Can You Build Deck Over Septic Tank?

Building a deck is one of the most enjoyable and rewarding home improvement projects you can undertake. No matter how complex or simple, a finished deck is a great place to spend time with friends and family and enjoy your home. There is one thing, however, that can throw a wrench in your deck building plans, can you build a deck over a septic tank?

You can build a deck over a septic tank, but that doesn’t always mean you should. Building your deck with a septic tank underneath requires more planning, building code knowledge, and has the potential for disaster. If you want to build a deck over a septic tank, you have to do it right!

In this article, you’ll learn the basics of building decks over septic tanks, the risks, and building codes associated with these projects, so you know whether or not this is a good idea for your home!

Can You Build a Deck Over a Septic Tank?

Building a deck over a septic tank is possible, but not always a good idea. Many different things should give you pause before you decide to build your deck over a septic tank. If you want to build a deck without disturbing your septic system, you have your work cut out for you!

However, it isn’t impossible or tricky- it just requires more planning and adjusting. Think of it like a unique design challenge, one that forces you to think more creatively and plan better!

Risks of Building Over a Septic Tank

Building a deck right over the tank will make it very hard, if not impossible, to pump the tank. Caring for the tank is very important, and covering the tank with a wood structure makes that work very difficult. Your septic tank needs to be pumped and emptied every three to five years, and older tanks might need more regular maintenance. For this reason, it’s not wise to build a deck over a septic tank unless you have no choice.

It’s also important to leave the septic lines and drain field untouched. The lines and drain field allow liquid waste to drain out slowly after it is treated by the tank. This liquid, known as effluent, flows out into the drain field and dissipates in the ground and air.

Most of the time, this happens without the homeowner ever knowing. When things interrupt the process, however, it becomes noticeable immediately. A damaged septic field will result in the ground becoming muddy and wet, and the air is filled with a putrid smell.

You should never build a deck over a septic field; doing so will prevent the natural draining and dissipation of the effluent. This can ruin the septic system, not to mention releasing foul smells into the air all around your deck. The dissipating effluent can also rot the deck from underneath. Nobody wants to hang out on a deck that smells like a sewer!

Guidelines for Building Decks Over Septic Tanks

In general, it is best to avoid building a deck over a septic tank. If there is simply no way around it, build your deck as tall as possible to give yourself and service workers ample room to work. It also helps to build a trap door or hatch into the deck directly, so that pumping and emptying the tank won’t be a hassle.

To avoid building over the septic lines and drain field, locate their exact locations and mark them off with flags while planning your deck. If you have recently purchased your home or have the paperwork from inspections, there should be a map or diagram displaying its location!

If your home has an old, empty, or decommissioned septic tank that is no longer in use, it is okay to build a deck over the tank, lines, and drain field. Keep in mind that this is only acceptable if the unused septic tank is empty, and the drain field is completely dry and is no longer any different than the rest of the yard.

Deck Over Septic Tank: Footings and Framing

If you are going to build a deck over a septic tank, know that each deck footing needs to be 5-10 feet – depending on where you reside – from the septic tank at every point. However, doing this means that the footings might be too far apart to build a structurally sound deck that meets code.

If the footings are too far apart, the deck beams will sag and the deck won’t last more than a couple of years. If you are finding that your deck plan requires that the footings are too far apart, consider framing the deck with steel rather than wood. This allows you to place the footings farther apart without risking deck sag!

How Big Is a Septic Tank?

Septic tanks range in size based on the size of the home; a two-bedroom ranch will have a much smaller tank than a 6-bedroom country homestead. The EPA recommends sizing the septic tank based on the number of users and the size of the home, as well as expected water usage.

The average size of a septic tank is between 750 and 1250 gallons. This is enough for the tank to filter and treat a few years of water and waste. Most of the time, these tanks will be rectangular and about 5 feet wide by 8 feet long and 6 feet tall. Every time the capacity requirement increases by 250 gallons, the tank will grow about a foot in each direction.

The largest residential septic tanks are 3,000 gallons in size and are designed for large families and multi-family homes like duplexes. Septic tanks are commonly precast concrete, plastic, fiberglass, or steel, although steel is the least popular due to cost and corrosion.

How Deep Are Septic Tanks Buried?

Septic tanks, contrary to popular belief, are not actually buried very deep. When a septic tank is buried too deep, the soil weight can crack or collapse the tank, causing the effluent to leach and soak into the soil around the tank rather than going to the drain field.

Most septic tanks are buried between 4 inches and 4 feet deep. This depends on the type of soil, the slope of the yard, tank design, and other conditions. Before building a deck over your septic tank, you should find out the exact location and depth of the tank and septic lines. This will help you avoid putting deck footings where they could cause damage.

Rules and Codes Regarding Septic Tanks

Before building your deck, you should check your locality’s building codes regarding septic tanks. In some places, it is acceptable to build a floating deck over a septic tank. In others, it is forbidden and can lead to fines and mandated removal of the deck.

For example, in Ohio, the septic tank, lines, and drain field must be located at least 10 feet from building slabs, roads, decks, and other structures. In Florida, that distance is shortened to just 5 feet.

You are never able to build over septic lines or drain fields; these rules are for building near tanks only. Building over lines and fields is a recipe for disaster; you should never consider building over your septic system’s lines or drainage field!

These regulations apply not just to decks, but to landscaping such as walls and trees, foundations, slab, and other construction. If these restrictions exist in your area, do not build close to or over the tank, lines, or drainage field.

How Close Can a Deck Be to a Septic Tank?

Depending on where you live, the footings of the deck should be at least 5-10 feet from the tank at every point. This could lead to the deck shrinking or growing in size to accommodate these rules. You can use this link to find state-level code about septic systems. For a better perspective, consult your local building codes or speak to a knowledgeable plumbing contractor or home inspector!

Even if your locality allows footings closer than 5 feet, it is still wise to keep the deck footings at least five feet from the tank. Doing so will ensure that you never compromise the integrity or effectiveness of your septic system.

Can You Build a Floating Deck Over a Septic Tank?

Floating decks, which are essentially freestanding wooden platforms built at grade or just above-grade, should not be built over a septic tank. The weight of the deck on the supports can displace and disturb the septic system’s ability to properly treat and drain waste. Ignoring this can result in your being the proud owner of the stinkiest floating deck in the city.

A simple solution to this is to build a hybrid floating deck: using buried footings like a normal deck, but keeping them short and separate from the dwelling like a floating deck. This will give the appearance of a floating deck without forcing the soil above the septic tank to support additional weight.

Can You Build a Deck Over a Septic Field?

You should never build a deck over a septic field. Septic fields are designed to allow effluent to drain out into the groundwater or evaporate into the air. Disturbing the septic field results in backup, making the ground muddy with contaminated fluid. The stench and appearance will be very noticeable and may require extensive repairs on the entire septic system.

Even if you are looking to build over an unused septic field, you should inspect the soil thoroughly to make sure it is no longer draining. Once you’ve confirmed that the drain field is not active, you are free to build over it as if it were normal ground.

What Can You Put Over a Septic Tank?

The only things that can go directly over a septic tank are decks or pergolas that have footings more than five feet away from the tank. Other structures, like concrete slabs, foundations, and even shrubbery can seriously affect the health and effectiveness of the septic system.

Conclusion

Building a deck over a septic tank can be tricky. It is possible but not always wise. Even when you do build the deck, there are many things to keep in mind. Before you build a deck over a septic tank, do your research, plan extensively, and always prioritize the septic system! Have you built over a septic tank in the past? Do you have any more questions about your upcoming deck project? Leave your thoughts in the comments section below!