What Size Downpipe Do I Actually Need?

It’s one of those questions we get asked quite alot... and i'm sick of it....  only joking!! 

“What size downpipe do I need?”

And, as much as I’d love to give you a one-size-fits-all answer, there isn’t one.

For a straightforward house, it’s normally pretty simple. But once you start dealing with bigger roof areas, long gutter runs, extensions, valleys or commercial buildings, just ordering whatever size was fitted before isn’t necessarily the best idea.

I've seen plenty of jobs where a good gutter has been specified, only for the downpipe arrangement to become the weak point of the whole system.

So, rather than massively overcomplicating it, here's what I'd actually look at.

Start with the roof, not the downpipe

This sounds obvious, but the downpipe only has one job: getting rid of the water your roof is collecting.

A little porch in Cheshire and a warehouse roof in Manchester clearly aren't dealing with the same amount of water.

The bigger the effective roof area, the more rainwater your gutters and downpipes need to shift.

Roof pitch matters as well. A large, steep roof can put a serious amount of water into a gutter very quickly when we get one of those lovely British downpours.

And if you're working somewhere that gets its fair share of rain like Greater Manchester, Lancashire, Cumbria, North Wales, Yorkshire, the West Midlands.... I'd rather see a system with some breathing room than one designed right on its limit.

A quick way to understand the numbers

There's a really useful rule to remember:

1mm of rain falling on 1m² of roof equals 1 litre of water.

So if the effective roof area draining into one section of gutter is 60m², every 10mm of rainfall puts 600 litres of water onto that area.

That's why sizing matters.

It isn't necessarily the normal drizzle you need to worry about. It's what happens when we suddenly get a serious burst of rain and all that water needs somewhere to go.

What size aluminium downpipe should I use?

For most domestic projects, you're generally going to be looking at the smaller and medium sizes within an aluminium rainwater system.

As the roof area and gutter capacity increase, so should the capacity of the outlets and downpipes.

But here's the important bit:

Don't choose the downpipe in isolation.

Your gutter, outlet and downpipe need to work as a complete system.

There's very little point specifying a high-capacity gutter and then creating a bottleneck at the outlet.

Likewise, going for the biggest downpipe you can find isn't automatically the answer either.

Sometimes another downpipe is the better answer

This is something worth considering on longer runs.

If you've got a substantial roof elevation feeding one long gutter, increasing the number of outlets can sometimes be more effective than simply increasing the diameter of one downpipe.

You're splitting the volume of water between two drainage points rather than asking everything to find its way to one outlet.

It can also make much more sense when you're planning the job from scratch.

Obviously, where those downpipes ultimately discharge needs considering too. Moving water from the roof is only half the story — it still needs somewhere suitable to go once it reaches ground level.

What about commercial buildings?

This is where I'd stop relying on rules of thumb.

Warehouses, schools, apartment blocks, offices and other commercial buildings can have huge catchment areas. Valleys and multiple roof sections can also concentrate large volumes of water into surprisingly small areas.

If you're pricing or specifying a commercial job in Birmingham, Leeds, Liverpool, Sheffield or anywhere else in the UK, work from the actual roof drainage requirements rather than assuming a standard domestic arrangement will scale up. 

An extra bit of thought at quotation stage is considerably cheaper than discovering the system can't cope once it's installed.

You could also ask us or our partners for some roof design calculations too Stafford Rainwater

Round or square downpipes?

From our point of view, this is often as much about the look of the building and the gutter system you've chosen as anything else.

Round aluminium downpipes work brilliantly across traditional and contemporary properties, while square and rectangular profiles can give a sharper architectural finish.

If you're matching an existing system, working on a heritage property or the architect has specified a particular profile, that may make the decision for you anyway.

The important thing is still capacity.

A beautiful rainwater system that can't handle the roof it's attached to isn't a particularly good rainwater system.

And don't automatically copy what's already there

This catches people out.

If you're replacing an old guttering system, it's tempting to measure what's there and order exactly the same sizes.

Sometimes that's absolutely fine.

But if the customer tells you, “Yeah, it does overflow a bit when it really rains…” that's probably your cue to have another look.

The existing system might have been undersized from day one. The roof may have been altered. An extension might now discharge into the same run. Or there simply might not be enough outlets.

Replacement is a good opportunity to put those things right.

Keep it simple

If you're trying to work out what size aluminium downpipe you need, I'd look at four things:

• How much roof area is feeding the gutter?
• How long is the gutter run?
• How many outlets/downpipes are taking that water?
• Does the capacity of the downpipe actually match the gutter system?

For a straightforward domestic job, the answer is normally pretty simple.

For a large house, unusual roof, long run or commercial project, it's worth doing the calculation properly.

And if you're staring at drawings thinking, “I've got absolutely no idea what size I need here,” send them over to us.

We'd much rather help you get the system right before you order it than have you standing on site in the rain wishing you'd gone bigger.

Need help choosing your aluminium gutters and downpipes? Talk to the Aluminium Gutters Direct team and we'll help you work out the right system for the job.