Loft extension to 1890s London mid terrace query...

[phb10186]

The existing roof has bitumen felt lining under the slates (replaced c. 1990), and is in great shape. There are two central vents, one on each of the pitches (back and front). Then there is a 150mm insulation layer installed in 2009 in the inside, with a breathable membrane securing it.

Do regs mandate that the bitumen membrance on the outer under the slates be breathable, or can we save the front side and leave as is.

The back side can be changed to a breathable type when a proposed dorma is installed.

Really, it would be a shame to have to re-lay a perfectly good roof.

 

[freddiemercurystwin]

The felt is there as a back-up should any tiles drop or leak and many older roofs do not have any at all anyway. If the roof covering is in good nick you don't need anything to get through the regs. If you have felt though you just need to keep a 50mm air gap between the top of any insulation and the underside of the felt. Continuous ridge vents may be required to get the ventilation to work. One assumes you have existing ventilation at the eaves.

Do you mean the insulation is at rafter level? Unless its Celotex or similar you may need to ditch the insulation as it won't get you anywhere near the required U value and you may need to beef up the rafters (amongst other things) so it may all have to come off anyway.