How to Survey a Commercial Flat Roof

Surveying a flat roof properly – and understanding what you are looking at when you get up there – is one of the most important skills in commercial building management and surveying. This guide covers the full process: from preparation before you leave the office, through the on-site visual inspection and core sampling, to materials and deck identification, insulation assessment and the environmental considerations that should inform your specification.

It was written by our Managing Director, drawing on over 45 years of practical experience in commercial flat roofing across London and the Southeast. Whether you are a building surveyor approaching a flat roof condition survey for the first time, or an experienced professional looking for a structured reference, it covers the detail that matters.

Already past the survey stage? Our guide to flat roof replacement covers systems, costs and what to expect in detail.

 

 

Before You Go to Site: Preparing to Survey a Flat Roof

Get a thorough brief

Before agreeing a date for your flat roof survey, make sure you have a clear brief. That means understanding the client’s long-term plans for the building. The advice you give will differ considerably depending on whether the property is to be redeveloped in five years, let on a long-term lease, or maintained as a long-term asset.

Find out all you can about the roof’s history – the problems the client and tenants have experienced, and obtain copies of any previous reports, quotations and details of repairs carried out. This background context is often as informative as the survey itself.

Desktop preparation

Use Google Earth or similar tools to familiarise yourself with the building before visiting. Prepare preliminary roof plans where possible. Establish which roof areas are to be surveyed and how they relate to the building as a whole.

Ensure you are properly equipped before leaving the office: appropriate PPE, ladders, keys, a core sample kit, a digital camera, measuring equipment and writing materials. Arriving prepared saves time on site and enables you to give your client the best possible advice.

Health and safety

Falls are the leading cause of death and injury for construction workers. You should not survey a flat roof unless you are competent to do so, or are being accompanied by a competent person. Always carry out a risk assessment before going up, and ask the client for input on site-specific hazards.

Key rules when carrying out a flat roof inspection:

• Let someone know when you are going onto the roof and agree a check-in time. If they don’t hear from you, they should call – and if they can’t reach you, raise the alarm.
• Never inspect a flat roof in high winds.
• Be extremely careful in wet or icy conditions. Postpone if the roof is slippery – a small patch of ice or wet membrane is all it takes.
• Ensure the roof structure and deck are safe before walking across them.
• Never walk backwards, and never approach the edge, parapet walls or rooflights without first getting down on your hands and knees.
• Never walk outside safety railings or barriers.
• Look out for rooflights that may have been painted over or obscured by dirt – they may not support your weight.
• Avoid trip hazards including cables, safety lines and raised fixings.
• Stop to make notes – do not walk and write simultaneously.
• Keep all tools and equipment away from the roof edge. Nothing should be left where it could be blown or knocked off.
• Take care around electrical cables and aerials. Establish whether any masts or transmitting equipment need to be turned off before you access the roof.

 

The On-Site Process: Surveying a Flat Roof

What a visual inspection tells you

A thorough visual flat roof inspection will identify existing defects, inform what system and design should be specified, and reveal factors that will affect how the work is carried out. Look carefully for:

• Ponding water or evidence of it (tide marks and staining). This may indicate inadequate falls, collapsed decking or insulation, or blocked outlets. The cause needs to be established before specifying a remedy.
• Holes, splits, blisters or rippling in the roof surface – all signs of fatigue and failure in the waterproofing membrane. Understanding what caused them ensures the new system does not fail in the same way.
• Plant, pipework, ducting and cables that are not sufficiently raised above the roof surface. These obstruct drainage, cause moss and debris build-up, and lead to premature localised failure.
• Details and junctions. Flat roofs most commonly fail at penetrations and where roof planes meet – at parapet walls, upstands and abutments. Examine all details carefully.
• Water-filled blisters, which indicate trapped moisture and almost always mean the waterproofing system has failed beyond repair.

Core sampling

Core samples are one of the most important tools available in a flat roof survey. A well-taken core sample can establish:

• The existing roof build-up – the number of layers, and the materials used for waterproofing, insulation and vapour control. On older buildings there may be several systems laid on top of each other over the years.
• What the deck is made of – which requires experience across the wide range of materials used historically and today.
• How the falls are created – whether by the structure itself, by a screed, or by tapered insulation.
• Whether moisture is trapped within the build-up. Trapped moisture will almost always require the existing system to be stripped out entirely before a new one is installed.
• The cause of blistering or deflection in specific areas – for example, interlayer blistering within a mastic asphalt membrane rules out the option of overlaying.

When approaching a building survey of a flat roof, core sampling is one of the most important tools available – but it exposes only a small area of the roof. The more samples taken, the more accurate the overall picture of condition. Relying on a single core can be misleading on larger roofs.

