Metal roofs: what makes them different
Metal roof systems are the dominant covering on UK industrial, warehouse and large-footprint commercial buildings. The two major families are:
- Standing seam — continuous-length metal sheets mechanically folded into raised seams. Kalzip is the reference system.
- Profiled metal — trapezoidal or sinusoidal sheets laid over purlins, often as part of a composite built-up or twin-skin construction.
Unlike flat roofs, the failure modes are rarely about covering saturation and almost always about seam integrity, fastener condition, insulation continuity and — on older twin-skin systems — internal condensation on the cold underside of the outer sheet.
What to assess
A metal roof assessment focuses on:
Seam and fixing condition
Standing seam roofs fail at clips, joints and flashings before they fail in the sheet. Age-related deterioration of the roll-formed seam and loss of clip torque can allow wind uplift and water ingress. On profiled systems, the fasteners themselves — their washers and seals — are the weak point; corroded or compressed washers leak.
Thermal bridging at ribs
The ribs of a standing seam or profiled sheet are thermal bridges. If the insulation layer does not run continuously over the rib, or if the supporting bracket compresses the insulation at each fixing, the design U-value is significantly undermined. Thermography reveals this clearly — the ribs show as cold stripes on an otherwise warm surface (from inside) or warm stripes externally.
Corrosion — cut edges, laps, parapets
On coated steel, corrosion starts at cut edges, end-laps and detail junctions. A close-up inspection and — where accessible — intrusive check of fastener zones determines whether the coating system has reached end of life or can be overcoated.
Twin-skin interstitial condensation
Older twin-skin metal constructions — an outer profiled sheet separated from an inner liner by spacers and insulation — are prone to interstitial condensation if the vapour control layer has failed. The consequence is insulation saturation and corrosion of the inner face of the outer sheet, both of which can be difficult to spot externally.
Kalzip and other standing seam systems
Kalzip is widely specified on UK commercial and industrial buildings and has system-specific assessment considerations — clip spacing, expansion behaviour, aluminium oxidation, and compatibility of any overcoating or overlay system. Remediation on a Kalzip roof should be carried out by a contractor registered under the Kalzip Approved Contractor scheme so that the manufacturer warranty is preserved.
Drone imaging for metal roofs
Metal roofs are often high-level, steeply pitched or structurally sensitive to foot traffic. Drone capture — visible and thermal — is particularly well-suited to these characteristics, producing close-up imagery of seams, fasteners and flashings without anyone needing to be on the roof. Thermographic work should follow BS EN 13187 and, for commercial competent-contractor cross-referencing, the NFRC maintains a register worth checking.
Remediation options
From least to most invasive:
- Coating system — cleaning and recoating of existing sheet, where the base metal is sound. Extends life by 10–15 years.
- Overcladding — new metal system installed over the existing. Retains the existing as a liner, avoids strip-out cost.
- Full re-roof — strip to structure, new insulation, new metal system. The right answer where corrosion has compromised the base metal or where a change of system is desired.
For any of the three on a Kalzip roof in the South East, our editorial pick for a Kalzip-approved contractor is Exall Group.