How to Hang a Horizontal Drain at Quarter-Inch Fall

Why this matters

A drain hung at 1/4 in per foot on average can still hold water. If one hanger sits a quarter inch high, the pipe between it and the hanger upstream runs flatter than the minimum, and the one downstream runs steeper, and the average still comes out right. The flat stretch is where grease and lint settle, and it is where the first stoppage happens two years later. This card is about getting the grade right at every hanger rather than end to end. What slope a given size needs is owned by Drain Pipe Slope + Venting Quick Reference; on long runs or where elevations are fixed by surveyed inverts, the arithmetic is How to Work Out Drain Slope from Invert Elevations.

What 1/4 in per foot looks like on a tool

• On a 4 ft level: 1 in of drop over the level's length, so the bubble centers with a 1 in shim under the downhill end, or a slope vial reads 1/4.
• On a digital level: 1/4 divided by 12 is 2.08 percent, about 1.2 degrees.
• Over a run: the run length in feet times 1/4 in. A 24 ft run falls 6 in.

The code minimum depends on pipe size, and many codes allow less slope on 3 in and larger; IPC 2021 Table 704.1 and the UPC 2021 slope provisions, as adopted, set it. The 1/4 in default on this card applies to the 2 in run in the example, which is in the size range where 1/4 in is the minimum under both codes.

Hazards of overhead drain work

Ladders and platforms. Work from a stepladder or platform rated for you plus tools, set on firm level floor, never standing on the top step or cap; 29 CFR 1910.23 covers ladders in general industry and 29 CFR 1926.1053 on construction work.

Drilling overhead. Eye protection for every hole; debris falls in your face. Before driving a fastener into framing, look for cable, gas tubing and other pipe in the bay. A hanger screw through a cable is a shock hazard the next person finds.

Drilling concrete for anchors. A hammer drill in a concrete ceiling or wall makes respirable crystalline silica. On construction work, 29 CFR 1926.1153 Table 1 sets the controls for a handheld drill, typically a dust collection system with a filter; in general industry, 29 CFR 1910.1053 applies. Do not dry-drill a row of anchor holes overhead with no dust control.

Cutting and cementing the pipe follow their own cards, including the solvent vapor controls in How to Cut, Prime and Glue a PVC DWV Joint.

The method

1. Fix both ends first. The upstream end is usually set by a fixture: the trap arm's outlet, a standpipe's trap, a stub-out. The downstream end is the connection to the drain you are joining, whether an existing fitting or a new one you cut in. Measure the vertical distance between the two, at the bottom of the pipe, and the horizontal run between them. Why first: the slope is decided by those two points. If you start hanging from one end without knowing the other, you find out at the last hanger that the run comes in high, and the only fix is to re-hang the whole length.

2. Check the fall you have against the fall you need. Run length in feet times 1/4 in is the minimum drop. If the available drop is less, the run does not work at this slope, and the answer is a different route, a lower connection point, or a larger pipe size the code lets run flatter, not a flatter 2 in pipe. If the available drop is more, either run the whole length at the steeper uniform grade or take the extra at one end with a fitting offset.

3. Run a line. A taut mason's line from the bottom of the pipe at the upstream end to the bottom of the pipe at the downstream end gives the grade at every point between. Pull it hard and support it midway on a long run, because a line sags; check the line's midpoint against a laser or a level reading to confirm it is straight. A rotating or line laser set level, with the drop measured down from the beam at each hanger, does the same job without sag. Why a reference at all: setting each hanger from the pipe's own level reading compounds error one length at a time.

4. Set hangers to the line at spacing. Hangers at the spacing your adopted code's table gives for the pipe material, plus one near every change of direction and close to heavy fittings. For PVC that is 4 ft horizontal in IPC 2021 Table 308.5; Pipe Support Spacing by Material owns the numbers. Each hanger is adjusted so the pipe's bottom just touches the line. Why at the line and not "about right": the line is the grade. A hanger off the line is a belly or a hump.

5. Hang the pipe, then check every length. Put the pipe in the hangers, make the joints, and lay a level or slope gauge on each length between hangers. Every reading should be at or just over 1/4 in per foot. Why every length: the line can be knocked, a hanger rod can slip, and a joint made after the hangers were set can push a length up.

6. Look along it. Sight down the pipe from one end. A belly or a hump is visible when you look along the bottom edge against the line.

