What good looks like
Ground floor and slab: what good looks like
Beam and block or ground-bearing slab, the damp-proof membrane, insulation and the perimeter details that carry the thermal line.
The ground floor is where the damp-proof line and the insulation line meet, and where both are easiest to get wrong. The membrane must be continuous, lapped and sealed, and taken up the perimeter to lap the wall DPC so there is no gap for moisture to track through. Insulation is laid tight, with boards butted and any gaps filled rather than left for the screed to bridge. Beam and block floors need the void ventilated where the design calls for it, with airbricks and ducts clear of debris. Service penetrations are sealed as they are made, not at the end of the stage when access has gone.
Basis in regulation
Part C, clause 4
Floors: resistance to moisture from the ground
Read the sourceChecked against the live source on 20 August 2026
Key requirements
Garage floor design considerations
Garage floors must be designed to safely transfer all imposed loads to the ground or supporting structure without excessive movement. Key considerations include the ground's bearing capacity, resistance to moisture and hazardous gases, appropriate thickness, and the provision of construction joints
Garage floors must be designed to safely transfer all imposed loads to the ground or supporting structure without excessive movement. Key considerations include the ground's bearing capacity, resistance to moisture and hazardous gases, appropriate thickness, and the provision of construction joints and drainage falls.
Basis in regulation
BS 8103-1
Read the sourceBS 8204-1
Read the source
Key requirements: ground floor and slab
The checks that matter before the floor is covered.
Damp-proof membrane continuous, lapped at joints and sealed to the wall damp-proof course. Sub-floor void kept clear of spoil, offcuts and mortar droppings. Ventilation openings to suspended floors provided at the designed rate and left unobstructed. Insulation boards laid tight with gaps filled, and perimeter upstands in place before screeding. Penetrations through the membrane sealed with a proprietary collar rather than tape alone.
Basis in regulation
Part C, clause 4.4
Ventilation of suspended floors
Read the sourceChecked against the live source on 20 August 2026
Cavity protection at ground floor level
It is critical to maintain the integrity of the external wall cavity below ground floor level to prevent damp penetration and thermal bridging. When using beam and block flooring, the cavity should not be bridged by the beam ends; a damp proof course (DPC) must be correctly installed beneath the end
It is critical to maintain the integrity of the external wall cavity below ground floor level to prevent damp penetration and thermal bridging. When using beam and block flooring, the cavity should not be bridged by the beam ends; a damp proof course (DPC) must be correctly installed beneath the ends of any beams entering the external wall.
Basis in regulation
Construction joints in large garage floors
For garage floor slabs exceeding 16m² in area, either reinforcement must be incorporated within the concrete, or construction joints must be provided. These joints should divide the floor area into bays, each measuring less than 16m², to control cracking caused by drying shrinkage and thermal moveme
For garage floor slabs exceeding 16m² in area, either reinforcement must be incorporated within the concrete, or construction joints must be provided. These joints should divide the floor area into bays, each measuring less than 16m², to control cracking caused by drying shrinkage and thermal movement.
Basis in regulation
BS 8103-1
Read the sourceBS 8204-1
Read the source
Hazardous gas protection for integral/attached garages
Where hazardous gas protection is required for a property, the gas membrane system must extend continuously into and across any integral or attached garage, encapsulating the entire footprint. The membrane must be adequately protected from physical damage to maintain its integrity.
Where hazardous gas protection is required for a property, the gas membrane system must extend continuously into and across any integral or attached garage, encapsulating the entire footprint. The membrane must be adequately protected from physical damage to maintain its integrity.
Basis in regulation
Garage floor drainage falls
Garage floors should ideally be laid with a fall to direct any water or spillages towards the vehicle doorway. This prevents water pooling inside the garage and facilitates easy cleaning.
Garage floors should ideally be laid with a fall to direct any water or spillages towards the vehicle doorway. This prevents water pooling inside the garage and facilitates easy cleaning.
Basis in regulation
BS 8204-1
Read the source
Engineered structural screeds
Reinforced screeds intended to act as structural toppings must be specifically designed by a qualified engineer. Their design should account for structural performance requirements and interaction with the underlying slab.
Reinforced screeds intended to act as structural toppings must be specifically designed by a qualified engineer. Their design should account for structural performance requirements and interaction with the underlying slab.
Basis in regulation
BS 8204-1
Read the sourceBS EN 1992-1-1
Read the source
Critical dimensions at this stage
Original editorial guidance based on publicly available Approved Documents. Not building control approval, a warranty determination, or a substitute for the full source document.