Is there a standard for sewer depth below slab for a single story home?

[Tomject]

I'm converting my garage to an ADU (aka mother in-law home) and as part of the project, am connecting to the sewer in our house about 25' away. Not having designed a 1st floor bath on a slab before- I'm wondering if there is a standard for how deep I should be trying to get the sewer line when it hits the garage? I know enough to make sure it's graded at least 1/8"foot for 4" drain, and no more than 1/4" per foot.

Obviously trying to minimize how deep of a trench I need to be digging between buildings. Currently it's dug to just below the bottom of the slab in the garage, but clearly I'll need some more depth to account for plumbing under the toilet and shower drain.

Toilet is going to be right where the trench lands at the garage, and the shower drain is about 15' from the toilet.

Any suggestions off top of head? Or should I get some drawings going to explain better?
Thanks for your input ya'll.

 

[Tuttles Revenge]

There is no Minimum depth inside of a building for the Drainage. However, you will want to have crushed gravel over the top of your pipe between your slab to prevent weakening of your slab. I like to have at least 3" coverage at a minimum from the top of the drain to the bottom of the slab.

 

[Sponsor]

 

[Tomject]

There is no Minimum depth inside of a building for the Drainage. However, you will want to have crushed gravel over the top of your pipe between your slab to prevent weakening of your slab. I like to have at least 3" coverage at a minimum from the top of the drain to the bottom of the slab.

Good point, I'll make sure I've got that. I guess my question is more specific to how much depth the toilet closet bend and shower drain would require.

 

[Tuttles Revenge]

A lot will depend on your layout of fixtures in relation to walls where vent pipes will extend up and how you plan to vent each fixture. There is no required depth per se. But generally if kept below the slab, you should be able to vent vertically up into an adjoining wall. A 4x3 closet ell can be installed with the top of the pipe about 1" below the finished floor and still work but it ends up cracking a slab. Vent fittings.. IE Wyes and Combos used for venting have long radii so the deeper they are the less the radii come into effect, meaning that at the top of the bend the fitting is circular. So in essence, if your system starts with about 3" of clearance over the top of the pipe to the bottom of the slab, you won't have these to worry about.. again, in a well designed layout.

 

[Tomject]

Thank you for the help.

I've drawn up a layout for the plumbing in the new building, and it looks like I can make it with sewer bottom 12" below slab top.
Currently my limiting factor on overall depth is the toilet flange.

I also need to figure out where to put in the cleanout(s). The building is on the lot line, so I can't continue the main drain past the shower (left side) and add cleanout on the outside. I can add a 2 way where it exits the building (right side of drawing), but not sure if that will be enough for future maintenance.

Finally, I may need a floor drain? and possibly a drain for a nearby tank water heater. Is it a dumb idea not to put one in while I'm cutting up bits of the concrete floor already...

This is the first plumbing I've designed, and I'm sure I'm making some bone-head beginner mistakes... so please have a look if you
have time. Thank you all.

 

[wwhitney]

Comments:

1) You don't need any 4" in the system shown, you could use 3" anywhere you are showing 4". But 4" is OK, and people do like it for the WC riser to the closet flange (which can reduce to 3" on the horizontal via a 3x4 reducing closet bend, a rare allowance to reduce size going downstream).

2) The only times you can use a sanitary tee are (a) when all 3 connections are dry vents and (b) when the barrel (straight path) is vertical (at most 45 degrees off plumb) and the outlet is a drain. Elsewhere you need a combo. So that would be for your shower dry vent takeoff and where the lav 2" drain hits the 4" building drain

3) You show a shower dry vent that goes horizontal under the slab. Dry vents can't go horizontal until at least 6" above the fixture flood rim. [That applies at the kitchen sink, too.] If you need a shower dry vent, route the shower fixture drain/upstream end of the building drain next to/under a wall and use a combo for a vertical dry vent takeoff up into the wall. That riser into the wall could be full size, allowing you to put a cleanout access in the wall.

4) If the shower trap arm (from the trap outlet to the combo where the 2" lav drain joins the shower drain) is 5' or less, then the lav can wet vent the shower, and you don't need that shower dry vent. But you'd still have to figure out a cleanout solution.

5) No laundry, and just one bath?

Cheers, Wayne

 

[Jeff H Young]

I run everything 1/4 per ft minimum unless I have reason not to.
4 inch clean out at end of line, I dont run 4 inch to one bathroom or 2 inch trap arm or trap at kichen , nor run 4 inch all the way to the shower ( which has an illegal dry vent running flat) so a few tweaks to this.
If you have depth and fall Id be around 10 inches below slab at the W/C

 

[John Gayewski]

Generally exiting a building we would go below the frost depth or very near that depth and that's ideal. This also gives you a lot of flexibility on layout and future configuration changes. After it enters the building and based on the layout and 1/4" per foot slope your elevation can jump up.

 

[Tomject]

You guys are awesome.

I'll correct my errors and see if I can wet vent the shower instead.

