Caisson in the garage: device, purpose and step-by-step installation

What is a caisson and why is it needed in the garage?

A caisson is a sealed chamber installed below the garage floor level, which solves two key problems: protection of underground communications (pipes, pumps, meters) from freezing and mechanical damage, as well as organization of technical access to them without excavation. In the context of a garage, a caisson is most often associated with inspection hole, but this is not entirely true: if the pit is intended for inspecting a car, then the caisson is for infrastructure.

The main reasons why garage owners install caissons:

• 🔧 Protection of water supply and sewerage from freezing in winter (relevant for unheated garages).
• ⚡ Safe placement of electrical inputs and distribution units (for example, for charging batteries or connecting a compressor).
• 🚗 Convenient access to drain systems (oil sumps, septic tanks) without dismantling the floor.
• 📏 Space saving: all communications are “hidden” under the floor, without occupying useful space.

It is important to distinguish a caisson from pit or well: the caisson is always sealed (has a lid with a seal and is often insulated), while the pit can be open. For example, if a submersible pump is installed in the garage to pump out groundwater, it is placed in the caisson - this prevents moisture and dirt from getting into the mechanism.

Types of caissons for the garage: comparison of materials and designs

The choice of material for the caisson depends on the budget, climatic conditions and soil type. Let's look at the main options with their pros and cons:

Material	Service life	Corrosion resistance	Difficulty of installation	Price (from/to, rub.)
Metal (steel 3–5 mm)	15–25 years	Medium (requires anti-corrosion treatment)	Medium (needs welding)	20 000–50 000
Reinforced concrete (monolith/rings)	30–50 years	High (with waterproofing)	High (heavy equipment)	30 000–80 000
Plastic (polyethylene, polypropylene)	20–40 years	High (does not rust)	Low (light weight)	15 000–40 000
Brick (masonry + waterproofing)	20–35 years	Low (absorbs moisture)	High (labor intensity)	25 000–60 000

For most garages, the optimal solution would be plastic caisson — it is lightweight, does not require additional waterproofing and is easy to install. However, if groundwater is close (less than 1 meter from floor level), it is better to choose reinforced concrete or metal with enhanced processing. Brick caissons are rarely used today due to labor intensity and the risk of leaks.

⚠️ Attention: In garages with a high groundwater level, metal caissons rust in 5–7 years, even with anticorrosive. In such cases, external waterproofing is required (for example, coating mastic Technonikol or rolled materials Gidrostekloizol).

Where to place the caisson in the garage: optimal schemes

The location of the caisson depends on its purpose. Let's look at typical options:

1. At the entrance to the garage — for placement of water supply inlets, meters or pumping equipment. Advantage: easy access for repairs. Disadvantage: takes up part of the driveway.
2. Under the inspection hole — if the caisson is needed to drain oil or chemical waste. Important: the pit must be at least 30 cm wider than the caisson for ease of maintenance.
3. In the corner of the garage - for electrical panels or septic tanks. It is optimal if the caisson is adjacent to the wall: this simplifies the output of communications to the outside.
4. Under the floor in the center - a rare option used for large caissons (for example, for autonomous sewerage). Requires reinforced covering.

The dimensions of the caisson are selected based on its contents:

• 📦 For water/gas meters: 80×80×60 cm (l×w×d).
• ⚡ For the electrical panel: 60×60×50 cm (including space for cables).
• 🚜 For the pumping station: 120×100×80 cm.
• 🛢️ For the oil sump: 150×100×100 cm (with a bottom slope for draining).

Step-by-step instructions: how to install a caisson in the garage with your own hands

Installation of a caisson includes several stages: digging a pit, installing the structure, waterproofing and backfilling. Let's look at the process using an example plastic caisson size 100×100×80 cm.

1. Preparing the pit

The depth of the pit should exceed the height of the caisson by 20–30 cm for the installation of a sand cushion and a concrete base. Width - 30–40 cm wider than the caisson for ease of installation. The bottom of the pit is leveled and compacted.

