Stiffened Skin Loft Floor

[Michael Morgan]

I have a 1970s bungalow with 4x2 ceiling joists @ 400mm centres, a central corridor hallway running the length of the house and max ceiling spans of 3m. The loft is accessed via a loft ladder in the hallway and leads to 3 'loft rooms' (each measuring approx 3x3m) which look like they were put up in the 70s with 2x4 framed, wood panelled walls and textured plasterboard ceilings. There's a single window in the gable end.

The plan is to rip out the existing rooms, insulate properly (currently has broken up polystyrene (not asbestos)), add a couple of velux windows, refloor, frame and board out walls and ceiling. Won't be a bedroom as there's no room for fixed stairs, so planning to use as a home office and for storing guitars / bit of a music room.

Structural engineer I spoke to suggested gluing (tight bond?) and screwing 18mm ply to the top of the joists to create a 'stiffened skin' floor. He reckoned this would be the best and cheapest way to do it without the expense of adding independent floor joists. Is this a common method for stiffening loft floors? It seems very difficult to remove once it's been carried out, e.g. to access wiring. Has anyone else seen this done / done this themselves?

Thanks!
Mike

 

[foxhole]

You could add 2x2 (screwed and glued)on top of existing allowing deeper insulation. Are you sure existing room timbers are not part of roof support?

 

[Michael Morgan]

Yes, roof timbers / bracing is embedded into the wall structures. By rip out the rooms I meant remove the wall clashing but leave the structure intact and just reclad.

Do you think an extra 2" would be a big benefit? Is the gluing of the floor to the joists a common approach then?

Thanks

 

[catlad]

As Foxall, has mentioned, those room panels are holding your roof up.

 

[Deleted member 174758]

Structural engineer I spoke to suggested gluing (tight bond?) and screwing 18mm ply to the top of the joists to create a 'stiffened skin' floor. He reckoned this would be the best and cheapest way to do it without the expense of adding independent floor joists. Is this a common method for stiffening loft floors? It seems very difficult to remove once it's been carried out, e.g. to access wiring. Has anyone else seen this done / done this themselves?

The method is sometimes referred to as a stressed skin and it is exactly how you are recommended to install T&G chipboard flooring and T&G spruce ply flooring these days, although using a flooring PU glue (which contains a filler and can handle small surface voids which PVA/"aliphatic resin" can't) is actually a far better choice than an overhyped PVA such as Titebond (which is American and has no UK ratings for structural work AFAIK). One big difficulty you will experience is in getting the flooring material up into the loft - 8 x 4 ft sheets tend not to go through loft hatches, so for plywood you'd need to be able to rip down the 8 x 4s to 2ft wide consistently and then rout grooves in the edges to take loose , glued tongues (which can be narrow rips of 6mm plywood). With screws alone and without the glue you just won't get the stiffness you desire. Adding 44mm siftwood on top of the existing joists by glueing and screwing is one way to get more insulation in there, but just how often do people have to access wiring, I wonder? I've been under my attic floors exactly twice in the last 20 years, both times recently. It is always possible to cut removable hatches over junction boxes or lights if you think you need them