No1 Ancoats Green: Best practice for flat roofing designed to the Passivhaus standard – ADC

Designed by local practice Buttress for This City (the Manchester Council-owned housing development company), No1 Ancoats Green comprises 128 new-build, low-carbon homes on a brownfield site in Ancoats, north Manchester. Currently under construction, the project includes 118 one- and two-bedroom apartments split across two blocks (one five storeys high and the other seven storeys), together with 10 three-storey terraced townhouses overlooking Ancoats Green.

Sustainability is central to the scheme, with energy-efficient properties designed to Passivhaus standards, ensuring lower energy costs for residents and contributing to Manchester’s 2038 zero carbon ambitions. This posed a number of challenges for the project team – not least ensuring that the buildings’ large flat roofs (utilising both warm and inverted warm roof construction techniques) met the stringent demands of the international energy performance benchmark.

Fortunately, help was at hand from leading roofing expert AccuRoof, who played a key role in the design, specification, and installation of the high-performance [SP1] [SP2] roofs. Daniel Bosworth, AccuRoof Technical Director, in conversation with Architecture Today’s Technical Editor John Ramshaw, discusses how best practice was achieved for this critical aspect of the project.

When did AccuRoof become involved in the project?

We are one of Wates Group’s principal roofing consultant partners, so once the contractor was appointed on the project we became involved in developing the design of the two main residential blocks, as well as the townhouses.

How did the design and specification progress develop?

We worked closely with the architect, basing our waterproofing design off their detail drawings. IKO PermaTEC Hot Melt inverted roofs with IKO enertherm XPS insulation were specified for the main blocks, with an IKO reinforced bituminous warm roof with PIR insulation for the townhouses. Both systems are BROOF(t4) – the highest rating under the European Classification System BS EN 13501-5 for fire performance. AccuRoof is a system agnostic supplier, so we always take a systems-based approach to the entire roofing package to maximise component compatibility, performance, and cost effectiveness.

Part of the specification process involved determining the buildings’ airtightness line. In this case it was below the roof deck. However, the waterproofing systems chosen for the project are naturally airtight anyway by virtue of an air and vapour control layer that laps with the waterproofing, underlay and cap sheet (by 50mm) at all abutments and penetrations on the warm roof system; and the airtight and waterproof nature of the holt melt system used on the inverted roof.

What are the main design and installation principles for warm and inverted warm roofs that meet Passivhaus standards?

One of the most important points is to eliminate gaps within the roof insulation that are greater than 3mm. As you can imagine, there are often questions from roofing contractors about how roofing insulation can be installed to such tight tolerances. Typically, we recommend the use of a compatible expanding foam to fill any cracks and gaps that are greater than 3mm.

Foam is also to fill voids where the insulation has been cut around soil vent pipes and other necessary penetrations, thereby retaining an unbroken thermal layer. It is also used where insulation boards come to stop, for example, at the junction with a parapet wall – if the gap is more than 3mm. Any metal penetration on the roof will need a thermal break at its base, together with the correct waterproofing detail.

Insulation boards should be mechanically fixed where possible and acceptable as part of the roof system installation methods. Joints between insulation boards must be staggered to eliminate the possibility of continuous vertical joints running all the way through to the roof deck. Boards must also be cut at 90 degrees to ensure tight abutment when they are laid next to each other. Hot wire cutting – rather than sawing – is recommend on site to achieve 90-degree edges when using EPS insulation.

If any part of the roof changes direction, there is the potential for bird’s mouth gaps to occur as the boards open up. This should be taken into account when the boards in these areas are cut to size and shape. Last but not least, the boards must be fit-for-purpose, in other words: suitable for the type of roof, free-from manufacturer defects, and not damaged as a result of transportation or onsite storage.

What levels of thermal performance and airtightness were required for the roofs on No1 Ancoats Green?

U-values of 0.18 and 0.15 W/m²K were required for the residential blocks and townhouses respectively. An airtightness value was not needed due to the buildings’ airtightness lines being set below the roof decks, and we were therefore not responsible for this aspect of the design.

What and where were the main design/detailing challenges, and how were they resolved?

Thermal bridging is an ever-present threat on Passivhaus projects, so we insulated around the soil vent pipes and rainwater outlets below deck in order to minimise this risk. We also employed tape around these penetrations internally to maintain airtightness between the inside and outside.

Avoiding thermal bridging posed by a roof-mounted modular plant room located on one of the inverted roofs to the residential blocks was another challenge. These structures are typically made from steel sections which are bolted to the slab. The solution was to employ a ‘floating slab’ which was cast onto the drainage layer (directly above the inverted roof insulation) and therefore supported the plant room, while also maintaining the integrity of the roof build-up below. In addition to detailing this element, AccuRoof was able to advise on permanent static load maximum limit working loads in relation to the insulation specification.

Correct installation is clearly critical in relation to the Passivhaus standard. How did you ensure high levels of quality and accuracy on site?

BriggsAmasco, one of our hot melt-approved contractors, was the installer for main blocks A and B on the project, and we made sure that their onsite operatives had correct and up-to-date training documentation for the waterproofing system specified. AccuRoof’s area field technician carried out regular site inspections to ensure that the roofs were being installed in line with the approved drawings and specification. As part of their onsite role, technicians will typically check for gaps between insulation boards. If these are found, they will advise the roofing contractor to remove the boards, tighten the gaps and then reinstall the insulation, or if not feasible, fill with the approved expanding foam.

How should clients and estate managers approach the maintenance of flat roofs that have been designed to Passivhaus standards

Maintenance regimes for flat roofs – designed to Passivhaus standards or not – comprise a minimum of two inspections per year (normally in the spring and autumn) to clear leaves and other debris which could litter the roof and block any drainage outlets. Mastics – if they’ve been used on the roof – are also a maintenance item, which should be inspected regularly, as well as any other project-specific detailing.

What advice would you give specifiers who are about to embark on a project incorporating Passivhaus-standard roof designs?

First and foremost, use a reputable roofing manufacturer or supplier, such as AccuRoof, to insure full system compatibility. This applies to all roofs, not just ones designed to Passivhaus standards. The roof system(s) chosen should always be B_ROOF(t4) compliant. If EPS insulation is being used on the roof, it is important to ensure that the product in question is approved by the waterproofing manufacturer and is part of their standard system.

Roof penetrations, including soil vent pipes, should be kept to a minimum for reasons of thermal performance and airtightness. Also, as AccuRoof are part of roof waterproofing associations, such as the Single Ply Roofing Association (SPRA), Liquid Roofing and Waterproofing Association (LRWA), and the National Federation of Roofing Contractors (NFRC), we have the benefit of industry technical guidance in relation to such elements as Passivhaus flat roof design best practice recommendations.

Contact Details

If you require design assistance on a roofing project, please call 01509 505 714, email, or visit the AccuRoof website.