail, but the specification must be checked against the actual garage.

Measure the existing floor before choosing a system

Record the garage slab level against the house floor, ceiling and external thresholds. A few centimetres can decide whether a raised floor is comfortable or whether excavation, a new slab or a thinner insulated system is needed. Check for cracks, settlement, contamination, low spots and signs of moisture. Do not assume the old garage slab was built to the same standard as the house.

What the build-up has to achieve

Approved Document C and Approved Document L provide England-specific guidance on moisture and energy matters. They are not a substitute for a project design, and Scotland, Wales and Northern Ireland have their own technical regimes.

Thresholds and services are easy to miss

Show every doorway, step, drain and service route on the plan. A floor raised to improve insulation can create a trip at the house door or reduce the clearance below a window. Plumbing may require falls and access points; electrical cables and heating pipes need a coordinated route. If a garage door is being infilled, coordinate the new wall and floor edge rather than treating them as separate jobs.

Questions to put in the specification

• What is the proposed finished floor level and how was it measured?
• What is below the insulation and how is moisture stopped or managed?
• How are wall edges, doors, drains and service penetrations treated?
• What will be inspected before the floor is covered?
• Which approval route and completion record will cover the work?

A clear section drawing is more useful than a quote that only says “insulate and screed”.