High-performance waterproofing products from Radmat are central to a major residential development in London by RM_A Architects
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Designed by RM_A Architects for Telford Homes, The Forge is a major residential development near Upton Park in east London. The scheme, which is shortlisted for the AJ Specification Awards, comprises 192 homes split between five courtyard garden blocks, a 14-storey tower and six townhouses. It also incorporates a range of different roofs at various heights that are intended to minimise the project’s visual impact on its neighbours. Roof terraces provide private external amenity spaces for residents, while a communal roof terrace on the sixth floor overlooks Priory Park to the south.
A key constraint of the original planning consent was a maximum site discharge rate of 3.3-litres per second for surface water. The restricted run off rate, which is due to the site’s location in a ‘critical drainage area’, presented a significant challenge with regards to the design and specification of the roofs and drainage systems. With the exception of amenity areas, the majority of the roofs proposed in the planning scheme were green living roofs. There were also a series of courtyard gardens at ground and first-floor levels. While the soft landscaping would assist in reducing and delaying surface water runoff in storm events, this alone was not sufficient to achieve the planning target. Given the constraints of a contaminated and tight urban site, ground level SUDS were eliminated due to ground quality, lack of space, and priority given to usable amenity spaces. The team therefore had to pursue a combined system of blue roofs, buried attenuation tanks and a hydro break.
RM_A Architects quickly developed a series of drainage strategies and roof specification diagrams to identify the scope of the works, as well as the position and number of rainwater outlets needed. The architect also organised a series of workshops for Radmat Building Products and the design team to explore and finalise both the drainage strategy and roof specification. Radmat provided typical roof details, U-value calculations, Blue Roof and NBS specification documents for all the key roofs and terraces. The company’s Blue Roof drainage calculations and those of the other roofs were fed into the engineer’s design for the below ground drainage system. Radmat also assisted the engineer in determining the dead and live loads for all roofs.
Roof specification plan (level 14)
Central to the drainage strategy are blue roof water storage reservoirs located beneath green roof build-ups on the fifth and 14th floors. Although the roofs incorporate multiple gravity and through-wall outlets, the majority of these are excluded from providing primary drainage flow to facilitate controlled rainwater attenuation. The outlets that are used – in this case two at each end of the roof – are fitted with a Radmat flow control device that reduces the orifice diameter. The remaining gravity outlets and through-wall outlets are fitted with Radmat ‘snorkels’, which prevent the devices from functioning during normal rainfall. However, in the event of the blue roof reaching capacity they act as overflows.
Typcial roof and parapet details
Radmat’s flow rate modelling determined the size of the orifice restrictors, based on specific storm events for the locality. The outlets themselves are standard cast in drains with the addition of the Radmat snorkels and flow restrictors. This means that the time taken to drain the roof can be delayed by several hours, reducing the impact on the local sewage network. The system can accommodate a 1-in-100-year storm event, plus 30 per cent for climate change. It will half drain itself in under eight hours in readiness for a subsequent storm event, and in accordance with emerging Blue Roof design standards.
SWG Geocell from Radmat
The surface water drainage from the buildings feed into a below- ground drainage scheme, along with the surface water drainage from the hard and soft landscaped areas at ground level. The ground surface areas required additional attenuation in the form of below ground attenuation tanks that complement and work in parallel with the blue and green roof attenuation systems. The system feeds into a Hydrobreak before discharging into the public sewer system.
Radmat’s ProTherm G XENERGY SL-E XPS insulation
The thermal performance, and U-values for the roofs formed a key part of the SAP assessments and overall Energy Assessment for the scheme. Radmat was able to confirm the U-values for each roof based on the architect’s preliminary details. These were fed into the M&E consultant’s Energy Assessment and led to confirmation of the need for PV panels on the northern end of the fifth floor roof. In addition to reducing drainage runoff, improving thermal performance and electricity generation (PV panels), the roofs also incorporate ecological requirements for bio-diversity. Various growing medium formulations and planting are employed. The habitat roofs promote environments that support bees, butterflies and birds. Radmat and the landscape architect coordinated the specification of these areas.
PermaQuik block from Radmat
At the time of design, Blue Roof specification was relatively new to the British construction market and preceded inclusion within the London Plan. The regulations and design codes that underpin blue roof specification are still being developed, but are reinforced by the vast knowledge of the sustainable urban drainage industry, the UK roofing industry and various legislative bodies. Radmat’s Head of Technical & Operations, Mark Harris chairs the British Green Roof Organisation trade board, and assists in developing industry agreed regulations. This gave the design team a unique insight into the regulations before they were officially agreed and published in their current form.
Key Radmat waterproofing products used on the project include:
• PermaQuik PQ6100 to main areas
• Radmat Geocells, G12 fleece, Flow Restrictors and Overflow Snorkel of multiple heights and dimensions
• ProThem G XENERGY SL-EP XPS insulation to achieve 0.13 W/m²K U-value
• ProTherm Quantum PURE VIP insulation to overcome reduced height access door thresholds
• Green roofs comprising pre-grown sedum blankets for overlooked areas and wildflower seeds around PV panels on main roofs
Contact Details
For more information please call Radmat on 01858 410372, email sales@radmat.com or visit the website.
