Gutters & Flashings Calculator

Gutters and flashing from roof perimeter

Add up the linear meters of eave you need to drain and that is your total gutter length. In practice it works out to the perimeter of the projected roof once you take off the gable walls. Flashing (rufos) is a separate run: it follows ridges (cumeeira), hips (espigão), valleys (água-furtada) and the spots where the roof meets a wall. The basic relation is L_gutter = P_eave and L_flashing = sum(ridges + hips + valleys + wall junctions).

PVC gutters come in 70–125 mm widths, and ABNT NBR 10844 tells you how to size them based on the area collected and the local rainfall intensity. Figure on a 75 mm downspout for every 50 m² or so of contributing roof; once you go past that, either add more downspouts or move up to 100 mm. PVC accessories from Tigre/Amanco ship in 3 m bars, while metal gutters from Mota or a local sheet-metal shop are cut to measure in galvanized or pre-painted steel.

Applications

Costing out rainwater drainage, working out how many brackets you need (one every 50 cm), counting corners, end caps and downspouts, sizing the cistern intake, planning flashing around chimneys, skylights and gable parapets, or laying out ice/water shield in cold climates.

FAQ

How many downspouts do I need? Roughly one 75 mm tube for each 50 m² of roof. Space them out every 10–12 m of gutter so the slope stays manageable.

What slope should the gutter have? Aim for 0.5–1.0% toward the downspout. That comes to 5–10 mm per linear meter, enough to keep water from pooling.

PVC or metal? PVC wins on price, weight and how quickly it goes up. Galvanized steel holds up better under harsh sun or near the coast, though you will have to repaint it now and then.