Raise loft floor for insulation

[myknees]

Advice and help please!

Year of house construction is 1993. Trussed fink rafters ('W' strutting) at 605 centres. 6m span. Joist depth is only 70mm. 70mm rockwool insulation between.

The water tank is built up in the middle of the span on 200mm deep timbers resting on the joists. There are stud walls in the 2 rooms below presumable supporting this weight. 1 block wall below stud at ground floor.

Want to insulate to total of 270mm with rock wool and board over about 2/3rds of floor as a storage area.

Propose 200x50mm rough sawn timbers at right angles to trusses, screw fixed with brackets. Because the trusses divide the floor into 3 areas I intend to space the 200x50 timbers 1160mm apart, 2 200x50 lengths in each 3rd of the truss length. On top of the 200x50 there will be 50x50 cross battens @ 605 or 403 centres, spanning 1160mm with small cantilever each end. This is to allow a 50mm air gap and support a floor.

The idea is that the load on the 200x50 timber is close to the load bearing support below (outside walls and stud) away from the middle of the span.

Does this seem feasible? Will my roof collapse?
Is 50x50mm section suitable to span 1160mm with 400mm cantilevering each end? Will 605mm centres be suitable to span 1160mm and support floor load?

 

[chirpychippy]

Your trusses should have a white ticket on them probably in the ridge at the very top of the truss. On this ticket will be the company that made the trusses. You could always contact this company and ask them if what you are proposing would be viable.

My personal opinion is that it is not viable, I think the additional load will if nothing else damage all your ceilings under the trusses as the additional weight of all the additional timber alone will cause the trusses to move, not to the mention whatever you intend to put on the flooring once laid. When centering timber, always work on the principle of either 400c/c or 600c/c if its structural then 400c/c applies. If I were you I would talk to the Truss company, failing that get a reputable builder in that can advise you on your project.

 

[Deluks]

Get some 8x2, doubled up*, 6m long and sit them on the loadbearing walls between the trusses, then build your floor off of this. Get a test section first to make sure there is enough angle in the eaves to accomodate the 200mm depth of joist where it sits on the loadbearing wall. There will be some deflection in these, so packing under them at each end is advisable otherwise they will deflect onto your ceiling plasterboard.

Counterbatten the tops with 2x2 for rigidity and load spread, and that will give you a storage area.


*Getting a 6m length in the roof could be tricky unless you can either

a) remove some tiles and feed them in from outside.

or

b) cut them into a 4m/2m bolted to a 2m/4m. So the joists will be doubled up with staggered joins.

 

[myknees]

Thank you all for the excellent advice.

I had a look for the label on the trusses but it is not there. I phoned a truss manufacturer and spoke to a guy there but did not really get any answers from him .

I agree with Churpy Chippy that there would simply be too much weight using 200x50 plus 50x50 battens as I originally proposed. It now also looks doubtful that the trusses actual sit on the stud walls below. I saw another item on this forum where it was suggested that the plasterboard above the stud walls could be cut out and then packed underneath the trusses with timber to provide support. This was on a conventional type roof construction.

The ‘lattice’ frame supporting the water tank sits on 50x50 on top of the timber roof ties and spreads the load over 4 trusses each side.

Resting the 200mm timber on the walls and spanning the full length is not really an option for me because I can only get an absolute maximum of 3m (10ft) manoeuvred through the loft hatch.

I could bolt 3 sections of timber to each truss and rest the ends on the walls to provide a sound structure but this is costly and time consuming. The section at the bottom of the truss is only 38x70mm deep and would not leave much room for bolts.

I used 75mm Celetex between 4 trusses around the loft hatch. The Celetex was left over, 'over ordered by me' when insulating my parents dormer. I ordered the Celetex from a web sight at about £24 per sheet 2440x1200. I nailed 10mm strips of wood onto the top of the joists and boarded over. This was relatively easy. The Celetex won’t allow vapour to pass through into the loft space. I hope this isn’t going to be a problem!

I still think it would work out to be less costly raising floor level and using Rockwool, if it can be done

I have thought about reducing the weight of the 200x50 timber and ‘foxhole’ says that I don’t need an air gap between the top of the insulation board. Thus, the need for cross battens could be eliminated. I thought an air gap would help the insulation to breathe and remove moisture from the Rockwool. I don’t intend to remove and replace the existing insulation and put in a damp proof membrane. Does any one else know if it is better to have an air gap for this reason?

I know that a lot of the neighbours have boarded over their roof trusses without raising the height of the floor and they don’t seem to have had any problems with the weight. The intended storage area will only have a light loading on it.

On the underside of the trusses there are 10mm ‘timber planks’ spaced apart. The plaster board is fixed to these. Hopefully this will help to stop the plaster board from cracking due to the movement caused by any extra load on the trusses. That is providing there is not too much weight and movement.

I am now considering reducing the area that I will board. Then using 50x50mm beams cut to manageable lengths with tapering ‘legs’ from 150x50mm, screw fixed from the top. Legs at the ends to be cut straight down one side and tapering at the other (140mm at the top and 50mm at the bottom). The legs in the middle spans to taper both sides (200mm at the top and 50mm at the bottom). This would reduce the weight by nearly 2/3rds. I could then space these beams to fit the rockwool between.

Does any one have any comments on this possible solution?