Calculate Gutter Size: Table With Common Sizes

Calculate Gutter Size: Table With Common Sizes

Reliable drainage with the right gutter size – gutters are a small detail with a big effect. If the drainage of the roof is not reliably guaranteed, there is a risk of major damage in and around the house. Rain that always drips on the same spot on a house wall quickly causes moisture damage. Then there is a risk of mold and decomposition of the building fabric. Read this text to find out how you can efficiently equip your house with a powerful roof drainage system.

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Gutters – prior knowledge and information

The applicable standard for gutters is DIN EN 612. It is both a German and a European standard. When purchasing your new gutter, make sure that this standard is complied with. Even the best quality gutters are not a particularly high-priced product. There is therefore no reason to overdo it with thrift at this point. Only a gutter that is manufactured and designed in accordance with standards can guarantee the desired, reliable roof drainage. Therefore, do not buy a “bargain” from unknown sources, but only products that meet the standards.

Compliance with the standard also means that the gutter can still be repaired or expanded in pieces. Temporary patching is therefore not necessary, even if individual pieces come from different manufacturers.

What does “partial” mean for a gutter size?

Some manufacturers of gutters still use the term “partial” when describing their products. This does not mean the number of individual sections required to enclose a roof edge with gutters. The term dates back to the time when gutters were made from individual metal sheets and riveted together. However, this designation is no longer up-to-date for today’s roof drainage systems.

Basic dimension: “Projected roof area”

The “projected area of ​​the roof” is decisive for calculating the gutter size. “Projected” means that not the actual area of ​​a roof is taken, but the area that the roof covers from the property. This can also be thought of as the “shadow” of the roof when it is irradiated vertically from above.

It makes no difference to the amount of rain to be discharged, how pointed or how flat the roof is designed. The amount of rain is always the same, so the projected area is sufficient to calculate the basis for roof drainage. If the roof does not have large overhangs, the basic dimensions of the house can also be assumed. To be on the safe side, however, you should always assume the actual length and width of the area projected from the roof. This protects you from measurement errors.

In the case of a normal, rectangular floor plan, this is, as I said, the length times the width of the roof edges. However, if you have an exclusive floor plan with round bay windows, towers, trapezoidal extensions or other special shapes, you must adopt the appropriate area formulas.

For complicated or irregular footprints, it is advisable to break them down into simple faces for gutter sizing. Then the simple individual areas are calculated and afterwards counted together again. Here we have put together a selection of the most common area formulas for you.

  • Circle: diameter divided by four times pi
  • Right triangle: the length of one side that leaves the right angle times the length of the other side. Divide the result by two.
  • Trapezoid: short side plus long side. The result times the distance between the two sides. Divide the result in two.

rain donation

Now that you have calculated the roof area, you need the “rain donation”. The rain yield is the rain, which was measured in “liters per second times hectares”. This measurement is recorded by the responsible environmental agencies over a long period of time. An average value per year would inevitably lead to incorrect results: A heavy rain event is then sufficient to overload the roof drainage. That is why the rain donations tend to be oversized. It’s better to be a little more generous when calculating gutter sizes than to expect moisture damage every moderate thunderstorm.

The discharge coefficient

In most cases, the runoff coefficient is irrelevant for calculating the size of the gutter, as it is assumed to be 1. This factor gives an indication of how falling rain is absorbed by the subsoil. The more permeable and absorbent a substrate is, the lower the runoff coefficient. Materials for roofs are for the most part completely water-repellent. Regardless of whether glazed clay tiles, tin roofs, concrete tiles, plastic or other materials are used for the roof covering, none of them are absorbent. The rain flows off in exactly the same amount as it was previously “collected” from the roof. Only the thatched roofs are an exception. They are optically a real highlight but very expensive in execution.

Calculation of downpipe and gutter size

Even though each roof area gets its own gutter and connection, the water is ultimately brought together in a single, shared downspout. It is very important that this pipe is adequate, ideally also oversized. Otherwise there will be a backwater in the pipe with every medium-heavy rain. The water then sloshes out at the top and runs down the wall of the house – with undesirable consequences. Anything is possible, from leaching of paint and plaster to softening of the building fabric and penetrating mold spores.

