Last week, I received a technical query from the Managing Director of a housing and development consultancy, which read as follows:
Hi, I wonder if you could help. We are building a single storey extension with a flat warm roof. I have been seeking quotes from roofing companies and its has been suggested the Section A drawing attached is not a warm roof and not a cold roof it’s somewhere between the two. I appreciate the difference between the warm roof and cold roof i.e. the cold roof requires ventilation the warm roof doesn’t but as the roofing companies are suggesting it’s neither one or the other I am not sure if I need to get the architect to revise the drawing. Are you able to provide any guidance/best practice on this please?
The ‘Section A drawing attached’ was of little use, but the annotation that detailed the proposed flat roof construction was eye-opening:
Roof to comprise of 200 x 50mm C24 softwood joists at 400mm ctrs or use easy joist as manufactures specification. Firing strips at a fall of 1:80 fixed to top of joists. Lay 100mm thick [rigid polyisocyanurate (PIR)] insulation over joists and 50mm thick fixed to underside between firing strips. Cover with 18mm thick exterior grade plywood and cover with [single ply] waterproof membrane. vapour check and 12.5mm thick plaster board & skim.
A subsequent e-mail from the enquirer revealed that they had received conflicting advice from the Architect and a Building Control officer had passed the plans, which they admitted was ‘a bit worrying.’
The flat roof construction that the Architect had specified was, fundamentally, a cold roof construction, as the deck (18mm thick exterior grade plywood) would be on the cold side of the insulation.
Cold, flat roof constructions are not recommended in BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings. Code of practice.
Where they cannot be avoided, they must be constructed in accordance with BS 5250:2021 Management of moisture in buildings. Code of practice, which recommends that a minimum 50mm void is maintained between the thermal insulation and cold deck; cross ventilation is provided to the void via ventilation openings equivalent in area to a continuous opening of not less than 25mm on each side; and the span of the roof is a maximum of five metres.
The proposed flat roof construction would not conform to either standard.
Furthermore, rigid polyisocyanurate (PIR) insulation boards are not structural; are not designed to span between joists; and require a continuous supporting deck underneath. (Let’s assume that the ‘ vapour check and 12.5mm thick plaster board & skim’ were not intended to be installed above the waterproof covering.)
In short, my advice was to recommend strongly that the Architect revised the design of the flat roof construction in accordance with the relevant British Standards and flat roofing system supplier’s requirements.
Thankfully – in my experience, at least – such flawed flat roof specifications are rare, though they appear occasionally, nonetheless. Moreover, despite the construction industry’s focus on developing competence being aimed at the installer generally, it was refreshing to read that it was the roofing contractors who were invited to tender for the works who had raised the issues and they are to be applauded.
This technical query highlights two things: 1) the importance of the early engagement of a flat roofing system supplier by a designer at the outset of a project and 2) the importance of collective flat roofing industry guidance. There is a wealth of guidance available; existing guidance is reviewed regularly, and new guidance is drafted to meet the demands of new legislation. It is incumbent upon all of us to communicate this guidance to the market to ensure that a flat roof that is ‘not a warm roof and not a cold roof it’s somewhere between the two’ never makes it to site.
Anthony Hogan BA (Hons) MIoR
Technical Manager, Single Ply Roofing Association