Extending a room into roofspace

Hi All. I would like some advice on how building regulations will affect a project I am undertaking in my Victorian semi.

The house has 3 floors and I want to do work on the top floor. There are currently 2 bedrooms on this floor and also a store/box room and shower room. I have removed the partition wall between the store room and shower room. The wall was a modern addition and non structural. I want to make the room into a larger ensuite bathroom.

I now want to enlarge the space by moving the current back wall into the roof space towards the eaves by about 60-70cm. The extra floor space that will be created does not have floor joists in place, just ceiling joists of the room below. Floor joists will therfore need fitting.

The slate roof does not having roofing felt but otherwise the timbers are in good condition. There is no insulation between the rafters.
The roof space does have ancient insulation material between the ceiling joists.

After looking at the appropriate building regulations it seems that the insulation requirements will reduce the height of the section of the ceiling in the room that is sloping. Depending on the amount of reduction, it may make the conversion a waste of time.

Further reading has suggested that if I improve the insulation in other areas of the room and loft space, this would allow me to have a reduced
amount of insulation in the areas which affect the ceiling height. Is this correct? I also hope to fit a couple of velux windows in the roof slope.

The first drawing shows the current room shape and the insulation that is present. The second drawing shows what I could do to improve insulation elsewhere. Can anyone suggest if these improvements would enable compliance with building regulations and if not, what could be done to minimize the reduction in ceiling height?
Cheers

Don't forget the increase in floor height which will add to your room height problem

^woody^ said:

Don't forget the increase in floor height which will add to your room height problem

Click to expand...

The floor in the the existing room sits on proper sized original floor joists. The floor which is currently ceiling joints will be brought up to this level and this is accounted for. The addition for floor covering has also been taken into account.

To give you an idea of the room height, the flat part of the original ceiling in 250cm.

I've altered the drawing below.

You won't be able to improve insulation elsewhere, and have less insulation in that section of roof - you are thinking of a solution to a different problem

You have not stated rafter height at the point where the new bit of insulation will be, but it may well be that this is less than 1.8 m in any case and so is not an area which will be walked under - in which case is the height really an issue?

Cheers. Part of the new area was going to house a bath and the getting in and out will get too difficult if the ceiling is too low.

As it stands then, I can leave the wall where it is and have a less insulated room than if I made the changes. Building regulations are confusing

Having re-read the first post, it seems that the rest of the rafters do not have insulation? If so, then pragmatically why should the "extended" bit?

TBH, you should just do the work without bothering with submitting a b/regs application

Obviously ensure that things are done properly in terms of structural support and work in general, but you can make your little compromise in terms of insulation.

Your velux windows will require doubled up rafters each side

Thanks again. I think you are right. I obviously don't want to cut corners with the structural side (and won't) but the insulation bit is of lesser importance to me.

Are the insulation regulations solely concerned with energy conservation or do they affect fire regulations as well? i.e how fast a fire can spread through ceilings or walls etc.

Insulation to the rafters is purely thermal

Insulation to the floor will be for fire and sound, but as you are only adding a bit of floor to an existing, I don't think this will be a problem

1/2 " plasterboard and skim will give 1/2 hour fire resistance and presumably the habitable are of the room will be surrounded with plasterboard in any case

I would insulate the section of roof/ceiling above or around any bath to remove the cold surface and minimise the chances of condensation and mould. If you have 3" rafters, put 50mm of insulation in but maintain a 25mm gap below the felt