+372 641 8550
Price inquery

How deep should a sewage holding tank be installed?

Before the excavator arrives, someone has to decide how deep the pit will be. There is no single answer to how deep to bury a holding tank (reoveemahuti, or reovee kogumismahuti in legal texts; UK readers may know it as a cesspool or cesspit): your house sets the depth. The tank treats nothing and has no outlet. It stores wastewater until a tanker empties it, and the wastewater runs into it by gravity from the building’s sewer pipe. Below: how to work out the inlet depth from that pipe, how deep the pit then gets, and when a shallow tank needs insulation or a deep one a concrete slab.

The house sewer pipe sets the depth

The Ministry of the Environment’s 2015 guide to rural wastewater systems (official guidance, though dated) says the installation depth (paigaldussügavus) of a holding tank depends on the depth of the sewer pipe leaving the building. Because the pipe must fall towards the tank, the inlet is always deeper than the point where the pipe leaves the house.

inlet depth h ≈ pipe depth at the house + pipe length × fall ± difference in ground level

  • Pipe depth at the house: from finished ground level to the bottom of the pipe at the foundation — from the design drawings of a new house, or measured in a small hole dug at the foundation of an existing one.
  • Fall: the ministry guide gives 1–2 cm per metre for the pipe from a building to a septic tank and no separate figure for holding tanks; your designer or installer sets the fall for your pipe.
  • Ground level: add the difference if the ground is higher at the tank, subtract it if lower. Use finished levels.

Two examples on level ground (the distances are illustrations, not setbacks; where the tank may stand depends on your municipality’s rules):

Example A Example B
Pipe depth at the house 0.6 m 1.1 m
Length of the pipe run 12 m 20 m
Fall at 1–2 cm/m 0.12–0.24 m 0.20–0.40 m
Inlet depth h ≈ 0.72–0.84 m ≈ 1.30–1.50 m
What follows shallower than 1.3 m: insulate up to Vesmaco’s 1500 mm limit: ask before ordering

From inlet depth to pit depth

Vesmaco’s order drawings, downloadable from the model table, have a box for h, measured from the ground surface down to the inlet: “installation height, max. 1500 mm” on the English sheets, sisendtoru sügavus (inlet pipe depth) on the Estonian ones. When you fill in the box, state which point your figure refers to (for example, finished ground level to the bottom of the inlet pipe), so that the supplier and the installer work from the same measurement.

pit depth ≈ h + inlet height above the tank bottom + sand bed

On the five drawings opened for this article, the inlet sits 1000 mm above the tank bottom on D1100 tanks, 1300 mm on D1400, 1500 mm on D1600 and 1900 mm on D2000 — about 100 mm below the tank top. For other models, check the drawing for your model. Vesmaco’s installation guide (in Estonian, 2016) puts the tank on at least 20 cm of compacted, stone-free sand; an anchored tank sits on a 20 cm reinforced-concrete slab with 20 cm of sand on top.

Example A with an MK-5 D1400: 0.72–0.84 + 1.3 + 0.2 ≈ 2.2–2.3 m, and at least 0.2 m more with an anchoring slab. As an MK-5 D1100 the pit is 0.3 m shallower but the tank about 2 m longer; choosing a diameter is covered in holding tank dimensions and drawings. A deeper pit is also more likely to reach groundwater, which can lift an empty tank — see holding tanks and high groundwater.

If the inlet works out at 1.5 m or deeper, as at the upper end of Example B, do not flatten the pipe to make it fit: the fall is what keeps the wastewater moving. Give Vesmaco the real figure before you order and ask what they recommend for that depth.

Shallow tanks: insulate pipe and tank

The ministry guide is explicit: where the sewer pipe and the tank lie less than 1.3 m below ground, both must be insulated from above. It names moisture-resistant polystyrene board with good load-bearing capacity as a suitable material.

Because the tank top is about 100 mm above the inlet, the soil over a Vesmaco tank is roughly h − 0.1 m, so at most about 1.4 m. Unless the inlet is close to the 1.5 m maximum, the guide’s rule therefore means insulating the tank, and the pipe wherever it runs shallower than 1.3 m (in Example A, the whole run). Vesmaco’s installation guide does not mention insulation, so agree the board type, thickness and extent with your designer or installer, and ask for insulation as its own line in the installation quote.

