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Loft Floors

Loft floor construction: joists, insulation and boarding.

Ceiling Joists

Existing loft joists are designed only to support ceiling boards, so a structurally sound floor is needed.

Ceiling joists consist of two narrow, low grade timbers often bolted together (sistered) above a supporting wall. It's not acceptable to reinforce these, however they are fitted to the new structure using steel straps and this will stabilise the ceilings below.

The video shows original joists made unlevel as the house settled either side of the supporting wall.

Maximising Head Height

A new floor structure is designed by calculating the minimum floor thickness needed to maximised headroom. Techniques used to preserve internal height include:

  • interlaying new joists alongside the old
  • notching small sections of the noggin struts around existing timbers
  • using smaller but closer together joists

Our architectural surveyor draws floor plans with the best technical (and economic) option for each loft and our experience provides us with a considerable range of options.

The video shows the ceiling joists fitting into the new timber structure installed around them. Notches are cut into the struts to minimise encroachment, and metal straps support and integrate the existing ceiling joists.

Floor Insulation

Floor insulation provides 3 benefits: thermal insulation for energy efficiency, acoustic protection to reduce noise transfer, and fire protection for half hour fire resistance (see videos below).

How to insulate between new floor joists in a loft conversion using 100 mm rockwool fibre insulation and chicken wire.

Wire reinforced mineral wool is installed to Building Regulations specified depth. Floor is boarded - tongue & groove moisture resistant chipboard flooring.

Glass Fibre Loft Insulation Rolls

Glass fibre loft insulation rolls

How we Install a New Loft Floor

A new floor system is installed using a frame (trimmer joists) to support the floor joists that span the width of the loft.

A basic floor system is installed in about 10 stages from preparation to boarding the floor.

  1. Clearing the loft area of all debris, insulation, struts and temporary boards to expose the ceiling boards
  2. Trimmer Joists form the periphery of the new floor. The strong timbers (approx 225mm x 75mm timber) are secured to the wall plates (brickwork) and bolted to the walls (party walls and around chimney breasts) to form a stable edge to the frame. They must be precisely level and at a height that allows the new joists to fit above or around the ceiling joists. Around the stairwell and below partitions the joists are doubled up. Metal joist hangers are fixed to these depending on the floor plan at approximately 20mm above ceiling to allow for deflection.
  3. New floor joists are measured and trimmed to size, then dropped into the joist hangers and secured in position.
  4. Spacer noggins (solid struts) are installed between joists to reduce joist deflection, vibration, improve floor rigidity and performance.
  5. Ceiling joists are tied to the floor joists with steel straps to stabilise the ceiling.
  6. Wire chicken mesh is installed as cradles for the rockwool so, should fire destroy the ceiling below the insulation remains in place and provides fire protection (see video below).
  7. Under floor insulation (100mm mineral wool) is installed tightly into the cradle.
  8. Electricians 'first fix' - for laying of electric power and other cables in place from consumer unit up to the ceiling according to the building plans. Positioning cables for sockets, light fittings, control units, shower point, window controls, Internet, phone, TV cable or communications.
  9. Plumbers 'first fix' that identifies radiator positioning and existing flow pipes and the position pipes through floor for radiator supply. Position and fit inflow and outflows e.g. for en suite shower and sanitary ware.
  10. Buildings inspection. Building regulations standards have the option to inspect work at this stage to ensure the loft is structurally sound and building work is faithful to plans Approved plans.
  11. Floorboards are laid over the joists. Moisture resistant, tongue and groove chipboard flooring is installed as a base flooring material. Carpets or hardwood flooring requires good quality underlay to reduce impact noise to the rooms below.
Loft Insulation Wire Cradles

Loft insulation wire cradles

Loft Boarding and Building Regulations

At Boarshurst we don't offer boarding only services. But for those that do, any structural work to reinforce joists etc. can cause an upgrade in the loft to "habitable room" status. I.e. a room used, or intended to be used, for dwelling purposes including a kitchen (but excluding bathrooms) where it is large enough to stand up and lie down in.

4 Bed Examples

Many 4 bed houses have large lofts needing reinforcement to stabilise the floor and transfer loads.

  • An older houses with a central supporting wall may have a reinforced steel joists (RSJs) installed a metre or so from the eaves on dwarf walls, in what is usually the knee-wall area. The new floor structure is then hung off these, sunk to the height of the existing ceiling.
  • A modern house that lacks the supporting wall may need the roof structure replacing with A-shaped attic trusses where the floor is integral to the truss. These truss conversions usually requires planning permission, which we obtain routinely, to raise the height of the roof.

Contact Us

Contact us to discuss your ideas on 0800 505 3414 and receive a free, no obligation fixed price quote. We convert lofts in Greater Manchester and we provide an Architectural Planning Service throughout the whole of the UK.

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