How to Calculate Your Roof Area for Sizing your Guttering

It is worth making sure that your gutters can cope with the amount of water that will come off your roof!

There are complicated and accurate techniques that take into account where your house is and the expected rainfall to work out the size of the guttering you need. However we feel that this is generally ‘overkill’ and sometimes a simpler solution is better, or at least as good.

The simple Way to Calculate Gutter Sizes

There are two things you need to know:

1. The effective roof area that will collect the rain which will need to be removed by the guttering. As most roofs are not flat this needs to be calculated – we’ll show you a simple way.
2. The area that your chosen guttering system can cope with. Thankfully the manufacturers will be able to provide this information through your merchant or online. In fact it is not going to vary much (if at all) between manufacturers as the common styles are very similar.

So to step 1:

To calculate the Effective Roof Area you should use this formula:

Roof Length   X   Half Roof Width   X   Roof Height ÷ 2 = Roof Area

So for a roof that is 5 m long, with a height of 4 m and the roof width is 8 m (which makes the half roof width 4m) we can calculate the effective roof area like this.

5m x 4m x (4/2) = 40 square metres

Note that this is not the true roof area but just an approximation to get you a number that is close enough to work with for our purposes.

The next step is to compare this calculate roof area against the stated roof areas that each style of guttering can deal with. Here is a table for the Floplast gutter systems compiled by gutteringsupplies.co.uk:

As you can see with an effective area of 40m² there will be no issue using any of these systems and configurations. However if our effective roof area were 50 m² then we would not be able to use a level Half Round gutter with a stop end outlet (ie, the downpipe at one end of the gutter, rather than in the middle).

To increase the capacity of the gutter, we would have to add a fall, which we would always recommend doing as a matter of course, or moving the outlet to the centre of the gutter but using a Running Outlet (which allows you to position the downpipe in the middle of the gutter).

If none on this makes sense, we have a project all about how to fit gutters which explains everything here.

Increasing the Capacity of your Gutter System

As you can see from the table there are a number of things that you can do to increase the capacity of your guttering system to remove water from your roof:

1. Add a fall to the gutters, which we recommend you do anyway
2. Move the outlet (downpipe) from the end of the gutter to the middle
3. Change the style of gutters to one with a greater capacity.

Most merchants and online suppliers will assist you with sizing your guttering so it is not a big problem, but it is something you can do yourself if you want to double check their measurements. They do this so that they can then calculate how much guttering you will need.

For more information about actually fitting your gutting yourself, have a look at our project about fitting plastic guttering.