Installation and construction
How to reduce the risk of leaks in a new roof
In brief
Leak risk starts with the choice of covering. Matching it to the pitch, correct laps and tight details around openings and edges are essential. The layer beneath the covering must drain water away from the building. Each system has its own conditions of use – check the documentation for the specific profile, thickness and roof layout, as the T18 or T35 label does not set a single minimum for all products. Accept the roof in stages, because some faults become invisible after the sheets are fitted and the attic is finished. Agree the design of the details with the designer.
Why does the problem start with the choice of covering?
Every system has conditions of use. Too small a pitch, unsuitable longitudinal joints or an unapproved substrate can increase the risk of leakage. The designation T18 or T35 does not specify a single minimum for all products. The documentation for the exact profile, thickness and roof layout must be checked.
Similarly, an insulated sandwich panel may have different requirements for continuous sheets and for transverse joints. The presence of rooflights also changes the conditions. If the roof geometry does not suit the system, a different solution is needed. Additional sealing compound is not a universal way of circumventing product limitations.
What role does accurate measurement play?
Before ordering, the rafter lengths, diagonals and edge positions must be checked. Unevenness in the structure can make correct laying of the laps difficult. An error in the first row grows in subsequent rows. It is therefore worth checking the geometry at the outset rather than trying to align the final sheets by stretching the joints.
Long elements should be transported and lifted without deformation. A damaged interlock or a bent edge may not fit as specified in the instructions. Minor deformation cannot always be corrected on the roof. Acceptance of the delivery is therefore directly related to subsequent tightness, not merely to appearance.
How do you create a second drainage path?
Beneath the main covering there is often a membrane or other underlay. It is intended to take up water that gets under the metal sheet and direct it outwards. The laps, connections at the eaves and penetrations must maintain this direction. It is not enough to lay the foil between the rafters and leave its ends without a detail.
At a chimney or roof window, water from the higher part of the slope should be conducted past the obstruction. The areas requiring protection beneath the counter-battens must also be sealed. The type of material and accessories is selected to suit the system. A damaged membrane covered by the roofing does not cease to be a problem merely because the roof looks correct from the outside.
Why do fasteners require inspection?
The screw must match the substrate, the material thickness and the fixing position. Over-tightening can damage the washer or deform the sheet. Under-tightening will not provide adequate clamping. A fastener set at an angle may behave differently from what the design allows for. The fixing method for trapezoidal steel sheet and for insulated sandwich panels is not always the same.
Both the tool and the workmanship need checking, not just the number of fasteners. Drilling swarf must be removed from the surface. Its corrosion can damage the appearance and the coating. If an opening is damaged, the agreed repair method must be used. Adding sealant to the screw head does not eliminate every cause of leakage.
Which details deserve particularly close inspection?
Roof valleys, chimneys, windows, skylights and service penetrations require the most attention. Water must have a continuous drainage path even during wind-driven rain. Flashings should account for thermal movement and the profile height. A properly formed collar should not be replaced with an arbitrarily bent piece of sheet.
In a roof made of insulated sandwich panels, the seams and transverse joints also matter. The seals must be protected from damage while the panels are being pushed into place. Additional support may be needed at openings. A tight flashing alone will not prevent leakage if the element deflects excessively under load and tears the joint.
How can you tell a leak from condensation?
Water on the underside of the roof can come from moist air that has come into contact with a cool surface. A leaky layer from the interior and obstructed ventilation increase this risk. Repairing the flashing alone will not then help to remove the cause. An assessment of the thermal and moisture conditions is needed.
It is worth observing when the problem appears. A link with rain, wind direction or a cold night may be a clue, but it does not constitute a full diagnosis. The point of leakage may be far from the source of the water. That is why opening up and re-sealing random sections of the roof should be preceded by a structured inspection.
How should handover and subsequent changes be organised?
Stage-by-stage acceptance should cover the substrate, the underlay, the fixings and the details. It is good practice to keep photographs of areas that will be covered. At the end, the cleanliness of the gutters should be checked and leftover materials removed. The roof should not be left with protective film for longer than the instructions permit.
Later photovoltaic panels, an antenna or a new chimney require an agreed mounting method. New openings can change a system that previously worked correctly. It is worth keeping documentation of the profile and the accessories used. This makes it easier to select compatible components and to determine whether the planned change also requires structural reinforcement.
7 branches. Materials closer to your project.
