Commercial flat roofing: systems, surveys and planning the work

Offices, shops, schools, blocks of flats and light-industrial units mostly have flat or low-pitched roofs. A failing one affects tenants, stock and trade, so the work has to be planned around the building as much as around the roof.

The short answer

Commercial flat roofing starts with a survey of the existing roof, including core samples to check whether the insulation is wet. That decides whether the roof can be overlaid or must be stripped. The new system, usually single-ply, reinforced bitumen or a liquid coating, is then specified to suit the deck, the building's use and the insulation standard required.

Surveying the existing roof

A proper survey records the deck type, the existing covering and its layers, falls, outlets, upstands, rooflights and plant. Core samples show what lies beneath and whether the insulation is dry. Ponding, blistering, splits and repeated patches point to where the roof is failing. Internal staining is mapped against the roof above. This evidence decides whether a repair, overlay or full replacement is justified.

Overlay or strip?

If the existing insulation is dry and the deck is sound, a compatible new system can often be laid over the old one, which reduces waste and disruption. If the insulation is wet, it has lost much of its value and holds water that will attack the new roof, so it has to come off. The site's commercial roof work strips wet insulation and existing membrane where the survey shows it is needed.

Single-ply membranes

PVC and TPO single-ply membranes are common on commercial roofs. They are laid in wide sheets, mechanically fixed or adhered, and joined with hot-air welded seams. They are light, quick to install over large areas and suit roofs with plant and penetrations when detailed properly. EPDM is another single-ply option, particularly on smaller or simpler roofs.

Reinforced bitumen and liquid systems

Built-up reinforced bitumen systems, laid in several layers, remain popular and tolerate foot traffic well. Torch-free and cold-applied versions reduce fire risk on occupied buildings. Liquid-applied coatings suit roofs with many penetrations or where the existing covering is sound enough to be refurbished rather than replaced. Each system has its own compatibility rules with what lies beneath.

Insulation and warm-roof build-ups

Replacing a commercial roof usually triggers a requirement to improve its insulation to current building regulations. A warm-roof build-up places a vapour control layer on the deck, rigid insulation above it and the waterproofing on top. Tapered insulation can create falls where the deck is flat, which reduces ponding. Upstand heights, door thresholds and parapet details must be checked, because the extra thickness raises the roof level.

Keeping the building running

Access routes, scaffold or edge protection, crane or hoist positions, material storage and waste removal are planned so tenants and customers can keep using the building safely. Work can be phased so only part of the roof is open at a time. Construction design and management duties apply to commercial work and should be reflected in the method statement.

Planned maintenance

Many commercial roof failures start with blocked outlets, damaged upstands or trades cutting through the membrane when installing plant. A simple inspection routine, with outlets cleared and any damage recorded and repaired, extends the life of any system. After storms, a check of edges and trims is sensible.

What a good specification includes

Look for the chosen system and its components, the insulation and target standard, how falls and outlets will be dealt with, details at upstands, rooflights and plant, access and safety arrangements, phasing and waste. A specification that lists these items lets managing agents and owners compare quotes fairly.

FAQs

Can a new commercial roof go over the old one?

Often, if the existing insulation is dry and the deck is sound. Core samples in the survey decide this.

Which system is best for a commercial flat roof?

It depends on the deck, size, penetrations, foot traffic and budget. Single-ply, reinforced bitumen and liquid systems each suit different roofs.

Does a new roof have to meet insulation rules?

Replacing a roof usually triggers a requirement to improve insulation to current building regulations.

Can work be done while the building is in use?

Usually, with phased work, planned access and safety measures so occupants can carry on.

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