Cast iron vs PVC drain pipe

The matrix

Fire and firestopping

This is the difference that decides stacks in multi-story buildings, and it is a building code question before it is a plumbing one. IPC 2021 307.3 and 315.1 send every penetration of a fire-resistance-rated assembly to Section 714 of the International Building Code (IBC). What 714 then asks for depends on what the pipe is made of.

Cast iron is ferrous and non-combustible. IBC 2021 714.4.1 (walls) and 714.5.1 (floors) carry exceptions written for "steel, ferrous or copper" pipe: in a concrete or masonry wall, a pipe of up to 6" nominal through an opening of no more than 144 sq in may be packed with concrete, grout or mortar for the full thickness, and a fill that keeps flame and hot gases from passing under the ASTM E119 or UL 263 fire test is also accepted. PVC qualifies for none of those exceptions. A plastic pipe burns out of its own hole, so every rated penetration needs a tested through-penetration firestop system.

A tested system under IBC 2021 714.4.1.2 or 714.5.1.2 is tested to ASTM E814 or UL 1479 at a positive pressure of 0.01 in of water, and its F rating must meet the rating of the wall it passes through. On floors both the F and the T rating must be at least 1 hour and not less than the floor's own rating; the T rating is waived for penetrations inside a wall cavity and for floor, tub and shower drains in a concealed horizontal space. For plastic that usually means a listed collar or wrap sized to the pipe — a part with a listing number that the inspector will want to see on the submittal.

Expansion, support and standards

PVC moves. At about 3.6" per 100 ft per 100 °F it grows several times as much as iron, which matters on a tall stack taking hot discharge or a long run in an unconditioned space. IPC 2021 305.2 requires piping to be installed so expansion and contraction cannot overstress it, and 308.8 limits expansion joint fittings to places that genuinely need them. Offsets that let the pipe flex are the usual answer. Iron's movement is small enough that it rarely enters the conversation.

Support goes the other way. Table 308.5 lets iron hang at 5 ft (or 10 ft where 10 ft lengths are used) and stand 15 ft between vertical supports, against 4 ft and 10 ft for PVC. Slope is identical for both; the drain slope calculator does not care which you pick.

On standards, check the print line on the pipe. Schedule 40 marked only ASTM D1785 is pressure pipe and does not appear in the IPC 2021 drainage tables; DWV wants D2665, F891 or F1488. The IPC basics guide covers how those tables are adopted, and IPC vs UPC covers what changes if your jurisdiction runs the other code.

Where each one wins

Repairing old cast iron with PVC

Most of the time this comparison is not a new-build choice at all. It is a cracked hub, a rotted section of building drain or a leaking stack in a house plumbed in iron, and the question is whether PVC can go in for the length you are cutting out. Under the IPC it can, provided the joint between the two is the right kind.

IPC 2021 705.16 requires joints between different materials to be a mechanical joint of the compression or mechanical-sealing type conforming to ASTM C1173, ASTM C1460 or ASTM C1461, with an elastomeric seal. 705.16.4 then says a plastic pipe meets a cast iron hub with a caulked joint or a mechanical compression joint, and meets other materials through an approved adapter fitting. Solvent cement has no part in it: the only solvent joint the section allows between materials is one ABS-to-PVC joint where the building drain becomes the building sewer, which is a different story covered in ABS vs PVC for DWV.

Two things catch people out. First, hub-and-spigot iron to ASTM A74 comes in two classes, service weight (SV) and extra heavy (XH), and the extra wall is added to the outside, so the two have different outside diameters. Measure the pipe you actually have before you buy the coupling; a transition coupling is sized to both diameters it joins. Second, iron is heavy and the run above or beside the cut is held up partly by the piece you are about to remove. Hang or clamp it on both sides first, so the pipe cannot drop or shear a joint further along once it is cut.

What the repair costs, without a made-up number

We could not verify a current published cost band for cast iron replacement on the day this page was written, so there is no dollar figure here. The price is set by access more than by material: iron in an open basement ceiling is the cheap case, the same length under a slab means breaking and patching concrete, and a stack inside finished walls on an upper floor means opening the walls and, where it passes through a rated floor, firestopping it again. A permit is normally required either way — see plumbing permit cost — and if the iron runs out to the street, the sewer line replacement cost guide covers the outside part of the job.

Buying checklist