Water drainage from pitched roofs

When designing pitched roofs, an important component of the entire project is the roof drainage design.
Before selecting a specific gutter system, it is necessary to determine the effective - basic and maximum roof area. Depending on the effective area of ​​the roof, the recommended width of the gutters and the diameter of the down pipes are determined.

The effective area of ​​the roof is the area, which needs to be dehydrated.

The effective area of ​​the roof is calculated according to the formula:

EPD = (H/2+W) x L


EPD – effective roof area
H – roof height
W – horizontal distance from the corner to the ridge
L- roof length

effective roof areaWith a flat or pitched roof, the angle of less than 10 ° is assumed, that the effective area is equal to the area of ​​the entire roof.
The maximum area of ​​the roof is the area, which can be drained with a gutter of a given cross-section and one downpipe. The larger the gutter cross-section, the greater its capacity and the greater the roof area from which it can drain water.
Roof area, from which the gutter can drain the water also depends on the location of the downspout – the pipe fixed in the middle of the eaves allows drainage of a larger area than the one fixed at the end. If the corner of the gutter is closer than 2 m from the downpipe, the roof area that can be drained will decrease by 10% compared to the solution with the downpipe in the middle of the hood.
If the length of the eaves does not exceed 10 – 12 m, one downpipe at the end of the gutter is enough to drain the roof slope.
One downpipe can also be mounted in the middle of the hood, if its length does not exceed 24 m. With the gutter length from 10 do 24 m most often, two downspouts are used at both ends of the gutter. An expansion joint is mounted in the middle, which enables mutual deformation of the gutters and the outflow of water despite the opposite direction of the gutters' slopes.

gutter dimensionsRecommended dimensions of gutters and downspouts depending on the effective roof area.
The values ​​in the table are calculated for the following assumptions: rainfall intensity 75 mm/h na 1 cm², downpipe located at the end of the gutter, gutters mounted with a slope 1 cm on 6 m of the gutter.

tmp883f-2Location of gutters and downspouts in pitched roofs.