A salvaged metal roof is inspected for rust, lifted fasteners and gaps at flashing, first from underneath for daylight and stains and then from a crawl board laid across the framing. Heating and cooling make a 6 m steel sheet grow and shrink about 4.3 mm over a 60 K swing, which slowly loosens fasteners and opens laps, so tighten and re-seal each year and scrape and coat rust spots before they perforate.
Part of Building Maintenance Schedules
Metal roof inspection is the routine check of sheet roofing, its fasteners, laps and flashing, for corrosion, movement and water entry.
Metal Roofs Fail At The Joints Before They Fail In The Sheet
The maintenance guide gives the metal checks: rust, lifted fasteners and gaps at flashing points in spring, then in summer tightening or replacing loose fasteners and re-sealing flashing at chimney penetrations, ridges and wall junctions. Salvaged metal is the fire guide’s safest roof, since it cannot burn (Fire-Resistant Building Materials). The weak points are therefore the places where sheets are joined to the frame, to each other and to the walls, not the sheets themselves.
Worked Example: How Much A Sheet Moves
Linear expansion is ΔL = α × L × ΔT. The coefficient for steel, about 12 × 10⁻⁶ per K, and for aluminium, about 23 × 10⁻⁶ per K, are standard handbook values recalled, not read from this corpus. The temperature swing of 60 K is assumed for a roof that is cold on a winter night and heated by the sun in summer; roof sheets in sun are well above the air temperature.
- Steel, 3 m sheet: 3,000 mm × 12 × 10⁻⁶ × 60 = 2.2 mm.
- Steel, 6 m sheet: 6,000 × 12 × 10⁻⁶ × 60 = 4.3 mm.
- Aluminium, 6 m sheet: 6,000 × 23 × 10⁻⁶ × 60 = 8.3 mm.
A fastener driven tightly into a hole in a sheet that moves 4 mm twice a day works loose, elongates the hole and opens the lap. A sheet clamped hard at every purlin cannot move at all and tears or buckles instead. That is why fasteners should sit in slightly oversize holes with a washer and be left snug, not crushed, and why the guide’s yearly tightening matters.
What You Need
- A crawl board and a footed ladder
- A wire brush or scraper, and tar, pitch or oil paint for rust spots (Roof Sealing)
- Spare fasteners and washers, and fabric strips for re-sealing flashing
- A pointed tool for probing the purlins beneath
- A second person and the log
Method: Look From Below, Then From The Board
Step 1: From inside, look for daylight at laps and fasteners and for stains on the framing.
Step 2: From the ground, look along the roof for lifted edges, buckled sheets and open ridge caps.
Step 3: On the crawl board, check each fastener line for loose heads, elongated holes and rust streaks.
Step 4: Scrape rust to bright metal, brush it off and coat it with tar, pitch or oil paint before it perforates.
Step 5: Re-seal flashing at chimneys, ridges and wall junctions with tar and fabric.
Step 6: Tighten or replace fasteners, snug and not crushed.
Step 7: Check that a ventilation gap remains below the sheets, since trapped moisture rots the framing (roof guide).
Common Mistakes
- Walking between the purlins. The sheet is unsupported there and can buckle or tear; step over the purlins.
- Over-tightened fasteners. They crush the washer and tear the sheet as it moves.
- Coating over live rust. It keeps rusting underneath.
- Mixing metals such as copper or lead flashing on steel or zinc sheet, which corrodes the more reactive one where water lies between them.
Safety
Edges, falls and hot tar
Cut sheet edges slice skin; wear gloves. Wet, frosted or dusty metal is slippery and a fall from a roof is serious. Step only over purlins that you have probed and trust, and never work alone. Tar is flammable and hot tar burns badly; heat it away from the roof, away from thatch and away from the building, and keep sand at hand (Fire Safety & Suppression).
What’s Next
- Learn next: Earth Roof Care — the third kind of roof
- Combine with: Leak Detection & Emergency Patching — when a lap lets water in