How many downpipes do I need?

November 29, 2019

The number of downpipes you need will depend on the size of your roof and your chosen style of guttering. This is because larger roofs require bigger volumes of water to be diverted away from them during periods of rainfall. Also, the various styles of guttering that are available have different flow rates. The flow rate is the number of litres of water that they’re able to divert per second.

We’ve created a simplified method for working out the amount of guttering that you will need, which, in turn, will enable you to work out the number of downpipes.

If you already know how much guttering you need and would like to get a free quote, contact us today.

Step 1: How to work out the size of your roof

Cream fascia, cream soffit, cream bargeboard with decorative trim and black seamless aluminium gutter The first thing you need to work out is the area of the roof that will collect rainfall, which is known as the Effective Roof Area. This can be calculated using the following formula:

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

You can use a tape measure to calculate each of these measurements.

Step 2: Choosing the right style of guttering/downpipes

Now you know the size of your roof, the next thing to do is choose a style of guttering that has enough capacity. We have created a table below which contains the flow rates and the maximum roof area that our various styles of guttering can handle, so you can work out which style is suitable for your roof area.

Type of gutter	Flow litres per second*	Maximum area drained (m²)
Round	0.9	43
Square	1.6	76
Ogee	2.2	105
Deepflow	1.8	86

*when the outlet is positioned at the end of a run

So, if you had an effective roof area of 40m², any of these guttering options would be suitable for it. When you know how big your gutters are going to be, and the size of your roof, you can calculate how many downpipes you need.

Learn if uPVC or aluminium gutters are right for you in our related blog.

Step 3: Options to increase water flow

The above steps will help you to determine how many downpipes you need. However, there are areas where optional extra downpipes may me desired:

Deepflow Gutter

You can install high-capacity guttering and downpipes, which will increase the flow rate of water from your roof.
You can also increase the fall on the gutters. The fall is the measurement by which the gutter drops to allow water to drain towards the downpipes.
We do not advise adding extra downpipes as this can be very costly and disruptive as it may involve installing new soakaways. However, if there is a soakaway available in the middle of a run, positioning a downpipe here instead of at the end of a run could increase water flow.

It is likely that the calculations mentioned above, to determine the number of downpipes, would have taken place prior to your property being built, normally by the architect. Therefore, if you feel that your current system isn’t coping, you may need to check that there are no blockages along the course of the pipework as this is the main reason why rainwater systems do not perform at their optimum. Alternatively, call us on 01252 852336 for a free survey and quotation.