Why Warehouse Roofs Leak: Laps, Skylights and Penetrations

The roof that leaks is the roof that was detailed last

A leaking warehouse roof is usually blamed on the sheet: the gauge was too thin, the coating was cheap, the supplier was wrong. In practice the sheet is rarely the path. Warehouse roof leakage happens at the places where the sheet stops, overlaps, turns or is pierced. Those are the laps, skylights, ridges, gutters and penetrations, and they are the details most often left to the site.

For an operations head the cost is not the roofer's bill. It is wet stock, damaged packaging, a slippery floor in a forklift aisle, water on electrical panels and a leak that recurs every monsoon because the visible drip was patched and the entry point was not. On a large roof, finding the source also takes time, because water travels along purlins and sheet ribs before it drops.

Where metal roofs actually leak

Laps and slope: the quiet cause

On a through-fastened roof, laps are where two sheets rely on gravity and a strip of sealant to stay dry. Two things defeat them.

Low slope. Water moves slowly on a shallow roof and spreads sideways into laps. Each sheet profile has a minimum slope set by the manufacturer, and the end lap length usually increases as slope reduces. A roof built at or below that minimum will leak at the laps sooner or later.

Capillary action and wind. Water can be drawn into a tight lap, or pushed uphill under it by wind. Sealant tape in the lap, correct side-lap direction and stitching fasteners at the specified spacing are what stop it.

The choice of system matters here. A through-fastened trapezoidal roof puts a fastener through the weather surface at every fixing point; a standing seam roof holds the sheet with concealed clips and closes the joint by seaming. Our standing seam versus trapezoidal roof comparison sets out where each suits.

Skylights and penetrations need their own details

Skylight sheets. Translucent sheets bring daylight into a warehouse, and their junctions with the metal roof are a frequent leak point. The translucent profile must match the metal profile closely, laps need sealing on both sides, and the different thermal movement of plastic and steel has to be allowed for at the fixings. The differences between materials are covered under polycarbonate skylight sheet and FRP translucent sheet. There is also a safety point that matters as much as leakage: translucent sheets should be treated as fragile, and no one should stand or walk on them.

Penetrations. Every turbo ventilator, exhaust duct, pipe or cable through the roof interrupts the sheet. The reliable detail is a purpose-made base or upstand flashing that suits the sheet profile and laps under the sheet above, so water is shed over it. A ring of sealant around a pipe is not a flashing; it fails as soon as the sealant ages. See turbo ventilators for how they are fitted and why their bases need care.

Fasteners, gutters and the things that age

Fasteners. Each screw seals against the sheet with a washer. An overtightened screw crushes the washer; an undertightened one leaves a gap; a screw that missed the purlin leaves a hole with nothing behind it. Washers also harden and crack in sun and heat over years. On a large through-fastened roof that is a great many small seals, which is why fastener condition is the first thing a full inspection should record.

Gutters. A large roof delivers a great deal of water to its gutters quickly. If the gutter is undersized, the outlets are blocked or the joints are open, water backs up and overflows, sometimes into the building rather than outside it. Rainwater drainage provisions are dealt with in NBC 2016, Part 9, and gutter and downpipe sizing for a specific roof is a design calculation, not a site estimate.

Swarf and debris. Drilling leaves steel filings on the roof. Left in place, they rust and stain the coating and can start corrosion. Clearing swarf at the end of each day is a small habit with a long-term effect.

Wind is a structural matter, not a roofing detail

Many roof failures in storms begin at edges, corners and ridges, where wind suction is highest. Wind loads on roofs are governed by IS 875 (Part 3), and the structural engineer determines the design loads for the specific site, height and building shape, from which fastener type and spacing follow. Those values belong to the engineer and should not be taken from another project or a supplier's general table. The envelope as a whole is covered on our PEB roofing and wall cladding page.

Repair or replace: decide from the whole roof

When a roof leaks in several places, the instinct is to call a roofer for each one. That treats symptoms. The better first step is a survey of the whole roof before the monsoon, recording the condition of the sheet and its coating, the fasteners and washers, every lap, skylight, ridge, flashing, gutter and penetration, along with the actual slope.

The survey usually points one of three ways:

• Local defects on a sound roof. Laps, flashings and penetrations repaired to a proper detail; this is the most common outcome.
• Aged fasteners across the roof. Systematic replacement of screws and washers, often with resealing of laps, rather than chasing individual drips.
• A roof at the end of its life. Corroded sheet, failed coating, or a slope the profile was never suited to. Here overcladding or replacement is the honest answer, and the choice of system can be revisited.

The decision is easier, and cheaper, with photographs and a marked-up roof plan than with a list of leak complaints.

Common mistakes

• Patching the drip. Sealant is applied where water drops, while the entry point is several metres upslope.
• Overtightening fasteners, crushing the washer the screw depends on.
• Building below the profile's minimum slope to save height or steel.
• Sealant instead of flashing around ventilators and pipes.
• Mismatched skylight profiles that never sit properly on the metal sheet.
• Leaving gutters uncleaned until the first storm of the monsoon.
• Mistaking condensation for a leak, and resealing a roof that needed insulation.
• Walking on skylight sheets or inspecting without safe access.

What to ask

• What is the actual roof slope, and what is the sheet manufacturer's minimum for that profile?
• Are end laps and side laps sealed, and to what detail?
• How are skylight sheets lapped and fixed, and do the profiles match?
• How is each penetration flashed?
• Are gutters and outlets sized by calculation, and when were they last cleared?
• Who inspects the roof, how often, and with what safe access?
• Has the structural engineer set the fixing pattern at edges, corners and ridges?

Standards referenced

Wind loads on roofs under IS 875 (Part 3); galvanised steel sheet under IS 277; construction practices and safety, including work on roofs, under NBC 2016, Part 7; and drainage, including rainwater, under NBC 2016, Part 9. Design wind loads, fixing patterns, minimum slopes, lap lengths and gutter sizes for a specific roof must be established by the project's structural engineer and the sheet manufacturer's data.