Q&A on Driving or Paving Over Septic Tank or Fields

Building over the septic field Q&A
Drive over the septic field without causing damage FAQs
Pipes cross the septic tank or field Q&A
Plastic or weed killer over the septic tank or drainfield FAQs

Buildings over the Septic Drainfield FAQs

Can I put a little garden shed over my septic drainfield?

I want to put a small 7x7 garden shed in my yard. I have laid an 8x8 section of patio stones down for it that is covering a small portion of where my drain field is - the back corner farthest from the septic tanks

It’s covering maybe 1/8 of the entire bed on a corner. In the image attached, it would be the corner nearer to the tree at the back.

My question is whether ANY weight at all can go on any portion of the bed - I’ve read about it affecting respiration or something. Please advise. Thanks! On 2019-08-30 by Derek

by (mod) - It's not a good idea to put a garden shed over even a corner of the septic drainfield

Covering any part of the septic drainfield is a bad idea, Derek; the concerns are traffic and weight over the drainfield that compresses soil or even damages pipes, and when covered, reduction of evaporation and transpiration.

If the closest edge of a garden shed to the closest drainfield trench is 10 feet you may still be OK.

Watch out: But in your photo I see all those white PVC pipes right at ground level.

What are those? If that's part of your septic system something is seriously wrong; septic leachlines would never be left exposed at ground surface, and your shed is right on top of some of that piping - not something you should attempt.

Watch out: I also see that the edge of your apparent drainfield is just a foot or two from nearby trees.

That installation is likely to be rather quickly clogged by tree roots, leading to a septic failure.

Our barrel sauna is over the septic bed - convince my husband to move it

We purchased a barrel sauna for our cottage. My husband placed it onto of the septic bed. I hate the look of it.

Sometimes the smell seeps into the wood into the sauna and when having a sauna it stinks. There are ant hills on the septic bed and now I have them in my sauna.

Please tell me how to convince him to move it closer to the lake and not at the back of the property on the septic bed. Its 16' long with a change room and covered porch. On 2019-07-06 by Turtle

by (mod) - Sauna set atop the septic bed - an explosion hazard?

Turtle

Well you could try mentioning that we have occasional reports of an explosion when people put a fuel-burning or electrical heater type device over a septic tank or drainfield where sewer gases are present.

At OUTHOUSE / LATRINE FIRE & EXPLOSION we describe a methane gas explosion in an outhouse - admittedly an unusual event.

More details are at METHANE & SEWER GAS HAZARDS

Pipes Under or Over a Drainfield or Septic Tank FAQs

To build a shop we need a road across the sewer easement across our property

We have an off site drain field on our five acre piece of property.

The easement for the drain line crosses the whole width of our property and is about halfway through the middle of the lot.

We would like to build a shop on the other side of the line and have a road going to the shop.

Is this possible if we aren’t driving over the drain fields but crossing the 10’ sewage easement drain line? On 2019-06-30 by Lisa -

by (mod) - Use Schedule 40 or Schedule 80 PVC pipe to avoid vehicle damage

You may want schedule 40 or even schedule 80 pipe and you may need to take further steps to protect it from damage depending on just what vehicles are going to drive over it.

I would expect your building or plumbing inspector to approved the ability to drive over a sewage effluent line provided that the pipe is of a type that is protected from vehicle damage.

Can we re-route pipe beween the septic tank and drain field?

It is PVC pipe running from my septic tank to the drain field.

Can we dig up about 12 feet of it, nearest the septic tank, and run it through a culvert then rebury the whole deal so we can then drive over that pipe?

If so, do we fill the inside of the culvert, around the pipe, with dirt we dug out of there?

Or put some sort of other fill in there? Or leave it open?

I cant seem to find anything about this anywhere.

Also we live in the upper peninsula of Michigan, so below zero temps are definitely a factor. On 2019-06-24 by Amy

Reply by (mod) - slope & freeze risk concerns for sewage pipes

It's often the case that a building sewer pipe runs close to the ground surface and exposed to sub-zero temperatures in northern climates, without frequent freeze damage problems, but your pipe route has to either maintain the proper slope between house and septic tank (1/8" to 1/4" of fall per foot of horizontal run)

OR you'd have to convert your sewer line to a force main using a grinder pump to push sewage from the building to the septic tank, or an effluent pump to push effluent from the septic tank to the drainfield.

Also

Watch out: if your building ever, over the course of its life, suffers from a slow or small plumbing leak into the drain line, such as from a running toilet or even a dripping faucet, water moving at that much slower rate down the building drain means that where the sewer line passes close to the ground surface in a freezing climate (Michigan UP or northern Minnesota for example) the line can slow fill with ice, then freeze and burst, causing expensive damage.

Is there a problem with routing surface runoff intercept pipes across the septic draifield?