I did 1/8" per foot on the 4" main line because I'm constrained at the other end where it goes under the main home basement floor. Plus trying to minimize deeper trench digging if possible. Fortunately the soil is clay, and fairly easy to dig. Building is already below frost line as it is partially underground.

I didn't design in a laundry, but will think on that.

It's a 300sf garage with a sleeping loft, so no need (or room) for a 2nd bath in this project.

 

[wwhitney]

I did 1/8" per foot on the 4" main line because I'm constrained at the other end where it goes under the main home basement floor.

Pretty sure that if you would be allowed to use a 3" building drain (which you would be), then you only need a 3" vent through the roof, even if you use a 4" building drain.

Cheers, Wayne

 

[Jeff H Young]

note 3 inch pipe 1/4inch per ft fall

 

[Tomject]

You guys are awesome.

It's a 300sf garage with a sleeping loft, so no need (or room) for a 2nd bath in this project. I didn't design in a laundry, but will think on that.

I did 1/8" per foot on the 4" main line because I'm constrained at the other end where it goes under the main home basement floor. Plus trying to minimize deeper trench digging if possible. Fortunately the soil is clay, and fairly easy to dig. Building is already below frost line as it is partially underground.

Here's a new revision. Changed shower drain to wet vent, extended main sewer out back of the building for cleanout, changed san-tees for combo or wyes.


Question: do I need a long sweep 90 where the kitchen sink enters the wall?

 

[wwhitney]

- Kitchen sink trap at 2" would be very unusual, far more typical is 1.5" trap, which would limit your trap arm to 42". I'm of the opinion that a horizontal 90 on a sink trap arm needs to be a LT90 per the UPC, but I understand that a quarter bend is often used as a compromise.

- Likewise lav trap should be 1-1/4" or 1-1/2". Only the dry/wet vent path needs to be 2", the trap arm can be 1-1/2" or 1-1/4".

- The kitchen sink is dry vented.

- Your rendering still looks like a 4" san-tee on its back where the 4" vent stack comes off the 4" building drain, but it needs to be a combo. Of course, this may be a limitation of your rendering software.

- A 3" vent stack would suffice for your building, I believe.

- Just to double check, the shower trap arm is from the trap outlet to the crotch of the wye where the lav drain joins in. That's what needs to be at most 5'.

Cheers, Wayne

 

[Tuttles Revenge]

2" drain and 1.5" trap arm for the kitchen sink. We never install long sweeps as stub outs. Definitely a combo for the kitchen sink vertical from the main drain.. agree its likely a rendering issue.

The whole DADU can be vented on 2" per UPC since the drain connects to the main house on the same sewer, its cross sectional area is already met there.

 

[Jeff H Young]

Yes only need a 2 inch vent.
We all have our differant ways to plumb, Ive used a medium sweep 90 on the side a few times on lavs especially pedestal lavs . and maybe a time or 2 on a kitchen just guessing maybe I didnt have a long sweep or just really wanted to put a escutcheon on.
kitchen dirty arms running across the back of cabinet get an 1/8th bend here when I set finish or come out wall on a 45 for a kitchen. I respect Tuttles shooting for a cleanner looking product . often our pipe isnt in the wall but running just outside drywall to avoid drilling out king studs etc.

 

[Tomject]

Good news on the 2" vent- less holes to cut in wood. I think the 3d model I have of the 4" combo is wrong, but is hopefully adequate for discussion purposes.

Now I've added a drain for laundry, and am rethinking the cleanout location- the far end would mean more digging in a very narrow gap between building and neighbor's yard, which I'd love to avoid.

It also looks like a mess on the end with the shower, floor drain and laundry. There must be a simpler way to do that bit. This plumbing design is a lot harder than it looks!

Another question (they are endless aren't they?) - does a ptrap need to be higher than the main sewer line? For the shower and floor drain it's currently lower.

Thanks for your continued help.

 

[wwhitney]

The laundry drain can't join the 4" building drain until downstream of the sink, shower, floor drain, and WC. Those 4 fixtures are allowed to use horizontal wet venting, but the laundry drain isn't. So if the laundry drain joins where you show, it disrupts the horizontal wet venting, and you have no vent for your shower, floor drain, or WC.

Cheers, Wayne

 

[Reach4]

The center of the output of the ptrap needs to be higher than the center of the main sewer line. This would typically slope at 1/4 inch per foot.

 

[Jeff H Young]

My opinion but the cleanout in post number 12 makes more sence than revised drawing in post 16 . why 2 clean outs and one going up hill?

 

[Tomject]

The laundry drain can't join the 4" building drain until downstream of the sink, shower, floor drain, and WC. Those 4 fixtures are allowed to use horizontal wet venting, but the laundry drain isn't. So if the laundry drain joins where you show, it disrupts the horizontal wet venting, and you have no vent for your shower, floor drain, or WC.

Cheers, Wayne

Oh crap. That complicates things.