Steep Street, Chepstow, Gwent
Client: Private client
Project Overview
Steep Street, Chepstow, Gwent
223 m²
Service: Design & Build, Roofing
Sector: Residential
- Approved Contractor: MLG UK Ltd
- System Installed: Polyroof BBA-approved Protec Warm Roof System
- Client: Private client
- Duration: 6 weeks
- Work Carried Out: Warm roof build-up, liquid waterproofing, green roof, PV integration
Scope of Work
A 223 m² roof to a Passivhaus new build on Steep Street, Chepstow, a distinctive 10-degree mono-pitch rectangular form integrating a comprehensive green roof system, extended overhangs, onboard gutters and a photovoltaic array to the front. The architectural design follows Passivhaus principles: high insulation levels, airtightness, control of thermal bridging, triple glazing and mechanical ventilation with heat recovery.
- 223 m² roof, 10-degree mono-pitch rectangular form
- Polyroof BBA Approved Protec Warm Roof System with 200mm insulation
- Comprehensive green roof system in a buried application
- Extended overhangs and onboard gutters
- Photovoltaic array integrated to the front elevation
Systems installed
- Waterproofing: BBA-approved Protec Warm Roof System (Polyroof)
- Vapour barrier: Foil-faced self-adhesive vapour barrier (Polyroof)
- Insulation: 200mm Tissue-Therm (Polyroof)
- Carrier layer: Self-adhesive sand-faced carrier layer (Polyroof)
- Roof structure: Prefabricated cassette system, 10-degree mono-pitch
Challenges & Constraints
Where certification is tested
A Passivhaus roof is where the whole building’s performance is tested, airtightness, thermal bridging and insulation continuity all have to hold at the roof, or the certification does not.
A loaded, buried roof
The roof carries a comprehensive green build-up in a buried application and a photovoltaic array, so the waterproofing had to work under both.
A wind uplift question
The distinctive 10-degree mono-pitch geometry with 200mm of insulation made wind uplift a live question rather than a formality.
Project Execution & Techniques
The specification changed first
Polyroof’s technical team recommended moving away from the originally specified prefabricated cassette system towards a true warm roof build-up, and the drawings and specification were adjusted to suit.
Calculated before installation
Thermal calculation, condensation risk analysis and a wind uplift calculation were all completed before installation, the last of these determining the bonding requirements.
A true warm roof
200mm Tissue-Therm insulation was laid over a foil-faced self-adhesive vapour barrier, with a self-adhesive sand-faced carrier layer beneath the Protec membrane.
Green roof and PV
The green roof was installed as a buried application over the certified waterproofing, alongside extended overhangs, onboard gutters and a PV array to the front elevation.
Outcome & Long-Term Benefits
Thermal comfort year-round
The completed 223 m² roof delivers high thermal comfort year-round, in line with the Passivhaus principles the house was designed to.
Better than drawn
The client got a better roof through changing the design rather than building what was originally drawn.
LRWA Awards finalist
Finalist at the LRWA Awards 2024 for Liquid Roofing Project of the Year in a Buried Application.
The client came back
Passiframe returned to MLG and Polyroof for The Cottage specifically off the back of this project.
Conclusion
McGee House is a Passivhaus new build on Steep Street in Chepstow: 223 m², a 10-degree mono-pitch, a green roof, onboard gutters, extended overhangs and a PV array. On a building designed around airtightness, thermal bridging and insulation continuity, the roof is where the performance either holds or does not.
The most useful thing MLG and Polyroof did happened before anyone was on site, the prefabricated cassette system in the original specification was replaced with a true warm roof build-up, and thermal, condensation risk and wind uplift calculations were run to prove it. MLG then installed it as the Polyroof approved contractor. The project was an LRWA Awards finalist in 2024, and the client came back for the next one.
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