Further assessment points

• How will the work be carried out safely? Consider access, working at height arrangements and risks to people below.
• Can the existing system be overlaid, or must it be stripped? Retaining and overlaying an existing system reduces cost, landfill and carbon impact – and provides additional waterproofing redundancy during works. However, not all existing systems are suitable for overlaying.
• Is there insulation, and does it comply with current Part L requirements? Can it be incorporated into the new system?
• If overlaying, what is the condition of the existing surface? What pre-treatment will be needed?
• Consider the building’s use – particularly where industrial processes or condensation risk (swimming pools, food production) are present.
• If additional insulation will raise the finished roof level, check that minimum upstand heights under building regulations will still be met. Plant, rooflights and other structures may need to be raised or adapted.
• Check soffits, fascias and roof ventilation.
• Identify plant that may obstruct access. Establish whether it can be temporarily moved or disconnected. Any works affecting gas flues must be carried out by a Gas Safe Engineer. Hot flues require protective shields.
• Check for telephone masts, satellite dishes and aerials. Strong signals can be harmful – establish the equipment and providers and confirm whether it needs to be turned off during works.
• Record the condition of all associated roof structures – masonry, doors, windows, skylights.
• Identify and measure all outlets.

Relevant regulations

All flat roofing work must be specified and carried out in compliance with current regulations and codes of practice. The key references are:

• BS 6229:2018 – Flat roofs with continuously supported flexible waterproof coverings: Code of practice
• Building Regulations Approved Document Part L (2021 edition, effective 2022) – thermal performance requirements
• The NFRC Safe2Torch campaign – fire safety requirements for torch-applied systems

As NFRC Competent Person Scheme members, Maguire Brothers can self-certify qualifying flat roofing works under building regulations, removing the need for separate local authority building control involvement.

 

 

Flat Roofing Systems – What You Will Encounter

A thorough knowledge of flat roofing systems – both current and historic – is essential to interpreting what you find during a flat roof survey and specifying the right replacement or repair solution.

Single ply membrane

The most common single ply systems are EPDM rubber and PVC. They offer an economical and fast solution on large, relatively simple roof areas and are widely specified on new build. Because there is only one waterproofing layer, a small failure can have significant consequences – particularly where regular maintenance access is expected or where future tenants may install plant without understanding the membrane’s characteristics.

Single ply membranes arrive in large rolls to minimise joints, but lap seams remain the system’s principal vulnerability. Joints are most commonly heat-welded and require a precise three-part weld – any imperfection can cause failure. Hot air guns present a fire risk, though lower than naked flames.

High-performance built-up felt

Modern high-performance built-up felt systems use polyester-reinforced carriers and modified bitumen, which substantially improves their performance over older felt systems. The multi-layer build-up means the system is far more tolerant of occasional access and maintenance than single ply, and the lap seals are less dependent on exacting installation conditions.

Many modern felt systems are available with flame-free application options, which is particularly important at junctions with cladding or pitched roofs. These are generally the most economical of the main flat roofing systems.

Cold-applied liquid waterproofing

Liquid-applied systems are applied without heat. They are seamless and highly adaptable – capable of being applied under low-lying plant and around complex details that would be difficult to waterproof with roll-applied systems. The seamless finish means there are no joints to fail, and the system is straightforward to repair or extend when plant is added or changed.

Material is delivered in small drums, making it suitable for transport via lifts rather than requiring hoists or scaffolding. Where heat or naked flames are a concern, cold-applied liquid systems are an excellent alternative. They are generally one of the more expensive options.

Mastic asphalt

Mastic asphalt is a hot-applied liquid system, melted and applied at temperatures up to 230 degrees centigrade. This limits its use in confined spaces, under low-lying plant or in narrow gutters. Like liquid systems it handles complex details well, but unlike them it is unreinforced and does not cope well with movement – it performs best on stable substrates.

Mastic asphalt is ideal where occasional foot access is required – it can also be used for car parks and walkways. It becomes soft in hot weather and should be coated every three to four years with solar reflective paint to reduce UV degradation. Typical lifespan is 7.5-15 years depending on exposure. It is one of the more expensive systems.

Fibreglass

Fibreglass resin systems are encountered primarily on small domestic roofs – extensions, porches and similar. They have not established themselves in the commercial flat roofing market for large roof areas. If you encounter fibreglass on a commercial building it is likely on a small secondary element rather than the main roof area.

 

What You Will Find When Surveying a Flat Roof: Decks, Screeds and Insulation

Insulation and Part L

Under current building regulations, when more than 25% of a roof area is being refurbished or replaced, the insulation must be upgraded to meet the thermal performance requirements of Part L (2022). This is a legal requirement, not a recommendation.

