Roof Coverings

A roof covering is the weathering layer on a pitched or low-pitched roof: profiled and standing-seam metal sheet, clay and concrete tiles, natural and fibre cement slate, asphalt and timber shingles, and traditional fully supported metals (lead, copper, zinc). Flat roof membranes have their own page. The covering works with what lies under and around it: battens or purlins, sarking or underlay, flashings at walls and penetrations, ridge and hip cappings, valleys, and the gutters it drains to. Most leaks arrive at those junctions rather than through the field of the roof.

What shortens life:

• Coastal, industrial and geothermal atmospheres, especially on unwashed surfaces under eaves and laps where salt is not rinsed off by rain.
• Low pitch and ponding at laps, end laps and behind penetrations.
• Dissimilar metals: copper or lead run-off onto zinc-aluminium steel, steel fixings in aluminium, inert catchment debris.
• Condensation on the underside of metal roofs over humid spaces: pools, cool rooms, laundries, uninsulated sheds.
• Foot traffic to plant and solar arrays; plant mounted directly on sheeting.
• Heavier replacement coverings on structures designed for lighter ones, such as concrete tiles replacing slate.
• Neglected gutters and valleys that back water up under the covering.

Coated metal corrosion

Sheet steel corrodes from the edges and laps inward. The coating chalks and loses gloss, then cut edges, end laps and fixing holes show red rust, then the sheet perforates where water and debris sit, usually in the bottom metre above the gutter, at end laps and around penetrations. Underside corrosion from condensation can perforate a roof that looks sound from above.

Fixing failure

Screws back out under thermal cycling, washers perish, and the hole elongates. Lifting sheets then admit water and can be torn off in wind. Concealed clips on standing-seam roofs can bind and tear seams if thermal movement was not allowed for.

Tile roof failure

Tile bodies rarely fail. What fails is the system around them: ridge and hip cappings lose their bedding and pointing and become loose; valley irons corrode; tiles slip when nails or battens fail; broken tiles go unreplaced; sarking tears and deteriorates. Concrete tiles erode at the surface and become porous; poor-quality terracotta spalls in frost.

Slate failure

Slates fail by "nail sickness": the iron nails rust through and slates slip out of place, usually scattered across the roof rather than in lines. Poor slate delaminates and flakes. Battens decay where the underlay has failed. Fibre cement slates curl, crack and delaminate.

Asbestos cement weathering

The cement matrix erodes and lichen roots into it, exposing fibre on the surface. Sheets become brittle and crack at fixings; they cannot bear a person's weight at any age. Coating can extend the life of sound sheet but not of eroded sheet, and pressure cleaning is prohibited.

Lead, copper and zinc

Lead creeps and cracks by thermal fatigue when bays are oversized or fixed too rigidly; it corrodes from the underside where condensation forms, and white carbonate appears at laps. Zinc corrodes from the underside where unventilated. Copper is durable but its run-off attacks other metals.

Asphalt shingle ageing

Shingles lose their mineral granules, curl at edges, crack, and lose wind resistance as the sealant strips age. Hail bruises them. The end is usually determined by an insurance inspection rather than a leak.

Flashings and junctions

Wall flashings pull out of their chases; apron and step flashings lift; valley gutters overflow; penetration flashings crack around pipes and plant. These junctions are where most roof leaks start.

From the street and from aerial imagery

• Colour change across bays, patches in a different material, rust at laps and bottom edges, sections already replaced.
• Slipped, missing or broken tiles and slates; scattered slipped slates (nail sickness); lines of slipped tiles (battens).
• Ridge and hip cappings with missing pointing or out of line; lichen and vegetation in valleys.
• Grey, lichen-covered corrugated sheet on industrial roofs of the relevant era: likely asbestos cement.
• Sagging ridgelines and roof planes (structure; see Roof Structure).
• Debris in valleys and gutters; staining under gutter lines.

On the roof

• Fixings: backed out, rusted, missing washers; elongated holes.
• Lap condition: corrosion at end laps and side laps; perforations at the gutter edge.
• Flashings lifted, chases open, sealant failed.
• Tiles cracked under foot traffic routes; loose cappings.
• Surface erosion of concrete tiles; fibre exposed on asbestos cement.

