Loft Conversion advice

I’m after some advice concerning insulation in a pitched roof loft conversion I’m currently planning to start in my 1950’s semi, if anyone can help please. This is to be an occasional room without BC which will be used as an office / chill out room, with a sofa bed for if we have anyone stay over. I believe Building regs would require 170mm of PIR board be used for the roof, but this isn’t practical due to the restricted head room it will present once the floor joists have been upgraded to 6 x 2’s. Currently the centre of the room measures 2400mm. The rafters are 80mm deep, so I was planning on adding 50mm batons, placing 80mm Celotex between (leaving a 50mm air gap below the roof tiles) & then adding a further 30mm below. Will 110mm be enough to keep the temperature stable in there?

Being a quite competent DIYer, I’m planning on doing the majority of the work myself & keeping as close to BC as possible for safety purposes, with a couple of top hung Velux escape windows, linked fire alarms, approved stairs etc. I’d prefer to work within the regs, but I believe I wouldn’t get approval with the reduced insulation & the construction of the new floor? The purlins are 12 x 6 steels, so I was planning on running 4 x 2’s below them & across the top of the original 3 x 2 floor joists – 2 x 3m long on each side, recessed into the wall & joined above a load bearing wall in both positions. Was then going to bolt 6off 3 x 2’s onto the steels & hang down & bolt to the new 4 x 2’s for extra support. 6 x 2’s joists would then be hung from these, which would drop 2” down alongside the original joists leaving 1” space above the plasterboard. The width between the steels is approx. 3.6m & again, there is a load bearing wall along the centre, so the new 6 x 2’s would be supported at either end & packed up 1” onto the wall below the centre. I’m not going to upgrade the joists under the eaves at either side, as this will be used for storage only & will be separated off. Not planning on moving for the next 20 years or so, so the improvement is not for financial reasons - just more space

Hopefully you can understand my ramblings & offer some advice on the insulation & whether the floor construction would be acceptable to BC or not.

Thanks in advance.

If your going to work within regs, then why not do it legally?

Smudge79 said:

with a sofa bed for if we have anyone stay over

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Don't forget to tell them they they will be sleeping in an unauthorised, uninsured, potential death trap.

A loft conversion with BR approval adds resale value to the house.

A non-compliant job doesn't

There is no good reason a job properly done would not have approval.

The regs don't require you to put excessive thicknesses of insulation in the roof; the thickness of insulation has to be 'reasonable', and as it is in a loft, thickness has a bearing on useable floor space.

If you're going into this project with this sort of structure 'acceptable to BC', why not make it fully compliant and do it properly?

Because a builder who can do the work to regulations would cost a stupid amount of money and wouldn't add any value whatsoever by the time the immense cost of doing the conversion is taken into account. I did mine 25 years ago so that I had a bit more space for storage. If what I have done impacts negatively on my house price, if and when I come to sell it, then I will go up there with a sledge and put it back to being just a dusty, crappy old loft area. Nobody sleeps up there nor uses it, other than us for storage. I have had my house valued twice in the 30 years that I have lived here and the valuations have come out in line with the current house prices in the area at the time. The valuers were told about the loft and they did look at it and whilst they did say I couldn't sell the house with any description other than it being a loft with "posh" storage that was fine by me. I was going to build a model railway up there at first and that was what prompted me to do it but then I bought a narrow boat and the model railway idea went out of the window.

tony1851 said:

The regs don't require you to put excessive thicknesses of insulation in the roof; the thickness of insulation has to be 'reasonable', and as it is in a loft, thickness has a bearing on useable floor space.

If you're going into this project with this sort of structure 'acceptable to BC', why not make it fully compliant and do it properly?

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Thanks for the positive response tony. I'd like to get it compliant if the solutions to get it there are practical. In your opinion, do you believe 110mm of celotex could be sufficient (20m² useable floor space) & does the floor construction look like an acceptable solution to you? I'm looking at getting professional advice, but a quick heads up might stop me wasting my time & money getting someone in & I'll just shelve the project. Hence the reason for my original post.

If you have any spare headroom on the first floor have you considered dropping the ceiling height to gain loft headroom? In the grand scheme of things when doing a loft conversion I never think it's much of a big deal. You might also gain a tiny bit with grade (C24), width and spacing of your floor joists. (and I can't follow your idea for the floor but if it involves hanging off some existing steel purlins they may not be adequate to carry the extra load)

It was some years ago when I did mine (started off as a bit of extra storage and ended up as a full blown 2 bedroom dormer job that eventually became BR compliant, (via a regularisation application) - but worth doing as the project cost me less than 5k and added tens of thousands to the value when I sold the house).