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Question DetailsAsked on 7/21/2017

Can I find out how many inches wide the cast iron pipe is under my house without having to dig?

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3 Answers


Well, if you make the usual, but not guaranteed, assumption that the pipe size where it enters the slab is the same all along under it, you could just measure it there.

A Sewer Cleaning contractor could also run in a router head (rigid type, like a root cutter) through the pipe - should be able to tell if it opens out after the entry point by the feel of the cable (because the head would flop around and "kick" more in the larger pipe - though if someone did something hokey and upsized more than one pipe size he probably could not detect that difference - liek the difference between 3" to 4" versus 3" to 5".

I suppose an innovative sewer cleaning contractor could tape a piece of something to a sewer camera, sort of a target or whiskers of certain diameter (presumbly almost 3") in front of the camera and then run it in the sewer line - using the camera to look at the target moving ahead of it and see with the camera whether the target just barely clears the ID of the pipe or if the pipe opens out more along the way, and measure how far is so. Target would have to be something that would not block the line if it fell off, but not go wet and floppy right away either.

You could also measure the pipe size where it enters the concrete, and again where it comes out at the cleanout (if you have the usual cleanout a few feet outside the foundation) - bearing in mind the first would likely be an outside diameter measurement (unless pipe size is labelled where you can see it) whereas at the cleanout riser you might or might not have a stickout at the cleanout riser pipe to measure the OD - might be you would have to measure the ID using a tape measure, or maybe a long stick or such with a nail sticking out of it to the side, to feel for the top and bottom ID of the pipe and mark those points on the stick at the top of the riser casing, then measure the distance between the two marks you made to determine the ID.

It is possible (though unlikely unless there is a wye junction underground) that the pipe size would increase en-route - normally would be same size from where it enters the slab to the street or septic tank, if there is an underground junction then would (at least if a waste pipe rather than just drain) be one size larger from there on - so maybe 3" going up to 4" at the wye.

I won't ask WHY you need to know the dimension - assuming you are planning on having materials on hand before you dig ? Why not just find a store which will refund your money for clean unused materials and buy both initially ?

Answered 3 years ago by LCD


BTW - here are links to a few previous similar questions with answers about replacing/repairing cast iron pipe which might be of help, FYI:

Answered 3 years ago by LCD


One thing I did not mention - 2 actually - if your normal household DWV (drain, waste, vent) pipe carrying the fluid is going to be 3 or 4 inch inside diameter (so around 1/4-1 inch greater OD depending on piping material, and even more at flanges/connecting hubs) - 3 inch commonly in pre-70's homes, mostly 4 inch since then.

The in-ground portion would commonly be 3" ID pre-60's or so houses but some 4", almost always 4" after that time. For a large house with more than two branches to the in-house sewer lines or more than about 4 toilets the in-ground portion may be 5" ID, and up into 6-8" in mansions and such. Commercial buildings commonly 4-5 inch ID in-building and 5-8" underground (and of course on up for very large buildings like hotels and highrise buildings and large hospitals and such).

Note that in digging up sewer lines under slabs, commonly the excavation is made about 10-12 inches wide bare minimum anyway - both to provide working room and to make it more likely that the run of the line (which may not be prefectly straight between known points) falls within the excavated trench, not in under the slab.

Answered 3 years ago by LCD

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