Remove fertile soil layer|

Dig a pit 30 cm deeper than the caisson |

Arrange a sand cushion (10–15 cm)|

Sprinkle sand with water and compact it |

Check bottom level (tolerance: ±1 cm)

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2. Base structure

Placed on a sand cushion concrete slab (10 cm thick) or fill the screed. An alternative is gravel backfill (fraction 20–40 mm) with a layer of 15 cm. For caissons for pumping equipment, a concrete base with anchor bolts is required.

3. Installation of the caisson

The caisson is lowered into the pit using a winch or manually (if the weight is up to 200 kg). Important:

• 🔹 Align the structure according to the level (misalignment of more than 2° is unacceptable).
• 🔹 Fix the caisson to the base (for plastic - anchor loops, for metal - welding).
• 🔹 Remove all communications (pipes, cables) through sealed inputs.

4. Waterproofing and insulation

For plastic caissons, it is sufficient to treat the joints with sealant (Soudal Fix All). Metal caissons are coated with bitumen mastic on the outside and pasted over Technoelastom. Insulation (expanded polystyrene 5 cm) is attached to the outer walls.

5. Backfill

The pit is being filled up sand-cement mixture (ratio 5:1) in layers of 20 cm with a tamper. The top 30 cm can be covered with soil. The caisson cover is installed last, checking it for leaks.

⚠️ Attention: If the caisson is installed in a garage with a high groundwater level, backfilling is carried out only sand-cement mixture (without soil!). Otherwise, in the spring the caisson may “float” under water pressure.

Typical mistakes when installing a caisson and how to avoid them

Even experienced craftsmen make mistakes that shorten the service life of the caisson or make it unsafe. Here are the most critical of them:

1. Ignoring waterproofing - especially relevant for metal caissons. Without treatment, corrosion destroys the walls in 3–5 years. Solution: use epoxy primers (for example, Zinga) and coating waterproofing.
2. Incorrect bottom slope — if the caisson is intended for draining liquids (oil, water), the bottom should have a slope of 2–3° to the drain hole. Otherwise, the liquid will stagnate.
3. Lack of ventilation — gases accumulate in the sealed caisson (for example, from decomposing oil), which creates a risk of explosion. Solution: install a ventilation pipe with a diameter of 50 mm with an outlet on the roof of the garage.
4. Weak cover — if the caisson is located under the roadway, the cover must withstand a load of at least 2 tons. Optimal material: 5 mm corrugated steel or cast iron hatch.
5. Backfilling with soil without compaction — leads to subsidence and deformation of the walls. That's right: fill in layers of 20 cm and compact with a vibrating plate.

Another common problem is freezing of pipes in winter. To avoid this, the pipes inside the caisson are insulated foamed polyethylene (thickness 13 mm), and the caisson itself is made of polystyrene foam (5 cm). For regions with frosts below –20°C, it is recommended to use heating cable (for example, DeviPipeheat).

What to do if the caisson is already installed, but is freezing?

If the caisson freezes and dismantling is impossible, use the following measures:

1. Install inside the caisson infrared heater (power 100–200 W) with thermostat.

2. Wrap the pipes self-regulating heating cable (for example, Raychem Frostex).

3. Replace the caisson cover with an insulated one (with polyurethane foam filling).

4. If the caisson is metal, weld it to the lid thermal insulation box made of galvanized steel with mineral wool inside.

Caisson maintenance: cleaning, repair and prevention

The caisson requires regular maintenance, especially if it is used to drain oil or chemical waste. Basic procedures:

1. Cleaning

• 🧹 Mechanical cleaning — once every 6 months, remove sediment from the bottom of the caisson (use a plastic scraper and a vacuum cleaner for liquids).
• 🧪 Chemical washing - use solvents for oil sumps (White spirit) or special compounds (Liqui Moly Oil-System-Clean).
• 🚿 Washing with water - after cleaning, the caisson is washed with water pressure (you can use Körcher with a “mud cutter” attachment).