In order to drain away surface flow water that is partially saturating the drainfield, is there any problem with running drainage lines from surface intakes over top of a drainfield from the high side to below the low side?

Note that the distribution box and lines are approximately 4' deep while the drainage lines would be only approximately 1' deep.

Thanks, Chris On 2018-11-30

by (mod) - Yes, that's a bad idea.

Assuming you're talking about solid piping that isn't leaking into the drain field I don't think that the pipes themselves would be the issue. But

Watch out: digging the trenches to route those pipes across your drainfield will act as a surface water and runoff catchment that will actually increase the flow of rain and surface water into the drainfield, so I think it's a really bad idea.

Watch out: However there is a risk that if you drive equipment order the drainfield to dig trenches you're risking compacting soil and damaging the fields.

I'd prefer an intercept swale or drain on the up-slope side of the drainfield and that intercept drain or swale must then carry water around and well away from the drainfield.

Can septic pipes run under a road?

Is it common to build a septic system where the pipes will be laid under a road? What problems could occur over time. If there is no other option, what measures should be taken to prevent damage to the system? (Sept 15, 2015) Joseph

Reply: yes with proper depth, protection, and type of piping - piping yes, actual drainfield, no.

If we're discussing solid piping to move effluent to a drainfield, proper pipe type (schedule 80 or better), depth, trench preparation and backfill can protect the pipes from damage from the road above.

We wouldn't put part of the drainfield itself under a road - it would lose its evaporation capacity for that area and repairs would be quite costly.

Can you run underground pipes over the septic tank [Or draifield]?

Can you run underground pipes over the septic tank but avoiding the lids? I know its not preferred but the site has no other way around it. It will be inspected ( Santa Cruz Ca.) On 2017-05-28 by Marc

by (mod) re:

Marc

What sort of pipes and for what purpose. Sanitary codes specify, for example, a minimum clearance distance between potable water pipes and septic components. And running other pipes such a as a waste line over a septic tank, depending on the tank lid depth, may invite breakage of the line.

But

Watch out: don't drive heavy equipment over an existing septic drainfield in the course of laying new pipes over the septic tank or field as doing so risks damage to the tank or fields.

How do I put a fence around my septic drainfield without a fencepost hitting a septic pipe?

How do I find out how to fence off my septic Drainage Field without hitting anything in the ground? On 2019-05-10 by Molly

Answer by (mod) -

Molly you'd either keep a generous obvious distance away based on a site inspection or you'd actually locate the drain lines using pipe locating equipment or a sewer line camera or similar means.

See details at SEPTIC DRAINFIELD LOCATION

Plastic or other Weed Killers over the Septic System FAQs

Can I kill weeds over the septic tank by putting down plastic?

We want to kill all the weeds growing over the tank and field and plant grass and clover. Can we lay down black plastic sheet for one season to kill the weeds and seeds in the soil? On 2019-05-17 by Samadhi

Reply by (mod) - Temporary plastic as a weed killer is OK over the septic tank, but NOT OK over the drainfield

Samadhi:

Plastic over the area of the septic tank itself won't interfere with septic operation - as long as you're not putting plastic over the drain field area you should be okay.

How do we get the weeds off of our drainfield?

We have a huge drain field full of weeds. I was wondering what kind of equipment we can to make it easier and more efficient to get rid of the weeds so we can plant a lawn?
Thanks Leah On 2012-05-20 by Leah H.

Reply by (mod) -

Leah, most shallow-rooted grasses and weeds don't themselves usually damage the drainfield, but if you want a nice lawn and intend to do so by removing all of the weeds using a machine, it needs to be light enough not to compact the soil or break a drainfield pipe.

A small lightweight walk-behind garden tractor might work.

Remember to re-seed with suitable shallow-rooted grass - a critical part of protecting the drainfield from erosion and helping its process of transpiration.

See SEPTIC SYSTEMS, PLANTS OVER

How to Drive Over a Septic Tank or Drainfield Safely Q&A

Watch out: Also see DRIVING ON or OVER a SEPTIC DRAINFIELD FAQs - or parking on it - is a bad idea.

OK to lay down boards to distribute the weight of an RV (14,000 lbs) to safely cross over septic tank?

Can you lay down boards to distribute the weight of an RV (14,000 lbs) to safely cross over septic tank? On 2017-06-06 by Rhonda -

by (mod) - septic tank covers rated for vehicle traffic - safety concern

Rhonda,

You ask an important question for which I don't have an absolutely authoritative answer, in part because I have no idea what boards are to be laid down and thus how much they will flex or will distribute or not distribute the RV weight. I also don't know what sort of septic tank you want to cross over.

