Cast iron downpipe sizes

[GMM]

Does anyone know whether cast iron downpipes (from gutters, not soil) are measured on the internal or external diameter? I need to repalce a short run, and bit I have is about 80mm external diameter (more or less, accounting for the paint and rust), while all the CI pipe I find online is 75mm, so I'm wondering whether I should be measuring the ID or whether the new stuff is actually a different size. (Of course, it would have been either 3" or 3 1/4" in the day.) Just to confuse the issue, some suppliers have plastic pipe in 80mm, which may be a cost-efective way forward (as 6' of CI seems to be £70-80!).

If it wasn't 20 feet plus up the wall and above the conservatory roof, I'd rip it all down and replace with plastic, despite the historic aspect, but that will have to wait until we have some scaffolding up for the roof job, which will probably be next summer now. I don't much fancy dismantling CI pipework off a ladder, given its weight and the likely need for 'destructive dismantling'.

[harryagain]

In general in the UK all pipework (but not electrical conduit) is sized by the nominal bore. So 75mm pipe will have a bore of about 75mm but bigger outside. (Depending on wall thickness) It's nominal because it varies due to having to make the pipe fittings interchangable even if the wall thickness vary. The pipe fittings are often fixed size even if the wall thicknesses vary. Thicker walls yherefore reduce bore size (though it's still referred to as the same bore size.)

It's not 75mm it's 3". BTW mm comes from the EUSSR frogs

[Andy Burns]

The frogs do tend to use myriad different sizes, e.g. for copper elbows compare and contrast ...

Surprised their unions haven't found a way to outlaw push-fit, try to buy a "pince à emboiture" over here, they don't seem to make a 15mm "matrix" for them ...

[Andy Burns]

I take that back, the manufacturer does make a 15/22/28 set

[Nightjar]

While generally true for pipework based upon the original wrought iron and cast iron pipe sizes, (i.e. today steel and plastic but not copper) the problem is that 80mm OD does not correspond to any standard pipe.

2.5 inch bore is nominally 73mm OD, while 3 inch bore is nominally 89mm OD. I suspect that, as rainwater pipes never needed to have a BSP thread put on the outside, they don't comply with that general rule.

[Andrew Gabriel]

Although they call them something else, you will find their threaded plumbing fittings are 1/2" BSP, 3/4" BSP, etc. This is useful for buying thermostatic shower mixers, which are much cheaper in France than they are here.

For copper tube, their equivalent of 15mm is 14mm, but unlike here, many sizes are more commonly used too. (If you go to somewhere like BES, you will find all the sizes here too, but you won't find them in stock in a standard plumber's merchant.)

[GMM]

Indeed, which is sort of my problem: Although they don't need a thread, CI pipes are produced with a connector and wall bracket integral to one end. If that doesn't accept the existing pipe, then it can't be compatible, and that's 80 quid (and all the asociated trouble of sourcing it) down the swanny, for one short length. What seems to be difficult to establish is whether all these various sizes are similar and compatible or whether they are all specific to individual manufacturers, in which case everything needs changing (so I may as well replace it with plastic). If I assume a nominal wall thickness of 1/4 inch / 6mm (I don't hink it's that much, but it's more than 1/8), then the 80mm OD becomes 68mm ID, oddly the size of new plastic pipe.

[Nightjar]

I don't like plastic rainwater goods. I much prefer cast aluminium, such as Alumasc:

You could probably do worse than contact their technical support, say you want to match an existing CI downpipe and ask them which of their products is the right one.

[Tricky Dicky]

From my experience of cast iron piping is that they are quite a sloppy fit in their connectors and slight differences in overall diameter are not goin g to be a problem. Getting hold of sections might be a bigger problem and I suspect you will have to source them from salvagers.

Richard

[Nightjar]

You can buy cast iron rainwater goods at Jewsons.

[Timothy Murphy]

I've been following this thread with interest, as I have been vaguely thinking of installing a long downpipe - around 50ft - and would probably be required to use cast-iron as the property is a listed building, and the downpipe would be in the front of the building.

I'm wondering if this would be a feasible project?

The reason I am considering this is that at present the rain water from a very large roof goes over a valley in the roof of our part of the property - an old orphanage - and it is slightly disturbing to have what sounds like quite a large river going over our heads.

[Vir Campestris]

I'd be cautious about aluminium in contact with iron. Make sure you have a gap of some sort - a good thick coat of paint might do - to prevent any funny galvanic stuff going on.

Andy

[Nightjar]

They recommend a thick layer of bitumen, but I was thinking more of simply asking their technical support about sizes.

[Nightjar]

Cast aluminium, which is available as visually the same, would probably be accepted, unless it is Grade I.

Has it caused problems? If not, I would be inclined to leave it as it is.

[Timothy Murphy]

Thanks for your helpful response.

The roof leaks slightly under very heavy rain. The quantity of water that goes over the roof is astonishing. This is a very large building - we only live in a small part of it - and all the rain falling on the roof goes over our valley.

Our builder offered to put in a downpipe some time ago, but when he came with a colleague to look at it they decided the quantity of water falling was too great for a soak in the front of the house. I wanted to collect the water in a tank/cistern, but they decreed that this was impractical.

[Bill]

I don't know if this info will be any use to anyone, but the new cast iron that my son is putting up is approx 70mm inside dia and 80mm outside. The other end has the mounting and larger dia for the next section to fit into.

I meant to take a micrometer, but forgot so just measured with a rule.

He has 2 of these pipes that take water from a large roof in the next garden down a sandstone wall into a soakaway.

His brother and a friend came to help him last weekend. They spent 4 hours setting up the scaffolding tower and inserting wooden plugs into the soft sandstone, while he was "unfortunately" forced to take a long and important business call. His brother was not very pleased. The pipes are still lying on the ground, which is why I was able to measure.

[Nightjar]

I suspect that what would be of most use to the OP would be to know what size that was sold as.

[Nightjar]

That sounds as though the gulley is filling up and spilling over the edge of its lining, under the slates / tiles.

If there is not room for a soak away in front of the house, then there is not much you can do, unless there is a surface water drain in the area the water can be directed into. Soak aways need to be at least 5 metres from any building and of a size that can cope with the highest water flow likely.

You might do better looking at whether the peak capacity of the gully can be improved.

[Bill]

This is cut and pasted from an invoice. If I remember the micrometer, I will take more measurements

2 of R3001E Primed Eared 75mm Cast Iron Downpipe x 1.83m length @ £99.70 10 of R0004 100mm Pipe Nails @ £4.10

Sub Total: £103.80

Delivery £0.00 VAT @ 20% £20.76 Total £124.56

If you have any questions about your order, please don't hesitate to drop us an email at [e-mail].

[Bill]

In message , Bill writes

OK, this pipe is 69.4 mm internal dia, 79.96 O/D. The larger socket end into which the next section of pipe fits is 84.50 mm internal.

I also measured the old pipe from the wall, which we believe is the original from approx 1890.

It is internal 66.5, external 77.85 and the larger socket end (and the original cast hopper receiver 84.2 internal, all to include rust etc.