2. Leak test

Once a year, inspect the caisson for:

• 💧 Leaks in joints (especially after winter).
• 🔩 Corrosion on metal elements.
• 🕳️ Cracks in concrete caissons (if detected, repair them waterproofing putty Ceresit CR 65).

3. Corrosion prevention

For metal caissons:

• 🔧 Once every 2 years, update the anti-corrosion coating (use primer GF-021 + enamel PF-115).
• ⚡ Install tread protection (zinc anodes) - this will slow down rusting by 30-40%.

⚠️ Attention: If flammable liquids (gasoline, oil) are stored in the caisson, it is strictly prohibited to use open fire for heating or lighting. For inspection use explosion-proof lamps (protection class Ex d IIC).

Alternatives to a caisson: when is it not needed?

A caisson is not always a necessary element of a garage. In some cases it can be replaced with simpler solutions:

Situation	Alternative to caisson	Pros	Cons
Low groundwater level, warm garage	Wall cabinet for meters/machines	Cheaper, easier to install	Not suitable for drain systems
Communications take place outside the garage	External well with hatch	Doesn't take up space in the garage	Requires insulation for winter
Only for draining oil	Portable oil collector (container on wheels)	Mobility, easy cleaning	Takes up space, less airtight
Electrical communications	Wall panel with IP65	Quick access, no risk of flooding	Not suitable for plumbing

However, if the garage has autonomous water supply, sewerage or heating system, the caisson remains the most reliable solution. For example, there are simply no alternatives for a pumping station - it must be located below the freezing level of the soil.

FAQ: Frequently asked questions about caissons in the garage

Is it necessary to insulate the caisson if the garage is heated?

Yes, insulation is necessary even in a heated garage. The caisson is located below floor level, where the ground temperature in winter can drop to +5...+10°C. For water pipes this is enough to avoid freezing, but for sewers or oil sumps it is better to use insulation (expanded polystyrene 3–5 cm). The exception is caissons in regions with a mild climate (average winter temperatures are above –5°C).

Is it possible to make a caisson from scrap materials (barrels, Eurocubes)?

Technically yes, but such a caisson will not last long. For example, a metal barrel without anti-corrosion treatment will rust in 3-4 years, and a plastic Eurocube will not withstand soil pressure. If your budget is limited, it is better to use concrete rings (diameter 1 m) or weld a caisson from 4 mm steel sheets with stiffeners. Available materials are only suitable for temporary solutions.

Which caisson is better: ready-made or homemade?

Finished caissons (for example, from Triton-Plast or Aquatek) are superior in terms of tightness and installation speed, but are more expensive. Homemade ones (made of concrete or metal) are cheaper, but require experience in waterproofing and welding. The best option is to buy plastic caisson (it does not rust and is lightweight) and install it yourself. For difficult soils (heaving, with high groundwater), it is better to order a concrete caisson from professionals.

What to do if water appears in the caisson?

Reasons for the appearance of water:

1. Violation of waterproofing (cracks, loose joints).
2. Rise of groundwater (in spring or after rains).
3. Condensation (if the caisson is metal and not insulated).

Solutions:

• For waterproofing: treat joints waterproofing tape PLUG or liquid rubber Hyperdesmoc.
• For groundwater: install drain pump (for example, Grundfos Unilift KP) with float switch.
• For condensation: insulate the walls with polyurethane foam and organize ventilation.

Is ventilation necessary in the caisson?

Yes, ventilation is required if the caisson:

• Used to drain oil or chemical waste (accumulation of vapors).
• Has metal walls (risk of corrosion from condensation).
• Located in a garage with high humidity.

The optimal solution is supply and exhaust ventilation with two pipes (diameter 50–100 mm): one leads to the garage roof (exhaust), the other to the floor (supply). For forced ventilation you can install duct fan (for example, Soler & Palau TD-100).