A septic tank sporting a cover that is made of concrete reinforced with steel re-bar is a far different case from an old steel septic tank with a sheet-metal lid. In the latter case I'd say that no board-crossover-plan is going to be safe. In the former case ... it's still doubtful from the weight you give.

If you *know* that your septic tank is a "traffic-rated" tank and know who constructed it, you would still want to give the manufacturer a call to be sure that their "Traffic rating" included a 14,000 lb. vehicle and you'll need to know the dimensions of the tank and the spacing of the vehicle's axles.

Consider that unless you have installed a septic tank with a "vehicle traffic rated" or Highway Traffic Rated strength cover, a typical concrete residential septic tank, following the University of Minnesota design guide (as a typical standard) is built to carry the weight of the soil covering the septic tank and a vehicle wheel load of 2,500 lbs. That is a far cry from your 14,000 lbs.

Let's *pretend* that your RV is so long that only two wheels of it are on the septic tank at a time while you drive across (scared ***itless to forgive the pun) - you're putting a 7,000 load on the lid. Your boards would have to be both long enough to span well more than the septic tank AND to be so thick and rigid as to not deflect to place a significant load on the tank itself.

Septic tanks designed to withstand vehicle traffic ("HS20 vehicle traffic rated septic tanks") have not just a thicker lid but heavier tank sides and bottom too. HS20 loading is defined by the American Association of Highway and Transportation Officials (AASHTO) as follows:

A vehicle with an 8,000-pound (3,630 kg) front axle and one or more rear axles weighing 32,000 pounds (14,500 kg) each and spaced at least 14 feet (4.3 m) apart. This is a key measurement since it means that the vehicle's axle spacing is greater than the tank width, so only one wheel set is on the tank at a time. HS20 and H20 are used interchangeably.

Please see SEPTIC TANK COVERS at https://inspectapedia.com/septic/Septic_Tank_Covers.php

and also SEPTIC TANK DESIGN STRENGTH SPECS https://inspectapedia.com/septic/Septic_Cesspool_Design_Strengths.php

OK so what now: I consulted with an expert source on concrete septic tank design, the National Pre-Cast Concrete Association who, as is no surprise, have looked at the question of how to make it possible to driver over an existing septic tank. Here is an excerpt from the associations article on this that I'll cite below:

Engineering analysis demonstrates the tank design shown in Figure 1, initially designed for non-traffic use, can be made into a HS20 traffic-rated tank with careful engineering and many preconditions. Delta Engineers, Architects & Surveyors of Endwell, N.Y., performed the engineering analysis. The eight conditions are as follows:

1. 1 to 2 feet (0.3 to 0.6 m) of soil shall be over the top of the tank.

2. No more than one axle or two wheels shall be over the tank at any one time.

3. The minimum concrete strength will be 5,000 psi (35 MPa) compressive strength.

4. The top slab thickness shall be increased to 6.5 inches (165 mm). However, the bottom slab and wall thicknesses cannot be modified.

5. The tank contains a monolithic compartment wall.

6. The water table shall be at least 3 feet (0.9 m) below grade when the tank is empty.

7. The walls are supported at the roof by an interlocking recess that is cast into the top slab.

8. Reinforcing bar size, spacing and location shall be installed per Professional Engineer recommendation.

• Barger, Eric, Traffic Related [Septic] Tank Design" PDF we will provide] NPCA, National Precast Concrete Association, 1320 City Center Drive, Suite 200, Carmel, IN 46032 USA, Phone: (800) 366-7731 Website: http://precast.org/ 28 July 2010, retrieved 2017/06/06, original source: http://precast.org/2010/07/traffic-related-tank-design/

Is it safe to drive boom truck on drainfield once the ground is frozen

I have some trees to be removed. Is it safe to drive boom truck on drainfield once the ground is frozen this winter. I'm in southern Minnesota. On 2016-11-09 by Kevin

Reply by (mod): possibly, yes; here's how to decide

Kevin:

Good question. I have just arrived back in our lab from Two Harbors.

The frost line in southern MN has been studied (in 1993 - see this MINNESOTA GROUND FROST STUDY [PDF] original source: http://files.dnr.state.mn.us/natural_resources/climate/summaries_and_publications/minnesota_ground_frost_study.pdf ) teaching us that if you wait long enough for the ground to be frozen to the depth in your area (perhaps 36") you may be OK.

Back before global warming was hot on our necks, the deepest frost was found in the second and third week of February. If your area has a "sudden increase in snow depth" you can get to maximum frost depth earlier if the snow remains (through say February) without melting. The study said that there were wide variations in frost depth in Southern MN as snow cover increased so this is not a hard-fast rule.

I speculate that if you do not see the truck leaving ruts in the ground you may get away with little or no damage to the drainfield. In areas where I was worried about soil compression I bought a few sheets of plywood to lay down over critical spots.