### How to calculate the right size of a roof gutter?/How to determine the correct roof gutter size?

The size of the roof gutter depends upon the following 4 factors.

1. Surface drainage area of a pitched roof.

2. Pitch of a particular roof.

3. Maximum intensity of rainfall in that region.

4. Type or style of the roof gutter.

Now, let us understand the calculation steps in detail.

Calculation formula:

First, we have to calculate the adjusted square footage for the given roof  by the formula,

= [(roof drainage area) 🇽 roof pitch multiply factor 🇽 maximum rainfall intensity]

After calculating the adjusted square footage, we can select the required roof gutter size from the charts or tables.

Calculation procedure:

1. Drainage area:

The drainage area of a roof is determined as shown below.

Drainage area = [roof length 🇽 roof breadth]

Note:

a. If the roof has multiple facets, we have to add all the areas.

b. If the drained water from the 2nd-floor roofing comes over the roof of the 1st-floor, the total roof area of both should be considered.

2. Roof pitch:

The pitch of a roof is the ratio of no. of units it rises for every 12 units of horizontal run.

The roof pitch multiplying factor for a different roof pitch ratio is given in the following table.

3. Maximum rainfall intensity:

We can collect the data on maximum rainfall intensity/hr. for a particular region from the Weather Bureau of that country.

4. Roof gutter size:

After calculating the adjusted square footage, we can select the size of the particular type of roof gutters from the company-provided tables.

For the calculation purpose, I have given a chart as follows.

Now, let us go through the example to understand the calculation in a better way.

Eg:

Given data:

Roof length = 6m.= 19.68 ft.
Roof width = 5m. = 16.40 ft.
Roof pitch = 8/12
Maximum intensity of rainfall = 6.5"

From the above-given table, the roof pitch multiply factor for 8/12 pitch is = 1.1

Calculation:

= [(roof drainage area) 🇽 roof pitch multiply factor 🇽 maximum rainfall intensity]

= [ (19.68 х 16.40 ) х 1.1 х 6.5]

= 2307 < 2500

Therefore, we can select half round gutter of 5" in size.

( Taken from the above-given table)

Thank you for going through these calculation steps. Have a good day 😄.