Gutter Size Calculator
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Other tools you may find usefulRoof drainage - how to choose the right gutter system
An efficient gutter system is a key element of the structure of every building. Its main task is to effectively drain rainwater from the roof, away from the facade and foundations of the house. Errors in the design or installation of gutters may lead to flooding of walls, the formation of moisture, fungi, and in extreme cases, damage to the foundation structure. For the gutter system to function flawlessly during heavy downpours, its size (diameter of gutters and downpipes) must be matched to the size and geometry of the roof. The key concept in this process is Effective Roof Area (EPD).
Our online gutter calculator allows you to quickly calculate the effective roof area (EPD) and automatically select the appropriate gutter and downpipe diameters for your building, which makes shopping easier and minimizes the risk of flooding.
What is Effective Roof Area (EPD) and how to calculate it?
Effective Roof Area is a measure that takes into account not only the physical angle of inclination of the roof, but above all, how water behaves during rainfall carried by the wind. EPD is calculated according to the official building formula:
EPD = (W + H/2) · L
Where the individual parameters mean:
- W:The horizontal distance from the eaves to the ridge (width of the horizontal projection of the roof slope) expressed in meters.
- H:Roof height (vertical distance from the eaves to the ridge) expressed in meters.
- L:Length of the roof slope (along the eaves) expressed in meters.
For completely flat roofs (slope angle close to zero, i.e. H = 0), the effective roof area is equal to its actual horizontal projection area (EPD = W · L).
Table: Selection of the diameter of gutters and downpipes to the roof surface (EPD)
The table below presents guidelines for selecting the dimensions of the gutter system elements depending on the calculated Effective Roof Area (for one downpipe placed at the end of the gutter):
| Effective Roof Area (EPD) for 1 downpipe | Recommended gutter width (mm) | Recommended downpipe diameter (mm) | Application / Building type |
|---|---|---|---|
| up to 35 m² | 75 mm | 50 mm | Small garages, sheds, gazebos garden, small balconies. |
| from 35 to 70 m² | 100 mm | 75 mm | Summer houses, small single-family houses, two-car garages. |
| from 70 to 120 m² | 125 mm | 90 mm | Standard single-family houses, medium-sized gable roofs. |
| from 120 to 200 m² | 150 mm | 110 mm | Large single-family houses, multi-family buildings, small warehouses. |
| over 200 m² | 190 mm | 120 – 150 mm | Industrial halls, supermarkets, large agricultural and commercial facilities. |
Frequently asked questions (FAQ)
How to choose gutters if the downpipe is in the middle and not at the end?
Placing the downpipe in the middle of the gutter length increases the efficiency of the system by approximately 80-90%. This allows you to cover a much larger roof area with the same gutter diameter. For example, a 125mm gutter with a downpipe in the middle can cover a roof with an EPD area of up to 220 m² (instead of 120 m² with the pipe at the end).
What gutter material should you choose – PVC or metal?
PVC (plastic) gutters are cheaper, lighter, easy to install, completely resistant to corrosion and available in many colors. Metal gutters (coated steel, zinc-titanium or copper) are much more durable, resistant to extreme temperatures, snow accumulation and UV radiation, but they are more expensive and require professional installation.
What slope should gutters have?
Gutters cannot be installed perfectly horizontally. To ensure that the water flows freely into the downpipes and does not remain in the gutter, a slope of 2 to 5 millimeters should be maintained for every 1 linear meter of the gutter (0.2% - 0.5% slope).
Every how many meters should the downpipe be installed?
It is recommended that a single downpipe serves no more than 10-12 linear meters of gutter. If the roof eaves are e.g. 15 meters long, two downpipes should be installed (e.g. at both ends of the building).
How to protect gutters from clogging with leaves?
The best solution is to install special protective nets made of plastic or metal on the gutters, which stop leaves and branches on the surface, allowing the water to flow down. It is also worth installing the so-called cleaners (revisions) with a strainer at the bottom of the drain pipes.