Flooring: Damp-Proofing, Insulation, and Contaminated Ground

When you convert your garage, you'll likely find that its existing floor is robust enough for everyday household activities. However, you'll almost certainly need to enhance it to comply with current Building Regulations, particularly concerning moisture protection (damp-proofing) and heat retention (thermal insulation). You might also choose to adjust its height to seamlessly align with the floor level of your main living areas.

Upgrading a Solid Concrete Floor

If your garage already has a solid concrete base, you can typically utilise this as your foundation. Nevertheless, you absolutely must install a fresh damp-proof membrane (DPM) and add thermal insulation. For many garage conversion projects, liquid damp-proof membranes prove to be a highly effective choice. Always consult product manufacturers for specific recommendations.

• You should install a correctly gauged damp-proof membrane and thermal insulation. You can place these directly onto the existing concrete or over a layer of sand blinding.

• If you opt for a liquid membrane, make certain that the insulation material you select won't react adversely with it. In some instances, you might need to include an additional separation layer.

• For the final floor surface, you can either apply a screed layer – aim for roughly 75mm in thickness, and remember to incorporate reinforcement mesh to guard against cracks – or you can install a timber covering, such as a floating floor. The specific details of this finish will vary depending on the type of insulation you've chosen.

• Critically, do not block any existing air bricks that serve your main house. If your new floor design would obstruct them, you must extend these vents outwards through the new floor structure to ensure continuous airflow to the exterior.

Installing a Suspended Timber Floor

Perhaps the floor level of your main house is considerably higher than that of your garage. In this situation, installing a suspended timber floor, which involves joists creating an accessible void beneath, might be a more sensible and practical approach.

• You must ensure there's a minimum clearance of 150mm between the existing concrete slab on the ground and the lowest part of your new timber joists.

• When installing your timber floor joists, make sure you correctly calculate their size according to their length. You should position them to span the shortest distance from one wall to the opposite one.

• To decrease the required span of the joists and allow for thinner timbers, you might need to construct an intermediate wall, which would require a small footing for support.

• Remember to include a damp-proof course (DPC) on the underside of the timber components to protect against moisture.

• You'll need to fit insulation between the joists; the necessary thickness will depend on the specific product you select.

• Crucially, incorporate air vents beneath the floor to provide proper ventilation for the void. This ensures that air can freely move and circulate from one side of the structure to the other.

Addressing Contaminated Ground

It's possible that the ground beneath your property in certain locations may contain contaminants that emit harmful gases. Should this be the situation, you are required to ventilate these gases away and fit a specialised gas membrane to stop them from penetrating your new living space.

• If you're dealing with land that has general gas contamination, you should consult the comprehensive Building Research Establishment (BRE) and Environment Agency report: BR 414 - Protective measures for housing on gas-contaminated land, published in 2001.

• When specifically addressing the presence of Radon gas, your reference point should be the Building Research Establishment report: BR 211 - Radon: Guidance on protective measures for new dwellings, which was issued in 1999.