Blue Roof Design Considerations

Development and expansion of towns and cities has seen exponential use of impervious surfaces causing artificially high rates of rainwater runoff. With the trend of urban growth coinciding with an ever-increasing number of ‘extreme’ weather events, flooding is becoming a common and expensive problem.

Designing a blue roof is complex and we will work with the client, architect and drainage engineer to provide a single point solution for waterproofing, blue roof and green roof layers, and system guarantee.

Blue Roof Design Considerations Overview

Legislation and drivers

Sustainable Urban Drainage is now part of legislation and is a critical part of planning. Blue roofs form part of the options available for SuDS.

Many local planning authorities (LPAs) are adopting early perspectives that encompass Schedule 3 of the Act to bring in measures that prevent flooding. Within construction and development, planners are restricting the amount of rainwater leaving a site via the drainage system, limiting water egress to 5-10 litres per second per hectare, the same flow rates for regional greenfield sites.

The Environment Agency and lead local flood authorities provide data and flood maps to assist drainage engineers and architects identify the level of risk by categorising the possibility and hazard levels of flooding by zones.

Incorporating a blue roof

The inclusion of a blue roof into a project can be at rooftop or podium level. A blue roof is designed to attenuate storm water within a void which sits directly above the waterproofing layer and beneath a surface finish such as a vegetated green roof or hard landscaping.

A blue roof should not be designed as a water storage facility but should allow all the water to evacuate from the roof over a 24-hour period from the end of the projected maximum rainfall event, with over half of the attenuated water having evacuated from the roof after a 12-hour period.

Key aspects are:

Single point source and guarantee
Adequate void space or capacity of stormwater
Meeting British Standards and Codes for Best Practice
Minimise roof penetrations
Inclusion of an emergency overflow

Roof deck construction

The implementation of a blue roof will have considerable loading implication on the roof and its waterproofing. The roof deck construction will need to be designed not only to accept the dead and imposed loads, but also the weight loading associated with the water to be attenuated on the roof.

A blue roof can be designed on zero falls providing the waterproofing system holds relevant certification and the roof is designed in accordance with British Standards.

When the deck construction is complete, a deck deflection survey should be carried out to ensure the accuracy of the construction and that no depressions or back falls occur. Certification should be gained before the installation of the blue roof proceeds. Any falls created should be taken into account as they will impact on the effective void space for holding rainwater.

Waterproofing system selection

Consideration must be given to the appropriate form of waterproofing so that it can meet the demands placed on it by the blue roof, as such performance characteristics for durability and thickness of membrane (density), tensile strength (EN 12311-1) and elongation at break (EN 12311- 1) need evaluation before appropriate selection can be made.

If the blue roof construction has a finish where germination of any plant seedlings is possible the membrane should be tested and approved to the current FLL and Green Roof Organisation (GRO) guidelines.

Our blue roof solutions utilise two robust waterproofing constructions; a bituminous warm or cold roof build-up with the Bauder Total Roof System (Green for soft landscaping) or a Bauder Hot Melt cold roof construction. Both systems carry BBA certification and are suitable to maintain the integrity required for blue and green roof applications.

Outlets

The design of every blue roof is individual to the geographical location, construction project and the total allowable discharge defined by the LPA within the planning approval.

The calculations for drainage requirements are a specialist discipline and should only be carried out by a qualified professional. Bauder produces roof specific discharge reports for the blue roof specifications we are engaged with.

Both restrictor outlets and emergency overflows need to be incorporated into the design with robust detailing and precise waterproofing installation.

Stormwater attenuation capability

There are different options available to the construction designer to achieve a blue roof depending on the finish desired and the volume of water to be attenuated.

Create a simple void space with a hard landscaped finish (paving) on a pedestal support system. Courtyard podiums or terraces are ideal locations for this blue roof solution with a completely paved finish above the void space.
Go to the BauderBLUE STORMcell system.
Utilise a green roof where the substrate and water attenuation layer attenuate low levels of stormwater. Extensive or biodiverse vegetation finishes can be coupled with a biosolar PV array for maximum environmental advantage.
Go to the BauderBLUE STORMsub system.
A capacity zone is created beneath a green roof finish for high volumes of stormwater attenuation. The surface finish allows stormwater to drain without restriction into the attenuating cavity that allows free-flowing movement of water to the flow restrictor outlets.
Go to the BauderBLUE STORMvoid system.

Detailing

British Standards and Codes of Practice for waterproofing detailing should be followed and will provide a guideline to detailing principles, even if they are not specific to a blue roof application.

The minimum recommended height for constructing waterproofing details is 150 mm above the finished surface of the landscaping, surface decking, ballast or paving. All waterproofing detailing should incorporate a suitable waterproof termination to the building structure.

Post installation inspections

An electronic or suitable integrity test should take place on the completed waterproofing, prior to the installation of any blue roof void forming or any landscaping components. The satisfactory waterproofing integrity certification must be retained, and the testing company must operate to the guidance issued by the NFRC.

A BauderBLUE roof will have the waterproofing system and blue roof system inspected by our site technicians who will produce individual reports confirming the installation of the system is in accordance with our recommendations and specifications.

Maintenance of a blue roof

Maintenance of a blue roof is critical to the viability to its function, including all the standard maintenance for the waterproofing and green roof, with additional attention to the restrictive outlets and emergency overflow elements.

Supplementary checks should made after:

Any significant storm event, the outlets should be visually inspected to ensure no blockage has occurred.
Any significant traffic or remedial works that take place on or around the roof to ensure outlets remain free-draining.

Download our Blue Roof Maintenance Guide.