A well-laid roof can still cause damp problems if the gutter or downpipe is not properly sized: water that isn't drained in time ends up seeping in at some point other than the slope itself.
Gutter: PVC, zinc or aluminium
- PVC: the cheapest, enough for standard housing.
- Zinc and aluminium: last longer and don't fade in the sun, a difference noticeable especially on very exposed roofs.
Usual diameters are 80, 100, 125 and 150 mm, with a purchase margin of 5% for joints over the measured length.
Calculation example: for 18 metres of gutter: 18 × 1.05 = 18.9 lm to buy. This is the same calculation the PresupuestoYa calculator applies automatically for the Roofer trade.
Downpipe: sized per downpipe, not for the whole roof
Size the diameter of each downpipe according to the roof area that particular downpipe collects, not that of the whole building. Several downpipes spread out need a smaller diameter each than a single one for the whole roof — it's a common mistake to oversize all downpipes the same without considering how many there are and how the area is shared between them.
Industrial units and buildings: the metal purlin
On industrial-unit roofs over a steel structure, the purlin rests on trusses or portal frames and directly supports the sheet or sandwich panel. The spacing between purlins is set by the sheet or panel manufacturer according to its load capacity and the snow/wind expected in the area — don't widen it to save material without first checking that data sheet, it's a structural calculation, not an estimate.
Frequently asked questions
Is a PVC or a zinc gutter better? PVC is enough for standard housing and cheaper; zinc or aluminium last longer and don't fade, a real advantage on roofs very exposed to the sun.
Must all the downpipes on a roof have the same diameter? No: each downpipe is sized according to the roof area it specifically collects, not the whole roof.
Can I increase the spacing between metal purlins to save material? Not without first checking the data sheet of the sheet or panel manufacturer: the spacing depends on the load capacity and the snow/wind expected in the area.
Conclusion
Choosing the gutter material well according to exposure, and sizing each downpipe according to what it really collects, is what avoids water build-up that ends up seeping through an unexpected point. On industrial units, respecting the purlin spacing given by the manufacturer is not a recommendation, it is a structural requirement.
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