The formula for calculating the gutter and downpipe size is therefore:

Q = roof area x rainfall yield x runoff coefficient

This results in an “expectable rainwater runoff, which is given in liters per second. This must be intercepted by the gutters and the downpipe so that the roof drainage can do its job reliably.

materials for gutters

Zinc sheet was the standard material for gutters for many decades. Even today, zinc sheets offer a durable solution with a good price-performance ratio. Basically, this material is just metal sheets, which are then kept permanently rust-free by hot-dip galvanizing. They can be painted as desired, but then require some care.

The nobler variant is the gutter made of copper. It’s significantly more expensive. Visually, it definitely makes a somewhat upscale impression. Unfortunately, this is not permanent: the copper, if it has not been covered with a clear coat, tarnishes and turns greenish or black.

Today, plastics for gutters are another widely used option. They are very durable, extremely inexpensive and very easy to assemble and process. However, unlike the sheet metal variants, they cannot be recycled, but end up in the incinerator at the end of their service life.

Finally, there are gutters made of aluminum or stainless steel. They are by far the most expensive to purchase and are also difficult to install and expand. Both metals can only be repaired or enhanced by expert hands. Your advantage is an extremely clean and durable look. Stainless steel will not rust. When it is dismantled, it can be sold again for a lot of money.

shapes of gutters

Gutters are either semi-circular or box-shaped. The half-round variant has significantly better flow properties. Installed correctly, they are largely self-cleaning. With box-shaped flow channels, you always have to expect dirt to collect in the corners. Of course, it gives a very individual impression, especially if it is made of a more precious metal than zinc sheet. Nevertheless, we would always recommend the semi-circular variant.

Gutter Size: A typical selection

Once you have determined the amount of discharge to be expected, you can use tables to select the appropriate gutter size. The manufacturers and dealers also offer calculators on their websites, with which you can calculate the gutter size you need for your building with just a few mouse clicks.

Projected roof areaMinimum diameter of the downpipematching gutter sizealternative size designations
up to 37 m²60mm200s10 pieces / RG80
up to 83 m²80mm250 or 2858 pieces / RG 100 or 7 pieces / RG125
up to 150 m²100mm333er6 parts / RG 150
up to 243 m²120mm400s5 parts / RG 200

Calculation example:

A house is 15 meters long and 10 meters wide. The roof overhangs 0.5 meters. This results in a projected roof area of ​​15.5 x 10.5 meters 162.5 m². In the case of rainfall, local average values ​​apply. In any case, the downpipe should be over 100 millimeters in diameter. A value between 333 or RG 150 and 400 or RG 200 should be selected as the gutter size. In this case, however, we would recommend the 400 RG 200 variant. This guarantees sufficient security.

Accessories for gutters

The water caught by the gutters is drained into the sewage system. Actually, it’s very unfortunate. Even if drinking rainwater is not recommended, there are still some things you can do with it.

Rainwater is very soft, which means that it is low in lime. It is therefore well suited for operating washing machines and dishwashers. It doesn’t cause deposits. However, in order to be able to use it for this purpose, a very expensive and complex additional installation is necessary. It’s easier to collect rainwater to flush the toilet. Again, a tank must be installed in the attic. This is quite heavy and must be statically secured.

The easiest way to use rainwater is to equip the downpipe with a diverter flap. This allows rainwater to be collected to be used later for cleaning or watering garden plants. Such a flap costs only about 15 euros.

A problem with gutters is their pollution. This can entail considerable work, especially in the vicinity of deciduous trees. The trade offers protective grilles and downpipe protective balls for this purpose. They keep coarse dirt particles such as leaves or branches from falling into the gutter. Nevertheless, these organic residues must be removed regularly, otherwise there is a risk of bad smells in the house. And nobody wants that.

Cleaning gutters is one of the most dangerous jobs in the house. However, there are now some tricks and tools that not only defuse this danger point, but also make the work much faster. Various manufacturers of vacuum cleaners offer special devices or attachments for industrial vacuum cleaners, with which the channels can be sucked free from the floor. Other solutions are conveyor belts that are inserted into the channels. If the channel is full of dirt, the tape is simply pulled off on one side and the rubbish is removed.