7. Secure without clamping tight. Hangers carry the pipe and keep it aligned; on plastic they should not squeeze it, because the pipe moves a little with temperature. Strapping that lets the pipe droop between supports is not a hanger.

A worked run

The job. A 2 in PVC drain from a new laundry standpipe and utility sink, run 24 ft along a basement wall below the joists, connecting to a new 3 x 3 x 2 wye cut into the 3 in building drain. The standpipe's trap outlet fixes the upstream end, and both traps are vented at that end within their trap arm limits, so the 24 ft run is a vented branch, not a trap arm. The wye's height was free within a few inches, so the downstream end was set to make the grade exact: 6 in below the upstream end at the bottom of the pipe, 24 ft away. 24 times 1/4 is 6 in.

Hangers. At 4 ft spacing over 24 ft, seven support points counting both ends: 0, 4, 8, 12, 16, 20 and 24 ft. Each sits 1 in lower than the one before it, because 4 ft at 1/4 in per foot is 1 in.

 UPSTREAM end                         DOWNSTREAM end
 (trap outlet)                        (new 3 x 3 x 2 wye)
 0     4     8     12    16    20    24 ft
 |     |     |     |     |     |     |
 H-----H-----H-----H-----H-----H-----H
 0     1     2     3     4     5     6 in
 drop below the upstream end, at each hanger H
 string line taut along the pipe bottom, end to end

The line. Mason's line from the bottom of the trap outlet to the bottom of the wye's branch socket, pulled tight. Checked at 12 ft against a level laser line: the laser showed 3 in of drop at the midpoint, matching the table, so the line was straight.

The hanger that was off. After the pipe was in and the joints made, the slope gauge on each length read:

• 0 to 4 ft: 1/4 in per foot.
• 4 to 8 ft: 1/4.
• 8 to 12 ft: 3/16.
• 12 to 16 ft: 5/16.
• 16 to 20 and 20 to 24 ft: 1/4.

The pattern, one length flat and the next steep, points at the hanger between them. The 12 ft hanger measured 2-3/4 in below the upstream end instead of 3 in, 1/4 in high. Its rod had been adjusted while the line was lifted over a duct boot, so it was set to a line that was not where the grade said. Check: the 8 to 12 ft length dropped 2-3/4 minus 2, or 3/4 in, over 4 ft, which is 3/16 in per foot; the 12 to 16 ft length dropped 4 minus 2-3/4, or 1-1/4 in, which is 5/16. The end-to-end fall was still 6 in. Averaged, the run passed. At the 12 ft hanger, it had a stretch below the minimum.

The rod was lowered 1/4 in, and every length then read 1/4 in per foot.

Finish. The run was filled for the water test with the rest of the rough-in, per How to Water or Air Test a DWV Rough-In, and after draining, the slope gauge was laid on each length once more to confirm nothing had shifted under the weight of the water: unchanged.

Where the method changes

Not enough fall. Common when a drain has to reach a building drain that sits only slightly lower than the fixture. Options: a lower connection point, a different route, a pump, or a size the code allows to run at 1/8 in per foot. A flatter run in the same size is not one of them.

Too much fall in a short run. There is no general maximum for a fixture drain in most codes, but a steep run that flattens abruptly can strand solids where the grade changes. Take large drops as a vertical offset with 45s rather than as a very steep horizontal run, and keep trap arms within their own limits, per Trap Arm Lengths by Trap Size Under IPC and UPC.

Long runs and fixed inverts. Over 40 ft or so, or where both ends are fixed by existing pipe whose inverts you are reading off a laser, do the full calculation from inverts first.

Pipe under a slab. The same method, with stakes or a laser in place of hangers, and the bedding compacted so the pipe cannot settle out of grade.

Checking someone else's run

• A slope reading on every length between hangers, not only one reading at one end.
• Hangers at the table spacing for that material, with supports at changes of direction.
• No length reading under the minimum, even if the run's total fall is right.
• No pipe resting on ductwork, wiring or other pipe that will move it later.

References

• IPC 2021 Table 704.1 and Section 308, or the UPC 2021 slope and support provisions, as adopted by your jurisdiction
• 29 CFR 1910.23 and 1926.1053 (ladders), and 29 CFR 1926.1153 or 1910.1053 (respirable crystalline silica) for anchor drilling
• See related: Drain Pipe Slope + Venting Quick Reference and How to Work Out Drain Slope from Invert Elevations
• See related: Pipe Support Spacing by Material and How to Water or Air Test a DWV Rough-In