Insulating a flat roof and levelling the ceiling

[mogget]

I plan to insulate the flat roof of my kitchen, and would like some advice as to how to do it please.

I have made a rough diagram of the existing construction

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• mogget
• 21 May 2009
• 1

The flat roof covering consists of straw boards, felt, tar and stone chippings.

What I want to do is to add insulation below the existing plasterboard, and also making the ceiling level - currently it slopes about 3 inches across 8 feet.

My plan is -
* attach 2x2 timber battens to the underside of the existing joists leaving existing plasterboard in place
* attach 8 foot long wooden wedges to the underside of the battens
* put up new plasterboard, filling the voids with rockwool as I go.

My questions are -
* are there any serious errors in my plan?
* is there a proper name for the wedges, and where can I get them from?

I am a complete novice at roofing, and any help would be great.

TIA

 

[Shytalkz]

Clearly your choice of colours was affected by the recent consumption of a Maccy Dees:

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• Shytalkz
• 21 May 2009
• 1

 

[stephenf8542]

The slope is for water to run off, if you level it you'll have more chance of water penetration

 

[Shytalkz]

Duh, he wants to level the ceiling, not the roof!

 

[stephenf8542]

Mabe I msunderstood when he said ''leaving the existing plasterboard in place''

 

[mogget]

Hi

I don't want to do anything to the roof, just to put insulation in and lower and level the ceiling, or am I barking up the wrong tree?

I'm a she not a he btw.

 

[stephenf8542]

Apoligies Ms Moggat
You may want to cut a hatch to see whats up there ,just to make sure your not going to screw the battons into cable runs etc

 

[mogget]

I think there's only one cable that's not surface mounted in the ceiling - I had a good look when the electrician was here.

Would it be acceptable to use a detector of some sort to see if there are any others? Are you allowed to have cables inaccesible in a flat roof?

 

[stephenf8542]

As long as theres no connection boxes then yes,if you feel confident then go for it

 

[mogget]

One thing I'm not sure about is the need to protect against condensation or damp causing rot in the timbers. Not wanting to create more work for myself in a few years, is this something I should be concerned about?

 

[regsmyth]

Yes you have to be careful to prevent condensation - use polythene behind the plasterboard.
However, is there any existing insulation?
Is it up to current standards?
You can find typical details for flat roof construction with insulation in various places on line.
The long taper bits are called firrings, but as far as I know, there isn't a standard angle, so you might have to cut your own ones.

 

[mogget]

However, is there any existing insulation?
Is it up to current standards?

The house was built in about 1880, and the kitchen originally had a pitched roof, which at some time was replaced by the flat one.

There is no insulation at all in there at present, at least not in the areas that were opened up.

I'm not sure if the straw boards were intended to provide insulation. It seems that outside air flows freely through the ceiling void so it wouldn't help much, I guess.

 

[noseall]

Fully fill the ceiling void with celotex or kingspan insulation ensuring a snug fit, even filling any joints or gaps with silicone sealant.

Firring strips or diminishing strips are sold at builders merchants and are used to provide falls to flat roofs. These may be the right size/pitch to correct your slope but you need to check first. If not, you could always fix some level timbers along side and attached to the sloping ones prior to insulating.

Use foil backed plasterboard on the ceiling.

 

[mogget]

Thanks everyone for the replies. I'll have a look on the web for extra resources too

 

[regsmyth]

So you'll have to take off the existing plasterboard...