Renovation and repairs
How to choose a repair for a leaking metal roof
In brief
Repairing a leak in metal roofing starts with establishing whether the water comes from rain or from condensation. Note when the dampness appears – a leak after rain driven by a strong wind may have a different cause from droplets seen in the morning in a cool garage. Check the fixings, sheet laps and flashings, as well as any services in the loft. Only after identifying the problem should you choose a sealant, replace an element or improve a detail, remembering that repair compound is not universal.
Does the water come from rain?
Note when moisture appears. A leak after rain with strong wind may have a different cause than drops visible in the morning in a cool garage. In the second case, condensation on the underside of the sheet must be checked. Sealing the external laps will not remove the vapour generated inside or improve faulty ventilation.
Water can travel along the membrane, a batten or another element. The inspection should therefore cover the area above the visible trace. Photographs of the underside of the roof slope and a description of the weather are useful. Before dismantling the covering, it is also worth checking installations routed in the attic. A leaking pipe can produce a similar symptom.
It also helps to check whether the trace occurs only with a specific wind direction. Such information narrows the inspection area at edges and flashings. However, it does not in itself prove which element is defective. The service should receive a description of the conditions, not merely an instruction to seal the spot indicated from floor level.
What to check at the fixings?
The screw may be set at an angle, insufficiently tightened or clamped so tightly that it has damaged the washer. A lack of proper support beneath the sheet can also be a problem. Not all visible fasteners should be tightened automatically. Excessive pressure may worsen the defect instead of restoring the tightness of the hole.
The choice of new fastener depends on the substrate and the condition of the existing location. Replacing the screw with a larger one alone is not always a correct repair. It must be assessed whether the timber or steel still provide a secure fixing. Damaged sheet at the hole may require a separate solution. The repair should also restore the transfer of wind loads.
When to correct the overlap or flashing?
If the sheets are improperly joined, the sealed area may still move and open under the influence of temperature. The direction of flow, the length of the lap and the required sealing for the given pitch must be checked. Solutions suitable for a steep roof may not work on a low-pitch slope.
At the chimney, roof valley and junction with the wall, the primary concern is an interrupted drainage path. The repair may mean changing the shape of the flashing or reinstalling it. Sealant will not replace a missing drip edge nor increase the capacity of an overly narrow roof valley. It should also not close the gap needed for ventilation.
How to choose repair compound or tape?
The material must be suitable for the given substrate and external conditions. Check compatibility with the sheet coating, resistance to water and the application temperature range. The ability to accommodate joint movement is also important. Labelling a product as “roofing” does not yet mean it is suitable for every location on the roof.
The surface should be prepared in accordance with the instructions of the selected system. Dirt, grease, moisture or loose paint reduce adhesion. A layer applied over such residue may detach together with it. Before using the tape, its position relative to the drainage flow must also be determined. It should not form a barrier that retains water and dirt.
When is replacement of a section needed?
A punctured or heavily corroded sheet requires an assessment of the extent of damage. A patch may be a solution provided for in a specific repair, but it will not always restore the original properties of the covering. If numerous holes, laps and edges are damaged, a series of small repairs may prove less sensible than replacing the element.
In a sandwich panel, not only the outer sheet but also the core and the second facing must be assessed. It must not be treated as an empty trapezoidal sheet. After moisture ingress or damage to the joint, a system repair may be necessary. It should be checked whether the continuity of insulation, tightness and the required properties of the entire assembly are restored.
How can the effectiveness of the repair be checked?
Agree with the contractor what has been identified and which element has been corrected. Photo documentation before closing the detail facilitates assessment of later events. The inspection should also cover the location from the interior side. Damp insulation or built-in components will not dry out immediately just because the ingress of new water has been stopped.
After subsequent rainfall, check whether the mark returns. Work on the roof requires secured access; wet sheet metal particularly impedes movement. Emergency covering of the damage can limit losses until the proper repair. However, it should not become a permanent detail that has not been checked for fixing, drainage and durability.
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