Shallow cover also leaves the tank closer to loads on the surface. On a green area, Vesmaco’s guide sets a safety zone 1 m larger than the tank in every direction so that nobody drives over it by accident. For vehicles the guides differ: the ministry guide says a holding tank must not go under a road or parking area, while Vesmaco’s guide allows it with a load-distribution slab — see holding tank under a driveway.

Deeper than 1.5 m: a concrete slab above the tank

For deep installation (sügavale paigaldamine), Vesmaco’s guide says: when the tank goes deeper than 1.5 m, lay a reinforced-concrete slab at least 15 cm thick 50 cm above the tank, extending 50 cm beyond its length and width on every side. It is the same slab the guide requires under traffic, and a qualified designer should confirm it for your site.

The guide does not say what the 1.5 m is measured to. Its illustration shows about 1.55 m over the tank top: three 30 cm layers, the slab, and 50 cm down to the tank. If the limit means soil over the tank, a tank at the maximum h of 1.5 m (cover about 1.4 m) stays just below it. If it meant the tank bottom, almost every installation would need a slab: a D1100 tank with h = 0.6 m already has its bottom 1.6 m down. If your tank is anywhere near this limit, ask the supplier and your designer how it applies to your installation.

The riser must reach the surface

The tank is emptied (purgimine) through its maintenance riser, which runs from the tank top to the cover at ground level. The drawings offer a PVC pipe D200 (standard) or a D400 pipe (option), each 500–1000 mm high, or a D600 shaft (option), 500–1500 mm. The riser has to bridge roughly h − 0.1 m. Read that way, the standard riser would suit an inlet up to about 1.1 m deep and deeper inlets the D600 shaft, but how the riser height is measured is specified by the manufacturer — check the drawing for your model and confirm the riser with the supplier when you order, especially for a very shallow or very deep inlet.

Two more things change with depth:

  • Level alarm. The Smartel Tank-Check sheet advises mounting the transmitter as high as possible, preferably above ground level and not under a metal cover. If you order the alarm, plan the riser top and cover with that in mind.
  • Emptying. The bottom of an MK-5 D1400 lies about 2.0–2.15 m below ground in Example A and 2.6–2.8 m in Example B. Ask the local emptying company whether its equipment works at your depth and distance.

What to measure before you order

  1. The depth and diameter of the sewer pipe where it leaves the house (the drawing offers a Ø110 or Ø160 inlet).
  2. The length of the run to the planned tank position, and the ground levels at both ends.
  3. Planned changes in ground level, such as landscaping.
  4. The surface above the tank: lawn, path or driveway.
  5. Groundwater and rock: what a trial pit or the neighbours’ wells show.

Installing a holding tank also requires a building notice (ehitusteatis) under the Building Code, and your municipality’s rules for on-site wastewater treatment and removal (Water Act § 104(7)) may add requirements, so check your own municipality’s rules for anything on depth, insulation or documenting the covered works. The whole installation sequence is in holding tank installation step by step.

Frequently asked questions

Can I install the tank shallower to save digging? Only as shallow as the pipe allows: the inlet has to sit below the house pipe by the fall over the run. You can reduce the part of the pit below the inlet with a smaller diameter (5 m³ as D1100 instead of D1400: 0.3 m less). A shallow pipe and tank then need insulation under the ministry guide’s 1.3 m rule.

Is there a legal minimum or maximum depth? No single depth applies everywhere. The ministry guide ties depth to the building’s sewer pipe; the 1500 mm inlet limit and the slab for installation deeper than 1.5 m come from Vesmaco’s drawings and guide. Municipal rules may add requirements, so check yours.

Can I raise the ground over the tank later? Ask Vesmaco and your installer first. Extra soil adds depth and load, may leave the riser too short, and can bring the tank into the guide’s deep-installation case. The cover must stay visible and reachable for the tanker’s hose.

Tell us your inlet depth. Enter the depth of the sewer pipe at the house, the length of the run to the tank and, if you have worked it out, your inlet depth h in the quote form, together with the volume you need. Vesmaco can then suggest a model, diameter and riser for that depth; excavation and installation stay with your installer. The site says replies usually come within 2–3 days. Phone: +372 641 8550.

Sources

Last checked: 17 September 2026.