Replace and Insulate a Slate Roof
I’m not sure we always think too much about roofs.
- This straddles the Damp & Mould hazard too and we’ve tried to incorporate some of the newer guidance around insulation requirements for thermal elements together with an older and through slate roof clause I had. You can adapt for tiles but it will “generally” be poorer conditioned Slate roofs we see in Camden.
In the wisdom of one of our E/H ancients, we did approach Excess Cold in the past and schedule writing in general with the philosophy that it is easier to compromise down from a very high standard than to start from something generalised and compromised.
It’s HHSRS of course so the emphasis is on removing the hazard and this can be achieved in many different ways but this should not dissuade us as professionals from asking for something that is a) going to last and b) is going to benefit the future of PSH properties. Particularly as Excess Cold is never going away and Cost of Living is appalling.
Main roof
Slate Roof Renew:
Strip the slated roof covering complete including slate battens. Carefully examine exposed structural timbers and replace all rotted, weak or broken members to leave the roof carcass structurally sound.
Renail all existing roof timbers to the ridge, purlins and wall plates.
Lay underfelt to the current edition of BS 747 and 25mm x 50mm treated timber battens set to an appropriate gauge. Re-slate the roof using new or sound second-hand slates securely fixed with non-ferrous nails to suitable battens, fixed to a proper gauge on approved felt lining. Renew any defective soakers, flashings and fillets in Code 4 lead. Re-bed loose hip tiles, where appropriate. Leave roof sound and watertight upon completion.
Dependent on the permeability of the slate under-felt used, ensure the roof void is properly ventilated in accordance with current Building Regulations and associated Codes of Practice. Upon completion, leave whole, sound and weather tight upon completion.
Strip off all felt/lead from the existing main roof centre valley gutter and strip out the centre valley gutter complete. Carefully examine roof timbers and replace all rotten or defective members using sound well-seasoned timber of suitable dimensions. Provide timber firring pieces to adjust the fall of the valley gutter so that it drains correctly to hopper outlets to the front and rear parapet walls. Strip parapet walls to prepare for re-rendering and provision of new metal flashings stepped into brickwork in accordance with current Building Regulations.
Renail all existing roof timbers.
Provide and lay new gutter linings in the most appropriate Code of lead sheet laid on a continuous layer of roofing felt and dressed-up roof slopes over tilting fillets to form gutter of 150mm depth, joined in length by welted joints laid flat in the direction of fall and to include fixing with nailed tingles. Where drips exist in gutter length, drip joints shall be reformed and lead dressed to gutter outlets.
Provision of roof insulation to the new roof and void space:
Provide at least 270mm thick loft insulation to the pitched roof void above the dwelling (100mm between the joists and 170mm across the joists) and to achieve a minimum U value of 0.16(W/m2K). The minimum thermal performance standards set out in Approved Document L of the Building Regulations as changed on 15 June 2022. After this date, the cold roof build-up of renovated roofs must achieve a U-value of at least 0.16W/m2K. This can typically be achieved by installing rolled mineral wool insulation at a minimum depth of 270mm but for maximum performance, we recommend installing insulation at a depth of 400mm.
Remember to layer insulation
Insulation must match the depth of the joist, so if the joist is 100mm deep, your first layer of insulation should be too. The top layers should be fitted at right angles to the layer below, completely covering the joist.
Unintended air gaps reduce performance, so make sure rolls are butted together, friction is fitted against joists and pushed down around the roof perimeter to meet insulation in the wall head.
Maintain ventilation
To prevent condensation, never cover air vents with insulation, and if the roof is lined with a sarking felt, allow for continuous ventilation of the roof void by leaving an air gap at the eaves.
Don’t cover cables
Reroute high voltage electrical cables over the top of the insulation to prevent the risk of overheating or fire.
Ensure the insulation is correctly fitted in accordance with the instructions. A high standard can only be achieved by insulating across the top of the ceiling joists as well as between the,, to avoid cold-bridging and including the loft hatches to the inner hallway and access hatch to the top landing. Pipes and tanks also need to be insulated to avoid frost damage.
Other key considerations include ventilation to avoid condensation in the loft, access and safety aspects of electrical wiring and fittings, and safety information.
Alternatives can be used to achieve the same u values as required in Building Regulations, approved document L 1 B (conservation of fuel and power in existing dwellings). See *Note regarding pitched roofs below.**
Notes: Building Regulations: Insulation and thermal elements
Making significant changes to thermal elements (walls, roofs or floors) would normally require Building Regulations approval and require the thermal insulation of the element to be upgraded to a reasonable standard. Walls are defined by Regulation 2(3) of the Building Regulations 2010 as being thermal elements.
The extent to which the work on the element is controlled and the amount of upgrading needed depends on the particular circumstances of the thermal element. Generally, when it is renovated then it should be upgraded, where it is cost effective to do so, to the standard set out in the Approved Document.
Further guidance on this is available in Approved Document L, Volume 1 covering:
- Guidance on the replacement of thermal elements and limiting heat gains and losses (pages 60-64)
- Explanation of when renovation works trigger requirement for upgrading insulation and what additional work may be required. (Appendix C and Table C1 pages 80-83)
You should fully consult the Regulations and the Approved Document and, if you are in any doubt, seek advice before commencing work. The definition of a thermal element does not include windows, doors, roof windows or roof lights.
*Note regarding Pitched Roofs
Insulation can be placed between the ceiling joists. Again, the thickness will vary depending on the material you choose to use.
If the roof has no ceiling, then the insulation can be placed between the rafters and ventilation maintained as described above - in which case the ridge should also have vent tiles installed to allow for through ventilation.
Further guidance on insulating roofs can be found in Approved Document L, Volume 1 Table C1.
For example: RIGID BOARD INSULATION with a U-VALUE: 0.15 W/M2.K between and below the rafter where a new roof has been constructed. Alternate methodologies may be discussed and agreed with the Environmental Health Officer and Building Control Officer/