A 100mm diameter downpipe will have a cross-sectional area that is almost 2.4 times larger than that of a 65mm diameter downpipe. However, it doesn't necessarily mean that a 100mm downpipe will collect 2.4 times as much rainfall as a 65mm diameter downpipe since the gutter and the entire drainage system come into play here – but it does explain the reasons why you wouldn't want to pick a downpipe by its looks.
Luckily enough, picking up the correct cast iron downpipe becomes significantly easier if you split the selection process into several stages.
The first question is what shape you need. Next comes size. Then comes whether your gutter outlet matches the downpipe. You will also need to decide on fixing style, offsets, bends, shoes and finishing.
When it comes to selecting the appropriate cast iron downpipe from the Alumasc Apex Heritage cast iron range supplied by us, the vast majority of our customers will be faced with the choice between traditional round cast iron downpipes in 65mm, 75mm and 100mm diameters or rectangular cast iron downpipes when the building requires.
If you are replacing the existing pipework rather than designing the entire new rainwater system, there is one important principle that you should bear in mind.
Struggling to identify what gutter profile you have? Read our help guide on choosing the correct cast iron gutter profile.
Here's a quick table to help you:
| Your situation | Where we'd start |
|---|---|
| Replacing an existing round downpipe | Measure the existing diameter and check the gutter outlet |
| Replacing a rectangular pipe | Measure both width and front-to-back depth |
| New traditional rainwater system | Round is normally the simplest starting point |
| Matching an architectural rectangular system | Keep the same pipe shape and proportions |
| Small to standard domestic roof | 65mm or 75mm may be appropriate, subject to the actual drainage requirement |
| Larger roof or greater flow requirement | Look at 100mm or have the rainwater system properly sized |
| Unsure what old system you have | Photograph the outlet, pipe, socket and fittings before ordering |
Round or Rectangular? Get the Shape Right Before the Size
For most traditional houses, round cast iron downpipes are the obvious place to start. They have the familiar socketed appearance associated with period rainwater systems and work naturally with many Half Round and Beaded Half Round gutter installations. The Apex Heritage round range we supply covers 65mm, 75mm and 100mm nominal sizes, together with matching bends, offsets, branches, brackets, shoes and access fittings.
Rectangular pipe gives quite a different appearance. Rather than disappearing into the elevation as easily as a smaller circular pipe can, it creates a broader, more architectural face running down the wall. That can be exactly right on larger period buildings or where you're replacing an existing rectangular rainwater system. Our online range includes 100 x 75mm cast iron downpipes, and the important thing with rectangular pipe is that both measurements matter. Have a look at our case study page for some inspiration.
Don't choose rectangular simply because you think it looks more substantial, though. Equally, don't replace an original rectangular system with round pipe purely because round is cheaper or easier to source. On traditional buildings, the proportions of the rainwater goods contribute quite a lot to how the elevation looks. Especially in cities like London.
If you're replacing existing gutters at the same time, our Cast Iron Gutter Profiles guide is useful here. Matching the gutter and downpipe as one rainwater system generally gives a much better result than choosing each part independently.
65mm, 75mm or 100mm? Size Isn't a "Bigger-Is-Better" Contest
Once you've settled on a round pipe, the next question is normally 65mm, 75mm or 100mm?
There isn't one standard cast iron downpipe size that every UK house should use. Roof catchment area, rainfall, the gutter profile, number of outlets and position of those outlets all affect the amount of water each downpipe has to deal with.
Rainclear's sizing guidance makes the same point: the roof area and rainfall requirement should ultimately determine the capacity required, rather than simply choosing a familiar diameter.
As a useful starting point, though, you can think about the Apex sizes like this:
| Round downpipe | General starting point |
|---|---|
| 65mm / 2.5" | Smaller and standard traditional applications where the flow requirement suits it |
| 75mm / 3" | A very common middle-ground size for domestic rainwater systems |
| 100mm / 4" | Larger roofs, higher-capacity systems or existing installations already using a larger pipe |
Our existing Cast Iron Guttering Sizes Explained guide goes further into how gutter size, roof area and downpipe size fit together.
One little detail is worth pointing out is if you're comparing different technical brochures, the smallest Apex circular pipe can sometimes be described as 63mm nominal in manufacturer literature, while it is commonly sold and searched for as 65mm / 2.5".
So don't panic if you see both numbers while researching.
The important bit is identifying the actual system and compatible fittings, rather than assuming two products from different ranges will join simply because their advertised dimensions look close.
The Outlet Calls the Shots: Don't Buy the Downpipe in Isolation
This is probably the easiest mistake to make. You've measured the wall, decided a 75mm pipe looks right and found the downpipe you want. Job done?
Not quite.
The downpipe has to receive water from something above it, normally a gutter outlet or hopper head, and that connection needs to be right.
