Extensions

FOUNDATIONS
Excavations must be taken down into firm, natural soil and the trench must be inspected by a Building Control Surveyor before placing concrete.

FLOORS
Solid Concrete Floor – remove vegetation and topsoil, lay hardcore with sand blinding over, 1200g polythene DPM, 100mm concrete, and insulation under a min 65mm screed (with chicken wire). Alternatively, the insulation may be under the slab. Lap DPM to DPC (Figure A). (View Diagram) Suspended Timber Floor - 150mm air gap between oversite concrete and joists, with insulation between or on top of joists. Sleeper walls may be needed dependant on joist size with gaps for air flow. Ventilation to be provided via air bricks every 2m (1500mm²/m), ducted to external air (Figure B). (View Diagram)

WALLS
Two skins of brick or dense concrete block below ground with horizontal DPC not less than 150mm above finished ground level. Above DPC, cavity construction may be of brick and lightweight block with stainless steel wall ties, with full or partial cavity insulation (partial fill requires a 50mm clear residual cavity). New and existing brick and block work to be properly bonded or tied and cavities maintained and kept 150mm below the DPC. Windows and external doors to be double-glazed to current standards and in area should not exceed 25 % of the floor area of the extension. Lateral restraint straps (30 x 5mm galvanised steel) required at 2m centres. Cavity tray required where the new roof joins the cavity wall of the existing, and vertical DPC’s at door and window positions.

STRUCTURAL STABILITY
Openings generally should not exceed 3m in width and a minimum of 550mm brick/block work is required at corners and suitable size piers provided where necessary. Suitable lintels are required over openings and calculations may be required for steel beams etc.

ROOM VENTILATION
Openable window area not less than 1/20th floor area of room, plus background ventilation e.g. trickle ventilation of 8000mm². Provide mechanical ventilation to bathrooms, kitchens toilets and utility rooms and background ventilation of 4000mm².

ROOFS
Pitched Roof - rafters and ceiling joists to suit spans. Insulation and ventilation as for flat roof; if mono-pitched, provide additional ventilation at high level, i.e. tile vents or air bricks (Figure C). (View Diagram) Flat Roof - Typically 12.5mm chippings on 3 layers roofing felt on 19mm or 22mm roofing grade boarding to a fall of 1:60 on roof joists (size and spacing depend on span). Holding-down straps (30 x 5mm galvanised steel) required to prevent wind uplift. Insulation is required with a 50mm air gap between the insulation and the underside of the roof boarding. Roof void to be cross-ventilated by 25mm continuous gap on two or more sides. Alternatively a warm deck roof does not need ventilation. (View Diagram)

DRAINAGE
Roof water drainage to discharge to an existing surface water drain (not to the foul), or to a soakaway at least 5m from any building, if the ground is suitable. Any toilets, baths, wash basins, sinks etc must connect to a foul drain.

INSULATION
Wall, floor and roof insulation to meet current standards.

MEANS OF ESCAPE
Escape windows will be required at first floor level or if you have to pass through another room to get out of the extension. Contact a Building Control Surveyor for advice before ordering your windows. For large extensions, mains-operated smoke alarms may be needed.

SAFETY GLAZING
Safety glazing to be used where necessary.

ELECTRICS
The electrical installation should be carried out by a competent person belonging to a Government scheme, or it will be necessary for you to pay us an additional fee. Please refer to our leaflet on ‘Electrical Safety’ for further information.

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EXTENSIONS