In the roof space and below

• Daylight through the covering; rust and perforation on the underside of sheets.
• Condensation drips and staining on the underside, uniform across the roof in cold weather.
• Water staining on battens, trusses and ceilings, localised under defects.
• Torn or missing sarking and underlay.

In the records

• Re-roofing dates and extents; warranties.
• Leak register; roof maintenance and gutter cleaning records.
• Hazardous materials register entries for the roof.
• Insurance inspections and storm claims.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A roof plumber, roofing contractor or building surveyor with safe access inspects. For large metal roofs and asbestos cement roofs, the inspection is planned around fragility and edge protection.

• Aerial imagery and drone survey for extent, material, patching and the pattern of corrosion across bays.
• Close inspection of laps, fixings, flashings, penetrations, valleys and the gutter edge, with sample fixings removed.
• Underside inspection from the roof space or plant deck for corrosion, condensation and daylight.
• Coating thickness readings where a coating or restoration is being considered.
• Tile and slate sampling: lift and examine fixings and battens; check sarking.
• Hazardous material sampling of sheeting, felt and mastics where the era warrants.
• Water testing of suspected leak points.

The report should give an extent plan, remaining life by zone, the junctions needing work now, any fragile areas, and the hazardous materials position.

• Repairs: replace failed fixings with oversized screws, reseal laps, replace individual sheets or tiles, rebed and repoint ridges, renew flashings.
• Coating or restoration of sound but weathered metal sheet; not an answer to perforation or failed fixings.
• Partial replacement of the failed bays, often the lower run above the gutter or the coastal-facing slope.
• Full replacement, with new battens or purlins where needed, sarking, flashings, safety systems and gutters.
• Re-slating with new stainless or copper nails and reused or new slates.
• Asbestos cement replacement by licensed removal, with edge protection and safety mesh during works.
• Condensation control: vapour control and insulation below metal roofs over humid spaces.

Interactions: re-roofing triggers insulation upgrades in some jurisdictions, renewal of roof safety systems, and replacement of gutters and flashings; it is the natural time to deal with solar mounting and plant penetrations.

Inspection is a lump sum. Repairs are itemised per sheet, per tile area, per linear metre of ridge or flashing. Replacement is priced by roof area for the covering, with separate lines for battens or purlins, sarking, flashings, safety systems and gutters. Asbestos cement replacement adds licensed removal, air monitoring and disposal. Access and edge protection are significant on any roof.

Where condition is unverified, the usual shape is two-part: inspection first, then a replacement envelope that falls away if the inspection finds the covering sound. Replacement is lifecycle; repairs are repairs and maintenance; asbestos removal carries a code compliance element.

• Australia. The National Construction Code calls up AS 1562.1 for metal roof cladding, AS 2050 for roof tiles and AS/NZS 4200.1 and 4200.2 for sarking; wind actions follow AS/NZS 1170.2, with much heavier fixing in cyclonic regions. Asbestos cement roofs fall under the work health and safety asbestos regulations, with licensed removal above set quantities; pressure cleaning of asbestos roofs is banned in several states.
• United Kingdom. BS 5534 governs slating and tiling for pitched roofs; lead sheet work follows the industry's rolled lead manual and BS EN 12588. Renovating a large part of a roof is a controlled service under the Building Regulations, and Approved Document L requires the thermal performance to be upgraded where more than half of the element is renovated. Listed and conservation-area roofs carry material constraints.
• United States. International Building Code Chapter 15 governs roof assemblies, with Section 1512 setting when a roof may be re-covered over an existing one; asphalt shingles follow ASTM D3462 and wind uplift ratings; hail resistance is rated under UL 2218. Insurers increasingly set the replacement point for shingle and metal roofs through age-based underwriting.

• Date and extent of the last re-roofing or sectional replacement, with invoice or warranty.
• Roof maintenance and gutter cleaning records.
• Leak register and any leak investigation report.
• Hazardous materials register entries for roof sheeting, felts and mastics.
• Roof plan showing penetrations, plant, solar and the safety system.
• Product and coating specification for metal roofs.
• Insurance roof inspections and storm claims.