If your gutter has a 65mm outlet, ordering a 100mm downpipe doesn't magically turn the whole installation into a 100mm drainage system. Equally, a correctly selected 100mm downpipe isn't much use if you've bought an incompatible outlet above it.
Think of the complete route:
roof → gutter → outlet → downpipe → ground drainage
The water doesn't care which part you bought first.
This becomes especially important when you're building a new system. Rather than choosing a gutter and then deciding on the downpipe afterwards, work out the rainwater requirement and choose the gutter, outlets and vertical pipework together.
Hopper heads work the same way. A 65mm hopper outlet is intended to connect into corresponding 65mm round pipework, while larger hoppers can use 75mm or 100mm outlets. Our cast iron hopper collection is separated by outlet size partly for this reason.
For replacement jobs, take a photograph of the existing outlet before dismantling anything. A straight-on photo of the pipe is useful, but an image showing how the gutter actually enters that pipe often tells us much more.
This is also why we'd avoid telling somebody that every normal house needs a 75mm downpipe. Sometimes it will. Sometimes it won't.
Choose the system, not just the pipe.
Old Pipe Detective Work: How to Measure an Existing Cast Iron Downpipe
When you’re replacing one part of an existing system which has gone wrong, don’t begin by asking yourself what kind of pipe you would use on the building today.
Begin by finding out what you’ve got.
With a round downpipe, remember that it is the outside diameter of the actual pipe itself which you need to know, not the wider outside diameter of the internal socket which goes on the top of each section of pipe. This is wider because the next pipe slips inside it. Measuring here will lead you entirely the wrong way.
With a rectangular pipe, you’ll need to measure both ways. If the pipe measures about 100mm across the front and 75mm out from the wall, then you want the 100 x 75mm pipe and not the “100mm downpipe”.
Photographs will help a great deal too. Try to take photos of the pipe, the socket, the wall fixings, the gutter outlet and any bends or offsets in the top of the elevation.
It’s the same again with the gutter above. The traditional cast iron systems have been around for a very long time and a building which is old may well have been patched up several times with various bits.
Ears, Bends and Shoes: The Pipe Is Only Half the Shopping List
Even once you have selected the correct profile and diameter, there are a few other features which will affect the choice between two seemingly identical downpipes.
The first of these is whether you have an eared or uneared pipe.
Eared cast iron downpipe has fixing lugs cast into the socket area. This provides a traditional method of fixing the pipe to the building. The uneared pipe has a more streamlined socket area without the precast ears and is then supported by separate brackets or holderbats.
Neither approach is inherently "superior."
If you are replacing an existing system, consider the fixing arrangements used previously. In the case of a new system, consider the desired appearance and fixing arrangements for the wall.
Then consider...
Is the gutter outlet precisely aligned with the point at which the vertical pipe needs to go? On older buildings, almost certainly not.
Enter bends and offsets. An offset moves the pipe away from or back to the wall in order to accommodate projecting eaves, stonework, cornices, or other changes in the elevation. Branches provide another means of routing rainwater into the downpipe, while a shoe provides a means of discharging the water away from the wall when the pipe does not continue directly into the drainage system.
This is why "three lengths of downpipe" is rarely sufficient to solve an elevation problem.
The Apex Heritage range includes pipe lengths as well as bends, offsets, branches, loose sockets, brackets, access pipes, and shoes, so it’s worthwhile drawing out the full route before ordering.
If you're installing the system yourself, our complete cast iron gutter installation guide explains how the gutter and downpipe installation fit together.
Pipe Dreams Sorted: So Which Cast Iron Downpipe Should You Actually Buy?
If you're replacing an existing cast iron downpipe, I'd make matching the priority. Identify whether it's round or rectangular, measure the pipe rather than the socket, check the outlet above and then work through the bends, offsets and fixing arrangement. Don't swap size or profile simply because another option happens to be easier to find.
For a new traditional system, round downpipes are normally the easiest starting point. The Apex Heritage round cast iron range we supply is available in 65mm, 75mm and 100mm sizes, so you can choose the pipe diameter alongside the gutter outlet and expected rainwater requirement.
For a building where the existing architecture calls for a broader, squarer rainwater pipe, look at the rectangular range rather than trying to make a round pipe imitate the original appearance.
The main thing we'd avoid is choosing from one measurement alone.
A good downpipe choice considers profile, diameter, outlet compatibility, roof drainage requirement, wall fixing and the fittings needed along the route. Get those right and the decision becomes much less confusing.
Still stuck? Send us a clear photo of the gutter outlet and downpipe, preferably with the pipe diameter or width/depth measurement. If there are bends or unusual offsets, photograph those too. It's considerably easier to work out the right system from a few decent photos than from “I've got an old black pipe and I think it's about three inches”.
A useful thing to do this week is simply measure the existing pipe and photograph the outlet above it. Those two bits of information answer an awful lot of questions before you spend anything.
