Checklist: designing and specifying flat roofs

What is a flat roof?

With reference to BS 6229, a flat roof is defined as a flat or curved roof with a pitch not greater than 10º to the horizontal, with a continuously supported flexible waterproof covering on a supporting structure that is both dense and heavy (for instance, a concrete slab), or consists of framing members supporting a lightweight deck of metal or of timber-based material.

Main types of flat roof

• Uninsulated – no insulation either under or over the substrate/deck.
• Cold – insulation located under the substrate/ deck. Depending on the type of substrate/ deck construction, ventilation may be necessary. 
• Warm – insulation located over the substrate/deck, with the waterproofing membrane located over the insulation.
• Inverted – membrane located directly on the substrate/deck with insulation above. 

Various materials can be used in the composition of the roof build-up, and may be used in multiple combinations: 

Membranes  

• Main types of membrane:
• Liquid applied – either cold (single-component polyurethane (PU)) or 
two-component poly (methyl methacrylate) (PMMA), with reinforcement layer or hot (either modified bitumen (with reinforcement layer) or asphalt-based material).
• Reinforced bituminous sheet – either styrene-butadiene-styrene (SBS) or atactic polypropylene (APP) modified. Generally SBS is most suitable for the UK.
• Single ply – composed predominantly of synthetic polymer involving either polyvinyl chloride (PVC) or flexible polyolefin (FPO).

There are advantages and disadvantages with each. Seek advice from the roof system manufacturer as to most appropriate use on specific projects.

Insulation

Main types of insulation:

• Cellular glass – manufactured from grinding down recycled/waste glass into a powder and mixing with a foaming agent. Kiln-fired, it creates a substance of closed-cell micropores, which can be formed into slabs.
• Mineral wool – manufactured from melting down of rock minerals (such as basalt, and recycled slag) and spinning them into fibres forming wool-like textured slabs.
• Polyisocyanurate (PIR) – rigid foam material that is formulated from various chemicals.
• Expanded polystyrene (EPS) – rigid, closed cell, thermoplastic foam produced from solid beads of polystyrene.
• Extruded polystyrene (XPS) – rigid, thermoplastic foam that is formed by extrusion.

Insulation can be used to create the falls required, primarily in warm roof construction. Again, there are pros and cons to the attributes of each type of insulation – seek guidance from manufacturers.

The specifier needs to be explicit in terms of use and required performance so the most appropriate recommendations can be given. The main attributes to consider are fire, resistance to loading, thermal performance and ability to perform if wet. For instance, thermoplastic products can be better at load resistance, achieving thermal performance in reduced zones, but are combustible, achieving, at best Class E to BS EN 13501.

We do not advocate the use of polystyrene-based insulation unless there is no alternative, but where necessary to use it, it should be encapsulated in non-combustible material. The overall system should also achieve BROOF(t4) as a minimum.

Surfacing

Main types of surfacing – can be combinations of the following:

• None – membrane provides the surface.
• Over slabs – may require the expertise of the structural engineer.
• Stone ballast – consider the size and necessary containment to ensure stones can’t, for example, be picked up by birds and dropped from height.
• Concrete pavers – bedded on over slab, or on proprietary pedestals.
• Natural stone pavers – bedded on over slab, or on proprietary pedestals.
• Timber decking planks – on proprietary support.
• Glass reinforced plastic (GRP) imitation timber decking planks – on proprietary support.
• Brown/ biodiverse – suitable substrate/growing medium to nurture natural colonisation of flora and fauna, often with prearranged biodiverse features such as log piles. Build-up includes filtration and drainage elements.  Fire breaks will need to be incorporated.
• Extensive green/biodiverse – options as to planting types (plug plants/seed mixes, pre-grown on matts) on suitable substrate/growing medium with filtration and drainage. Fire breaks will need to be incorporated.
• Intensive green/ biodiverse – all types of surfacing including but not limited to soft landscaping such as planting, turfing, shrubbery, trees and so on, on suitable substrates/ growing mediums.  Hard landscaping such as paving, paths, roads; fixtures and fittings.  

It is crucial to be clear about who assumes responsibility for designing and specifying surfacing. Other considerations include use of ballast for visual and stability and protection purposes, but correct size and grading is key. 

Key considerations

We always align our specifications with the procurement and design responsibility matrix (DRM). We need to understand if an element of the design is prescriptive (architect design responsibility) or descriptive (contractor/sub-contractor responsibility). Final design solutions would usually be descriptive, but this depends on contractual appointment. 
Due diligence should be undertaken with reputable manufacturers to ascertain the most suitable construction approach for the complexities and service conditions (including interface detailing), zones (in which any roof construction must fit), required design/service life, and cost constraints. 
Designers will need to understand the different approaches to construction and options to assist when in dialogue with roof manufacturers.