There are three main insulation configurations used in flat roof construction:

• Warm roof: the most common approach in commercial refurbishment. Rigid urethane insulation boards are laid over a vapour control layer on the roof deck, with the waterproofing on top. This keeps the insulation within the warm zone of the structure, avoiding condensation risk. Warm roof insulation can be manufactured with a built-in gradient to improve falls – these are tapered or cut-to-falls schemes.
• Cold roof: insulation between ceiling joists with the roof void ventilated above. The risk of condensation in the roof void is significant and this configuration is very rarely appropriate for commercial flat roof replacement.
• Inverted roof: the insulation (typically extruded polystyrene) sits on top of the waterproofing, held in place by ballast. Common in new build but rarely practical in refurbishment. The insulation and waterproofing both get wet in use, requiring greater thickness to achieve equivalent thermal performance.

Structural deck types

Assessing the structural deck is a critical part of any flat roof survey. The deck must be capable of supporting all imposed loads and providing adequate falls for drainage. If the roof covering is being replaced, the suitability of the existing deck must be formally assessed.

The following are the deck types most commonly encountered in commercial buildings:

• In-situ concrete: sometimes cast with a fall, or laid level with falls created by a screed or tapered insulation. On existing buildings these will have cured fully, though remedial drying action may be needed where decks have been saturated by leaks.
• Pre-cast concrete: various plank and block systems set between concrete beams. Falls provision methods and provisos are broadly the same as for in-situ concrete.
• Plywood and OSB: suitable for flat roof decking provided the correct structural grades are used. All edges must be supported and noggins installed where required. Falls are typically created using tapered firring pieces in the structure below.
• Chipboard: not suitable for flat roof decking and should be replaced if encountered. It degrades rapidly when wet and provides inadequate structural support.
• Woodwool slabs: boards made from compressed wood strands bonded with cement, popular from the 1950s to the 1990s. Woodwool slabs are now considered fragile construction materials, particularly when wet following roof leaks. They should either be replaced or over-sheeted. Risk assessments are required when working on roofs that incorporate them.
• Stramit board: compressed straw boards with paper facings, installed extensively during the 1950s and 1960s. Stramit board loses all structural integrity when wet, and removal destroys the upper face. Any stramit board encountered should be replaced. Risk assessments are required.
• Profiled metal decking: can be laid flat or to falls, which should be established during the survey. The corrugated profile means the deck does not provide a continuously flat surface – the crowns should represent at least 50% of the roof area for adequate bonding. The vapour control layer is laid across the crowns and must not be damaged by foot traffic in the troughs during installation.

Early post-war flat roofs frequently used chipboard, Stramit board and woodwool slabs. Any of these may be encountered when surveying buildings from the 1950s to 1990s. Particular care is required during removal due to fragility and the risk of partial collapse under load.

Screeds

Screeds are used to create falls on existing roof decks and to provide a surface for waterproofing systems. When surveying, establish what type of screed is present, whether it is creating the falls, and its current condition.

• Sand and cement screeds: generally robust. Once old waterproofing has been removed, the new system can usually be installed directly on top.
• Lightweight screeds (perlite, expanded clay, sintered PFA): usually laid to falls. Water drains through the material, making it difficult to establish whether the screed has been wetted by roof leaks. Moisture trapped at deck level causes blistering of new waterproofing. Lightweight screeds are also difficult to strip without damage, often making full replacement the only practical option.
• Lightweight aerated screeds: a grey, breeze-block coloured material made from Portland cement, water and foaming emulsion. Good insulation properties but difficult to dry once wet. Great care is needed during removal of existing waterproofing – if the screed is well bonded and dry, it may be preferable to leave it in place.

Where all screeds are saturated, replacement is likely to be the only viable option. Screed vents can be installed to allow trapped moisture to escape gradually, but they will not dry out a saturated screed completely.

 

 

Environmental Considerations

Strip or overlay?

When surveying a flat roof and specifying the replacement, the greatest environmental consideration is frequently disposal of the existing covering. Where existing waterproofing is suitable for overlaying, retaining it reduces landfill, carbon impact and cost – and the additional layers provide further redundancy during the works period.

The longer a roof is left to deteriorate, the less likely it is that the existing covering will be suitable for overlaying, and the greater the risk of damage to the deck structure below. Addressing a failing roof early almost always reduces the total environmental and financial impact.

Specifying for whole-life performance

Longer-lasting flat roofing systems must be replaced less frequently over the building’s life, reducing both environmental impact and whole-life cost. The distinction between the guarantee period and the system’s realistic lifespan is important – these are not the same thing, and specifying on the basis of guarantee period alone can lead to poor value decisions.

