I want to install a wooden loft ladder and install new “raised” floor joists in the loft space of a detached house. The existing roof is of a trussed construction at 400mm centres (32mm x 95mm ceiling joists support the first floor ceiling). Whilst I am not seeking to design a habitable room in the loft (because of cost & the fire regs compliance issues), I do want to do the job properly.

I have purchased some new joists (47mm x95mm C16) to attached to the existing trusses to elevate the loft floor which I propose to fit 2400 x 600 x18mm T&G flooring. I wanted to do this to maintain the 270mm loft insulation and to not apply any additional pressure onto the first flooring ceilings.

I am only proposing to lift and board the centre of the loft space (2.5M x 12M) which has a 3.4m headspace, so pretty useful for storage and perhaps a project room.

My question is whether it’s better (stronger) to attach the new joists to each of the existing joists in parallel, or at right angles to them forming a new subframe?. Either way I want to raise the floor by 95mm to accommodate the existing insulation with an air gap below the new T&G flooring to avoid any condensation?.

Any advice is gratefully received.

  • Malcolm Phillips

    Hi There

    Are your joists in need of strengthening to enable use of the loft? If not why not just put down layers (to sufficient depth) of a building regs approved insulation board (kingspan, polyurethane foam) (the first layer should be at 90° to the trusses and subsequent layers at 90°to the last to avoid heat leaks through the joints) and then finishing off with a batens to afix the flor boards to or skin the upper surface with a load spreading impenetrable flooring material, assuming you can get large sheets of materials into the loft?.

  • Martin Stewart

    Thanks for the response Malcolm,

    I already have sufficient loft insulation at 270mm throughout the loft so didn’t want to buy anymore & I think that whilst Kingspan is very efficient thermally for the depth it’s also quite expensive. Because I don’t want to risk any damage to the existing ceiling through the extra weight in the loft, my idea was to suspend a new floor off the existing trusses which are space at 400mm centres across the roof.

    I will only insulate the central area with lightweight ‘double bubble silver foil insulation) & I will board the 30 SqM area out with large sheets of 18mm T&G as you suggest to create a sort of storage room within the cold loft (this will allow air circulation around all of the roof rafters to avoid any condensation risks). The ridge height will be 3.4M above the new flooring so I can insulate with foil at 2,5M to create the air gap above. Also the sides will have cold loft area all aroud (it’s a large detached house).

    I need some knowledgeable expert advice because I am not at all sure if I need building control approval for this project (ie, a non-inhabitable room <50 cubic M), or the best way to suspend the new floor joists off the existing truss rafters to effectively spread the weight.

    To better explain….. my idea is to use new wooden joists was because I was skeptical about using loft legs (because of point pressure from the new floor & its effect on the existing ceiling) and I don’t want to use a metal joist system as it’s more expensive than wood & carries the condensate/heat leak risk.

    Thanks, Martin

  • Post a comment
    You must be logged in to comment. Log in