Testing/inspection

Testing/inspection should allow for:

Offsite testing

• Independently certified test data.
• Where not for the particular service conditions, testing may have to be undertaken.

Onsite testing

• Adhesion/ compatibility testing (prior to installation).
• Incremental inspection at key points of the system installation in the presence of client, contractor, sub-contractor and manufacturer for quality control purposes, before subsequent phases of the installation are permitted.
• Electronic and flood testing to check workmanship.

Best practice/technical details

Standards/bodies (general)

• BS 6229: 2018 – flat roofs with continually supported flexible waterproof coverings. Code of practice. Considered the primary overarching standard.
• REP BR 504 – roofs and roofing. Performance, diagnosis, maintenance, repair and the avoidance of defects, published by the Building Research Establishment (BRE).
• BS 8217 – reinforced bitumen membranes for roofing. Code of practice.
• Liquid Roofing and Waterproofing Association (LRWA).
• Single Ply Roofing Association (SPRA).

Standards/bodies (green roofs)

• Green Roof Organisation (GRO) Green Roof Code – code of best practice for the UK.
• Guidelines for the Planning, Construction and Maintenance of Green Roofing (FLL Guidelines for green roofs) published by German Landscape Research, Development and Construction Society (FLL).
• Fire Performance of Green Roofs and Walls, published by Ministry for Housing, Communities and Local Government (MHCLG).

Key considerations

‘Flat’ roof construction should be proprietary systems by a reputable manufacturer with independent third-party testing. Necessary performance should be understood and established, since this will form the basis of achieving the correct design solution:

• Weathertight.
• Reaction to fire – generally BROOF(t4) to BS EN 13501-5, though building regulations may permit lesser performance in certain locations. Roofing tested to only BS 476 should not be accepted. In some instances, as dictated by building regulations, non-combustible insulation must be used.
• Loadings (permanent and imposed) – compliant with BS EN 1991, including to resist wind loads applicable to location.
• Movement.
• Thermal performance.
• Condensation – must not form internally of the waterproofing membrane or interstitially.
• Sustainability.
• Acoustics – provide required level of sound reduction.

Consideration should be given to any potential planning restrictions/ requirements (may affect ‘surfacing’ requirements/ stipulations).

Thought should be given to fixtures and fittings interfacing with the roof, required in the broader building design, and how these need to be secured/ fixed, to installation of PV panels. Any breach of the waterproofing membrane should be minimised and appropriately detailed.

Similarly, roof penetrations should be minimised. Penetrations to be sized and coordinated such that they can be accommodated by appropriate detailing

Key contacts

Other members of the design team should be liaised with as appropriate in a range of instances (list below is not exhaustive).

Structural engineer

• To ensure that substrate/decking is also accurately specified, including but not limited to materiality: concrete (in-situ/precast), profiled metal with plywood overlay or concrete infill, plywood decking). Grades of materials finishes need to be correct for the intended purpose in terms of structure and also to receive the necessary roof construction. All compliant with appropriate standards for the associated materials.
• Appropriate gradients/falls will need to be achieved. No ponding should occur, with consideration given to deflections, coordinated with rainwater drainage.
• Understand where primary building construction/movement joints occur so appropriate detailing can be undertaken without compromising performance.
• Ascertain loads applied to the works when in use: this is a performance criterion that the roof system needs to achieve.  

Services engineer

• Rainwater drainage strategy needs to be considered early in the design phase, comprising the following options and need for integration and detailing.
• Rapid drainage through gravity or syphonic action, compliant with BS EN 12056 and BS 8490.
• Attenuation via ‘blue roof’ principles. Compliant with NFRC Technical Guidance Note for the Construction and Design of Blue Roofs, published by The National Federation of Roofing Contractors and BS 8582.
• Integration of photovoltaic panels and similar type fixtures and fittings.
• Penetrations of services. Refer to points regarding this raised earlier.

It’s important that installation shall be by operatives recommended/ approved by and trained by the system manufacturer, to maximise quality control through trusted partnerships.

The components of the entire assembly and full installation need to be covered by a single-source warranty, with approval from the manufacturer for all materials used in the works. The warranty should include an insurance company backing, paid as a single premium at the start of the policy. Guarantees shall be jointly provided by both the manufacturer and the contractor.

Flat roof specifications checklist

• What is the design responsibility? Will be a performance (descriptive) specification based on a system, or fully designed (prescriptive)?
• Is the proposed system pre-tested and certified for the conditions of use intended?
• Has input and advice been sought and received from manufacturers?
• Any there any planning restrictions/requirements.
• What is the surface finish?
• What are the performance requirements (fire/ acoustic/ wind loadings/ sustainability).
• Can client warranty requirements be achieved?
• Have quality control requirements been cleared described?
• Have all interfaces and co-ordination with other disciplines been considered?

SpecStudio is an independent specification writing team delivering specifications for architectural and design practices