Thermal performance and Part L

Upgrading insulation as part of a flat roof refurbishment is not just a regulatory requirement where it applies – it reduces the building’s carbon emissions and lowers the client’s energy costs. These are material benefits to include in any advice to building owners and managers.

Access and trafficking

All flat roofing systems require protection where regular access is needed – for maintenance, plant operation, or use as a terrace or balcony. The level of protection required depends on the frequency and nature of access. Edge protection and anti-slip measures must also be considered, particularly with single ply and cold liquid-applied systems in wet conditions.

Maintenance

A modest, well-planned maintenance programme can significantly extend the life of a flat roof and reduce the risk of premature failure. Regular inspection – at least annually and after severe weather – and prompt attention to minor defects are the most cost-effective investment a building owner can make in their roof.

Future-proofing – solar PV

Commercial roofs are increasingly being recognised as assets capable of generating income as well as protecting the building below. Solar PV installations can deliver a meaningful return on investment, particularly for businesses with high energy consumption. A flat roof replacement is the ideal time to assess solar integration – installing a PV-ready specification at the outset is far more economical than retrofitting later.

Future-proofing – green roofs

For buildings in urban settings, or where a roof may form part of a recreational or educational facility, a green roof can deliver significant additional benefits – stormwater attenuation, biodiversity, improved thermal performance and occupant wellbeing. Green roof specifications are growing in planning requirements across London and the South East and are worth raising with clients at the survey stage.

 

Working with Maguire Brothers

This guide was written by our Managing Director, drawing on over 45 years of experience in commercial flat roofing across London and the Southeast. Maguire Brothers are specialist commercial roofing contractors based in Surbiton, Surrey, established in 1979.

We work closely with building surveyors, facilities managers and property managers on all aspects of surveying a flat roof and managing the subsequent works – from independent condition surveys and technical input at specification stage through to full end-to-end project delivery. Our surveys are detailed, clearly written and supported by photographic evidence. Our advice is always manufacturer-neutral.

If you have a flat roof survey or project you would like to discuss, we would be pleased to help. Contact us on 020 8942 2324 or through our website.

 

Frequently Asked Questions

What is involved in surveying a flat roof? A thorough flat roof survey covers: pre-site preparation including desktop review of the building and its history; a health and safety risk assessment; visual inspection of the waterproofing, drainage, details and junctions; core sampling to establish the existing build-up, deck type and moisture content; assessment of insulation compliance; and identification of all factors that will affect specification and procurement. A written report with photographic evidence and a clear recommendation should follow.

What are the most common problems found in a flat roof inspection? The most common defects found during a flat roof inspection are: ponding water caused by inadequate falls or blocked outlets; membrane splits, blistering or delamination; failed seals at penetrations, upstands and parapet junctions; trapped moisture in the insulation or screed; and deck deterioration – particularly in older buildings with chipboard, woodwool or Stramit board decks.

What is a Stramit board and why does it matter in a flat roof survey? Stramit board is a compressed straw panel with paper facings, used extensively as flat roof decking in the 1950s and 1960s. It loses all structural integrity when wet, which makes it a serious concern on any building of that era. If stramit board is present and has been exposed to water ingress, it must be replaced rather than overlaid. A risk assessment is required before working on roofs that contain it.

What are woodwool slabs and are they a problem? Woodwool slabs are flat roof deck panels made from compressed wood strands bonded with cement slurry, popular from the 1950s to the 1990s. They are considered fragile, particularly when wet, and must be treated with care during survey and any subsequent works. They should be replaced or over-sheeted rather than used as a substrate for new waterproofing without full structural assessment.

Can a flat roof be overlaid rather than stripped? Yes, in many cases. Where the existing waterproofing is well bonded, the deck is sound and dry, and the build-up is suitable, overlaying is often the most cost-effective, lowest-disruption and most environmentally responsible approach. The key test is whether moisture is trapped in the existing build-up – a saturated system must be stripped. Core samples are the most reliable way to establish this.

What insulation is required when replacing a commercial flat roof? Under Building Regulations Part L (2022), when more than 25% of a roof area is replaced or refurbished, insulation must be upgraded to meet current thermal performance requirements. In practice this means installing a warm roof build-up with sufficient thickness of rigid insulation to achieve the required U-value. Tapered insulation can be used to improve falls at the same time.

How long does a flat roof survey take? This depends on the size and complexity of the roof and the number of core samples required. A straightforward survey of a single roof area of up to 500m2 might take two to three hours on site. Larger or more complex buildings – multiple roof levels, extensive plant, restricted access – will take longer. Allow additional time for report preparation.