3" or 4" main septic line?

[5030tinkerer]

3\" or 4\" main septic line?

I am building a new house and am at the point where I need to install the under basement slab plumbing. I have always been under the impression that bigger is better and have noted that all the homes I have ever toured used 4" septic/sewer main line. A builder that I spoke to today, however, indicated that with the low-flow toilets of today, that a 3" main line is better than a 4" since the 3" will allow a greater water level height in the pipe, thus doing a better job of getting the blackwater (the waste from the toilet) to successfully get flushed out of the line with each toilet flush. I can only set this line once. What do you guys think is better, a 3" line or a 4"? Does this opinion change for the line leaving the house going to the septic tank?

 

[5030tinkerer]

3\" or 4\" main septic line?

I am building a new house and am at the point where I need to install the under basement slab plumbing. I have always been under the impression that bigger is better and have noted that all the homes I have ever toured used 4" septic/sewer main line. A builder that I spoke to today, however, indicated that with the low-flow toilets of today, that a 3" main line is better than a 4" since the 3" will allow a greater water level height in the pipe, thus doing a better job of getting the blackwater (the waste from the toilet) to successfully get flushed out of the line with each toilet flush. I can only set this line once. What do you guys think is better, a 3" line or a 4"? Does this opinion change for the line leaving the house going to the septic tank?

 

[JJT]

Re: 3\" or 4\" main septic line?

We have 3 full baths, all low flow toilets and I used 4" pipe for as much as possible. 10 years later, no regrets. In highschool I worked for a plumber, we never ran 3" if we could run 4". Bigger is better and make sure you have the right pitch and keep your outside run as short as possible. I put in 2 1000 gallon tanks to keep my runs short. Overkill for sure, but I'd do the same today.

 

[JJT]

Re: 3\" or 4\" main septic line?

We have 3 full baths, all low flow toilets and I used 4" pipe for as much as possible. 10 years later, no regrets. In highschool I worked for a plumber, we never ran 3" if we could run 4". Bigger is better and make sure you have the right pitch and keep your outside run as short as possible. I put in 2 1000 gallon tanks to keep my runs short. Overkill for sure, but I'd do the same today.

 

[JimR]

Re: 3\" or 4\" main septic line?

I'll second that. 4" is the way to go. In 30 years we have never plugged ours up.

 

[JimR]

Re: 3\" or 4\" main septic line?

I'll second that. 4" is the way to go. In 30 years we have never plugged ours up.

 

[marks]

Re: 3\" or 4\" main septic line?

When building my 5 bdr/3 bath home several years ago, I posed this same question to the plumbing contractor regarding main lines and branch lines. His response was similar to the one you received. However, sizing requirement is a basic calculation and specified in the code.

 

[marks]

Re: 3\" or 4\" main septic line?

When building my 5 bdr/3 bath home several years ago, I posed this same question to the plumbing contractor regarding main lines and branch lines. His response was similar to the one you received. However, sizing requirement is a basic calculation and specified in the code.

 

[5030tinkerer]

Re: 3\" or 4\" main septic line?

There are no codes or inspectors where I am building. That is not so much the concern as what will be better. I obviously do not want clogs. The home will have four bathrooms (two full, one 3/4, one 1/2). All four toilets will be 1.6gpf or less.

 

[5030tinkerer]

Re: 3\" or 4\" main septic line?

There are no codes or inspectors where I am building. That is not so much the concern as what will be better. I obviously do not want clogs. The home will have four bathrooms (two full, one 3/4, one 1/2). All four toilets will be 1.6gpf or less.