Here is how to measure your roof area for the guttering?

DIY Improvement Ideas, Fascias, Soffits & Guttering, How To?

Here is how to measure your roof area for the guttering?

It pays to ensure your guttering can cope with the amount of water that will come off your roof while fascias supporting it’s fixture!

Here is how to measure your roof area for the guttering-

There are complex and correct techniques that are needed taking into account depending on the shape of your house along with its location and the expected rainfall in order to work out the type of fixture and size of the guttering your house needs.

However; at we believe this to be an ‘overkill’ so in most cases a simpler solution is far better, or at least as just as good as the solution that is pricey and overly complex.

Cheat sheet to calculate the size of your guttering

Before we start there are two most important things you need to know:

1. The entire roof area(s) that will effectively collect the rain water you want guttering for.

2. The guttering system must be able to cope with the roof top area you have chosen it for.

The good news is that you will be able to get that information before you buy the guttering system, almost all manufacturers provide this information along with their product, just ask the sales guy/girl in store.

So, we are ready! Are you?

Step number 1

To calculate your Effective Roof Area you should use this formula below:

Here is how to measure your roof area for the guttering

Roof Length X (multiply) Half Roof Width X (multiply) Roof Height ÷ (divided by) 2 = Your Roof Area!

So lets suppose your 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) you can calculate the effective roof area like this, easy peasy!

5m x (multiply) 4m x (multiply) (4/2) = 40 square metres

Note: Remember this is not the true roof area but just our way of explaining it to get you a number that is close enough to work with for illustration purposes.

Step number 2  

Compare your calculated roof area against the stated roof areas that each style of guttering can deal with. The most popular guttering system supplies in Ireland are from FloPlast so we attached their PDF Guide for your convenience ( is not affiliated with FloPlast in any way)

Here is how to measure your roof area for the guttering

With an effective rooftop area of 40m2 there will be no issue using any of their guttering 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).

So to maximise the capacity of your gutter, you 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 (this allows you to position the downpipe in the middle of the gutter).

Increasing guttering system intake

There are a number of things that you can do to increase the capacity of your guttering to remove water from your roof as you saw in the table.

1. Add a fall to your gutters, which we recommend you must do

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

Any sales representative in Woodies, B&Q or even in our local hardware store will be able to assist you with sizing your guttering so it is not a big challenge.

However if it is something you want to try doing it yourself then you must double check your roof top measurements to calculate how much guttering you will need, it will also be crucial if you are thinking of ordering your guttering supplies online.

Your thoughts?

Did you learn something new?

how did we do?

If you would you like to give us your feedback, then please do so, we will look forward to receiving your opinion.

It would be great to to find out how did you go about installing or repairing your guttering system. 


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