Bison slab flat roof, insulation and re-covering?

[thompo]

Hello

I have a flat roof in need of repair and wondering the best option for tackling it.

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• thompo
• 3 Oct 2013
• 1

Flat roof is 2.5m x 3.5m covering 3 cavity walls on a small extension. It seems to be a solid bison slab or concrete poured in place covered with old felt. Internally as a ceiling it had Battons nailed to slab, PB,then insualted PB (25+12mm). We took the ceiling down and discovered wet and dry rot tons of mould. The old plasterboard fell under its own weight!

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• thompo
• 10 Sep 2013
• 2

The Felt is knackered holes and blisters resulting in penetrating damp. Temporary fixed by putting a sheet of 1200 gauge plastic over the lot, walls now dry.

Need to put up a new ceiling and re-cover the outside so i have the option of either cold roof or warm roof. Only problem i see with warm roof is that i dont have a lot of space between the flat roof and the guttering of the pitched roof. If going cold roof, would the thermal bridge between the inner cavity wall and the roof slab encourage condensation?

As for final covering im thinking EPDM.
Had a roofer out for a look, he didnt rate felt or EPDM and recommended a paint on substance with a 15year gurantee. Some commercial grade stuff, not like bitumen paint etc
Quote approx £400, Same price as re-felting or EPDM but i suspect the amount of labour for him is vastly reduced so im cautious.

#edit - Roofer seems to recommend a cold bonded liquid roof applied directly over the old felt.

Cheers for any info

 

[freddiemercurystwin]

I would insulate against the soffit, use as thick a rigid insulation (Celotex or Kingspan) as you can afford, no air gap. At least this way the cold bridge will be minimized to the inner skin. If the insulation is foil faced have this facing down and tape the joints with foil tape. If not then use foil backed plasterboard. It would be prudent to waterproof the concrete deck and then hold off insulating until the slab has dried out or all that moisture will just get locked in forever/seep into your insulation.

You will never achieve an ideal (ie a proper warm roof) scenario when its an existing situation you just have to do the best you can.

 

[thompo]

Excellent, thanks for the reply.

I have a few sheets of 100mm Xtratherm left over from doing the floor so i might see if that will fit. Ill get up today and get a few pictures/dimensions.

We have had the ceiling stripped out and plastic over the felt for approx 2 months now with heating and de-humidifer running so now hoping its dry enough.

Planning on having External wall insulation done in the next year or 2. Would we be best of waiting until this is going on and possibly re-arrange the guttering to allow for better insulation?

Would putting up a ceiling of thin insulated board (25mm+12mm) to allow us to get the plastering finished pose any issues if we then add additional insulation to the outside in the future?

Thanks

 

[Alastairreid]

Get other roofers! that ones a cowboy! he wants to paint a blistered knackered roof and charge you 400 quid

 

[thompo]

Thanks for the replies.

Have to agree with you on the different roofer front, thought it sounded like a way for him to make a quick buck.

Had a friend of a friend take a look, (joiner by trade, but likes roofs), he has experience doing Felt and Trocol. Reckons the Trocol system is one of the best going but probably 3 times the price of felt. Hopefully get prices soon.

As for the insulation, my plan is to fit 100mm Xtratherm on top of the slab, remove gutters and fit 25mm Kooltherm to enclose the sides of the slab, then cover the lot and re-fit guttering to suit.

For the ceiling inside, what do you think the best approach would be? Lath/Battons @ 400 centers drilled and screwed to the slab then PB?
Given its a utility room with HW cylinder, washer and tumble dryer etc would it be wise to use a vapor barrier between the boards and lath? Or should the warm construction prevent condensation?

Thanks Neil