We install septic tanks and biological wastewater treatment plants (BIO plants) in Riga and the Riga region in the winter months too, if the weather allows. A septic tank including the tank and installation costs from €2,200; in winter, extra items may appear in the estimate.
How deep the ground freezes in Riga
The building climatology code gives a standard frost depth for clayey soil in Riga ranging from 59 cm, reached once every two years, to 120 cm, reached once every hundred years. Sandy soil freezes deeper, so a coefficient of 1.2 is applied to it. In mild winters the frost does not go that deep, but the system is designed for the harshest winter, not an average one.
This determines the pipe depth. For gravity pipes up to 500 mm in diameter, the depth to the top of the pipe is 0.3 m less than the once-in-100-years frost depth, but not less than 0.7 m. In Riga, in clayey soil this is about 0.9 m, and deeper in sandy soil. Pipes laid at a depth of 0.7 m or shallower must be protected from freezing, usually with insulation boards.
LBN 003-19 “Building Climatology” (Cabinet Regulation No. 432), Table 15: Riga, standard frost depth for clayey soil 59 / 80 / 91 / 101 / 112 / 120 cm (once every 2 / 5 / 10 / 20 / 50 / 100 years); coefficient 1.2 for sandy soils. LBN 223-15 (Cabinet Regulation No. 327), point 67: for pipelines up to 500 mm, the depth to the outer surface is 0.3 m less than the once-in-100-years frost depth, but not less than 0.7 m; pipelines laid at 0.7 m or shallower are protected against freezing through. Sources: likumi.lv, checked on 30 October 2026.
What is different in winter
| Task | In winter | Why it matters |
|---|---|---|
| Digging | The frozen top layer is broken up with an excavator ripper or a hydraulic breaker | Takes more time and machinery |
| Backfilling | Only with imported, unfrozen sand; frozen lumps are not tipped into the pit | The lumps thaw in spring, leave voids, and the tank or pipes settle |
| Compaction | Only possible with unfrozen material | Frozen soil cannot be compacted |
| Anchor slab | Factory-made concrete slabs are used instead of pouring concrete on site | Fresh concrete does not cure properly in frost without heating |
| Tank | Filled with water straight away during backfilling; the neck and cover are insulated | If the house is not yet in use, the water in the neck can freeze |
| Drain field | Drains and gravel are laid in winter; the lawn is restored in spring | The topsoil cannot be levelled properly in frost |
When winter installation makes sense
- The old septic tank has collapsed or is overflowing, and waiting until spring means frequently pumping an overfull tank. See the page replacing an old septic tank.
- On a new build, other work is continuing and the sewage system has to be ready by spring.
- On a soft plot where machinery gets stuck in autumn and spring, the frozen top layer holds it up.
- In dry frost, the groundwater is often lower than during the spring floods, so there is less water in the pit.
When it is better to wait
- During a long, hard frost, when the ground is frozen deep and unfrozen sand cannot be delivered.
- During a thaw in clay soil: the top layer is muddy, there is frost below, and water collects in the pit.
- If an infiltration test or a soil survey has to be done: in frost, the results are not reliable.
In spring after a winter installation
- Check for settlement Once the ground has thawed, check for dips above the tank and pipes, and top up the soil.
- Cover level The cover should be slightly above the lawn so that rainwater does not run into it.
- Lawn Restore the topsoil over the drain field and sow grass, but do not plant trees or shrubs.
How much septic tank installation in winter costs
The base price is the same as in other seasons: a septic tank including the tank and installation from €2,200, a biological treatment plant for 4–6 people from €1,750 and installation work from €800. In winter, extra items may appear in the estimate: breaking up the frozen layer, delivering unfrozen sand, insulating the pipes and cover, and clearing snow from the work area. If the pipe from the house to the tank has to be longer, it costs from €25/m.