Failure mechanism
Roof uplift and fastener failure
Wind flowing over a low-pitch roof pulls it upward rather than pressing it down, and the suction is strongest at the edges, the ridge and the corners — precisely where the sheeting is most often fixed at the same spacing as the middle of the roof. The load enters the structure through the fasteners, the smallest and least considered parts of the roof. When they let go the sheet leaves, the purlin loses the restraint the roof was giving it, and the bay next to it is weaker than it was.
How this failure works
A low-pitch roof under wind does not behave like a floor under gravity. Airflow accelerates over the roof and the mean pressure on the surface is negative, so the design case is uplift, and the external pressure coefficient in the zones near the windward edge, the ridge, the eaves and the corners is much larger than in the general field of the roof. The net pressure on the sheeting is the difference between that external suction and the internal pressure. In a closed, well-sealed building the internal pressure is small; in a building with openings, or with a door left open, the internal pressure becomes positive and adds to the suction, so the net uplift rises without anything on the roof having changed.
The load enters the frame through the fasteners and there are two ways for one to fail. It can withdraw from the purlin, which depends on the thread form, the purlin thickness and the length of engagement; or the sheet can tear over the washer, which depends on the sheet thickness and strength and on the washer diameter. Which of the two governs is a property of the assembly, not of the screw, so a fastener that performs on a thick purlin under heavy sheet can pull straight through light sheet on a thin purlin. Spacing is designed against the governing mode, and it is designed zone by zone: one spacing used across the whole roof is a design omission, not a simplification.
The sheet spanning between purlins carries the uplift in bending just as it carries downward load, but the profile is not symmetric and the ribs that give it stiffness in one direction are often open in the other, so its uplift capacity is not its downward capacity. The sheeting also restrains the purlin. A purlin acting in the uplift case has its top flange in compression, and that flange is held by the sheet through the fasteners. Once a few fasteners in a bay have failed, the sheet no longer restrains that purlin, the capacity of the purlin falls, and the bay that failed first makes the adjacent bay more likely to follow it.
Wind is not a static load. Gusts rise, fall and reverse, and every cycle works the fastener and the hole it sits in. A fastener that is not seated correctly — overdriven so that the washer is dimpled into the sheet, or underdriven so that it never clamps — allows relative movement between sheet and purlin; the hole elongates, the clamp is lost, and the assembly then fails at a load well below the one it was designed for. An elongated hole is the record of that history. Tightening a fastener that sits in an elongated hole does not restore the capacity, because the material that carried the load has already been removed.
Illustration
Conceptual illustration: a low-pitch roof under wind uplift, with suction arrows concentrated at the ridge, eaves and corner zones and the first fastener pulling over the sheet.
Typical causes
- One fastener spacing used across the whole roof, with the higher suction at edges, ridge and corners never treated as separate zones.
- Internal pressure ignored, or the enclosure changed after design — openings added, a wall removed — so the original assumption no longer holds.
- A fastener chosen for the sheet rather than for the assembly, so withdrawal from a thin purlin governs and was never checked.
- Fasteners driven to the wrong depth, overdriven into the sheet or left standing so the washer neither seals nor clamps.
- Washers omitted, undersized or deteriorated, reducing the pull-over capacity of the sheet around each fixing.
- A lighter sheet or a shallower profile substituted for the one the fixing pattern was designed around.
- Purlin bracing, sag rods or fly braces removed, loosened or never fitted, so the purlin lacks the restraint the sheet was assumed to provide.
Warning signs
- Washers cupped, or the sheet dimpled and creased in a ring around a fastener.
- Elongated, oval or torn holes around the screws, or a screw standing proud of the sheet surface.
- Polished or fretted witness marks around the fixings, showing that the sheet has been moving on the purlin.
- Ripples running across the sheet, a lifted corner or edge, or a side lap that no longer closes flat.
- Water staining or leaks along the fastener lines and at the laps after wind and rain.
- Sag rods or purlin bracing hanging slack, or a purlin line that can be moved by hand at midspan.
Inspection methods
- 01Walk the edge, ridge and corner zones after any significant wind, and check for lifted sheets, elongated holes and missing fasteners.
- 02Check the actual fastener spacing in each zone against the drawing, and record where the zones were not differentiated.
- 03Check fastener seating on a sample of each zone: washer compressed but not dimpled into the sheet, thread engaged through the purlin flange.
- 04Check the sheet profile and thickness as installed against the drawing, and check the side lap and end lap fixings.
- 05Check the internal pressure path — the enclosure, the openings — and whether the building is still the one the wind design assumed.
- 06Check purlin bracing, sag rods and the connection of the sheeting to the purlin, including corrosion at the washer line.
Preventive measures
- Compute the net uplift with the internal pressure appropriate to the actual enclosure, and design the fixing for the result.
- Design fastener spacing by zone — field, edge, ridge and corner — rather than with a single spacing across the roof.
- Select the fastener against the real assembly of sheet and purlin thickness, and check both withdrawal and pull-over.
- Control installation depth with a depth-nose or depth-sensitive tool, and do not overdrive the fastener into the sheet.
- Provide the washers or distribution plates the design assumed, and hold the sheet thickness and profile as specified.
- Maintain the purlin bracing and sag rods, and inspect them alongside the roof rather than only when the roof leaks.
- Inspect after wind events and replace fasteners sitting in elongated holes instead of tightening them.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- IS 875 (Part 3) — wind loads, including the pressure coefficients and the edge and corner zones.
- IS 800 — steel design and the connection provisions the purlin and fixing detail relies on.
- IS 277 — galvanised steel sheeting, specifying the base metal thickness and coating the fixing depends on.
- IS 801 — cold formed light gauge steel members, which is how the purlin and sheet sections are treated.
- Eurocode 3 (EN 1993-1-3) — cold formed members and sheeting in the European basis.
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.