Why was 4" the standard?

ChrisJ

Hi all,

Working on some projects at home and as usual my mind started wandering.

Why was 4" the standard pipe size for the drain in houses for years? And when and why did it end up dropping to 3"?

Obviously 3" is more than big enough for most houses, but if that's the case why was 4 "the norm" for so many years? 4" doesn't fit in a standard wall and costs quite a bit more money so I have to think there was a reason originally. Is this a cast iron vs PVC thing? 3" wasn't a standard size at one point?

I'm sure someone out there knows the history.

acwagner

From the engineering standpoint larger pipe allows for shallower slopes for the same flow capacity, which gives more flexibility making gravity flow work on longer runs. Material roughness also effects flow rate--rougher means less flow. A smaller, smoother PVC pipe can handle the same flow as a larger cast iron pipe.

Mannings equation is what is used for designing "open channel flow" or partially filled conduits such as sanitary sewers.

My guess is that the 4" was the best compromise between function, cost, and performance based on trial and error back in the day. Over time, changes in new materials such as PVC, flow from fixtures, and someone taking the time to value engineer all this determined the 4" was over sized.

Just my guess.

Intplm.

@acwagner That's probably the best explanation I have ever seen.
Now that PVC and ABS piping is so well established, some codes have allowed for a lesser drainage pipe size in some respects.
The smooth extrusion on the I.D. of plastic pipes makes for a much smoother flow as you mention above.
Looking into the I.D. of cast iron and galvanized drain pipes of the past show a much more rough internal surface, hindering flow.

JUGHNE

One thing I noticed out here in the no-code rural areas, only 4" CI, never 3", some 2" CI but galv 2" and 1 1/2".
Only "S" traps on everything.

Only one vent pipe thru the roof, most things drained most of the time. I believe the 4" had enough room for the air to move around and find the VTR. FWIW

ChrisJ

Was or is 3" cast iron even a thing or was that size introduced with abs or PVC?

Danny Scully

Just worked at a house today built in 1905 that had original 3” cast @ChrisJ ...if not original than from whenever they changed from cesspool to county sewer. I think plumbing was more of an experiment back than then the science it is today and that 4” was the norm, just because it was.

JUGHNE

IIRC, there was 3" CI in the city (inspected...to code). Possibly trying to compete with copper DWV. Which 3" was the largest I have seen. It dropped into 3" or 4" CI under the floor.

Now, where did 1 1/4" NP size come from.....it is the only 1/4" size??

Danny Scully

No @JUGHNE, 1/4” is another 1/4” size

SlamDunk

Thanks for the question and answers!
Now I don't have wonder about this while I conduct my planned 4" CI pipe inspection tomorrow morning for cracks. Got a whiff of something bad. Hope it is a dead mouse. Wish me luck; tight crawl space.

ChrisJ

@DanHolohan Needs to do a plumbing book on the history of plumbing. I'd buy it!

I I believe my 4" CI was installed in 1910.
My 2nd floor bathroom has all copper drains which I have no idea when they were done but I assume they came later.

mikeg2015

Older houses typically drained the downspouts into the sewer. To vent all the roof water you need plenty of pipe.

EBEBRATT-Ed

Depends in if it is a riser or horizontal

ChrisJ

> @mikeg2015 said:
> Older houses typically drained the downspouts into the sewer. To vent all the roof water you need plenty of pipe.

Where have you seen this?

mikeg2015

Almost all the homes here in my town. Sewers were originally combined, storm and sanitary. Maybe it’s just a Midwest thing.

City made you disconnect the Down spouts the lasts 10 years. Makes ground water issues worse.

Fred

My gutters drain into the storm sewer. I have never seen gutters drain into sanitary sewers (legally) but I suppose if the storm and sanitary are a combined system , it's possible. Most turn of the century or prior homes either drained into a storm system (if available) or a cistern.

710.1 Maximum Fixture Unit Load
The maximum number of drainage fixture units connected to a given size of building sewer, building drain or horizontal branch of the building drain shall be determined using Table 710.1(1). The maximum number of drainage fixture units connected to a given size of horizontal branch or vertical soil or waste stack shall be determined using Table 710.1(2).

Tables are on this link but probably vary slightly by jurisdiction. I would also guess that 4" was the standard because when indoor plumbing became a common amenity, toilets flushed 6 to 8 gallons of water, tubs were large and deep and people used a lot more water to fill them and showers were less common so the number of
"fixture units" connected to a waste pipe was likely far fewer but dumped far more volume than the more efficient fixtures that evolved over the years :
https://up.codes/s/drainage-system-sizing

Intplm.

ChrisJ said:

Was or is 3" cast iron even a thing or was that size introduced with abs or PVC?

Not sure about the size introduction.

I have noticed however, that when using a ratcheting C. I. cutter at times the chain cutter links don't match to the tightest link required to make a cut or the link needs to be tapped on gently with a hammer.
Sometimes when this is done the cutter will jam and not make a cut and you have to start the process over again. This could be a hint to what you are saying above.

Has anyone else had this problem ??

It doesn't seem to happen on any other dimension whether it's NH, older installed hubbed CI or even XH or XXH

BenDplumber

@FRED CITY OF KETTERING has multiple combo sewers. I've personally done at least 15 drain disconnects separating storm from sanitary, which the city paid for under the guise of chief plumbing inspector Mr. David Bunn RIP. Sumps and storms tied into sanitary drainage. Work involves excavation of concrete and redirecting storm drainage into sump pit, adding a check valve to laundry tray and backwater valve to sewer lateral. Oversized 4" on single bath home is common given the hydraulic flow rates when storm drainage is applied to the sanitary sewage system circa 1940's-1955
.

Fred

@BenDplumber I guess that doesn't surprise me. I'm in Dayton. Kettering was mostly farmland before it was Incorporated in 1955. Prior to that it was Van Buren Township and became a village in 1952. I'm sure the older infrastructure from the village days were likely to do it as cost effectively (cheap) as possible until it grew to its current population of somewhere around 60K, making it the largest suburb of Dayton.