How to prepare a site for a 7x2.7 m modular sauna. Step-by-step instructions from an engineer. Photos and videos.

🔍 Why the foundation is important even for a mobile sauna

Many customers mistakenly believe that a modular sauna  doesn't require site preparation. After all, it's a "mobile structure" that can be placed directly on the ground. This is a dangerous misconception.

Even if your sauna  has a reinforced frame and doesn't require a solid foundation, proper foundation preparation is critical for the following reasons:

1. Protection from moisture and decay

Wooden structures in contact with the ground absorb moisture due to capillary rise. Even rot-resistant Thermowood requires insulation from the ground to ensure maximum service life.

2. Stability of geometry

An uneven surface can cause the structure to become uneven. Even a 3-5 cm difference over a 7-meter length can cause:

• Differences in seams between boards.
• Doors and windows jamming
• Damage to internal communications (electricity, plumbing)

3. Durability of the roof and walls

A level foundation ensures proper load distribution. This is especially important for a pitched roof, where uneven loading can lead to deformation.

4. Operational safety

A properly prepared area with drainage prevents water stagnation around the bathhouse , which is critical for the safety of electrical wiring and the comfort of visitors.

5. Saving money

Site preparation costs (12,000–55,000 UAH) account for 2–8% of the sauna's cost . However, a lack of preparation can lead to repairs, which will cost 30–50% of the initial price within 2–3 years.

💡 Expert opinion : "A modular bathhouse  doesn't require a permanent foundation, but it does require a level, stable base with drainage . This isn't construction; it's engineering preparation that can be completed in 1-5 days."

 

📊 3 Foundation Options: Comparison Table

There are several ways to prepare a site for a  7x2.7 m modular sauna . The choice depends on:

• Soil type (sand, clay, black soil)
• Groundwater level
• Budget
• Deadlines
• Plans for the future (do you plan to move the bathhouse )

Comparison table of foundation options:

Parameter	Concrete slab	Strip foundation	Screw piles	FBS blocks
Price	25,000–45,000 UAH	30,000–55,000 UAH	18,000–35,000 UAH	12,000–22,000 UAH
Installation time	3–5 days (plus 7–14 days for concrete to gain strength)	5–7 days (plus 14–21 days to reach strength)	1–2 days	1–2 days
Complexity	Medium (need a concrete mixer or concrete order)	High (requires formwork and reinforcement)	Low (special equipment for screwing in)	Low (crane or manipulator required)
Suitable for weak soils	✅ Yes	✅ Yes	✅✅ The best option	⚠️ Limited
Slope suitability	❌ No	⚠️ It's difficult	✅✅ Perfect	⚠️ It's difficult
Possibility of dismantling	❌ Impossible	❌ Impossible	✅✅ Easy (dumps can be unscrewed)	⚠️ Possible, but difficult
Durability	50+ years	50+ years	20–30 years old	15–20 years
Drainage	Additional required	Additional required	✅ Natural (the space between the ground and the bathhouse )	⚠️ Limited

Engineer's recommendations:

✅ Choose a concrete slab if:

• You have a flat area with a low groundwater level.
• You plan to use the bathhouse  for 20+ years in one place
• The budget allows spending 25–45 thousand UAH.

✅ Choose a strip foundation if:

• The bathhouse  will be large (over 20 m²) or two-story
• The soil is unstable (clay, loam)
• You want maximum reliability

✅ Choose screw piles if:

• A site on a slope or with a high groundwater level
• Needed quickly (1-2 days)
• It is possible to dismantle and move the bathhouse  in the future.
• Best value for money and speed ⭐

✅ Choose FBS blocks if:

• The budget is limited (12–22 thousand UAH)
• Needed quickly (1-2 days)
• The soil is level and stable

💡 Our choice for a  7x2.7 m sauna : screw piles or FBS blocks . This is optimal in terms of cost, speed, and reliability for a modular structure of this weight (~2800 kg).

 

🛠️ Step-by-step installation: From marking to connecting utilities

Stage 1: Preparatory work (1–2 days)

Step 1.1: Determine the installation location

Accommodation requirements:

• Distance from the site boundaries: ≥1 m (according to DBN)
• Distance from other buildings: ≥3 m (fire safety)
• Access for a truck: width ≥3 m , height ≥3.5 m
• Distance to the network: ≤15m (for convenient connection)
• Distance to water and sewerage: ≤10 m (for shower area)

Step 1.2: Clearing the Site

1. Remove vegetation (grass, shrubs, trees)
2. Remove the fertile soil layer (10–15 cm) – this prevents plants from growing under the bathhouse.
3. Level the surface (height difference ≤3 cm per 7 m length)

💡 Lifehack : Use the removed fertile soil layer for your vegetable garden or flower bed – don't throw it away!

Step 1.3: Markup

Tools :

• Colas (8 pcs.)
• Rope or cord
• Tape measure (minimum 10 m)
• Building level (2 m)
• Goniometer or Pythagoras' theorem

Process :

1. Drive the first stake into the corner of the future bathhouse.
2. Measure 7m along one side, drive in the second stake.
3. Measure out 2.7g perpendicularly, drive in the third stake.
4. Check the diagonals: they should be equal (tolerance ±2 cm)
5. Stretch a cord between the stakes - this is the outline of the future foundation.

Stage 2: Installation of the foundation (1–5 days depending on the type)

Option A: Screw piles (fast method)

Materials :

• Screw piles Ø89 mm, length 2–2.5 m (12–16 pcs.)
• Pile head 100×100 mm (12–16 pcs.)
• Antiseptic for metal
• Sand for backfill

Step 2.1: Screwing in the piles

1. Determine the installation points for the piles:
   • Around the perimeter: step 1.5-2 m
   • In the center: 2-3 additional piles for even load distribution

Layout for a  7×2.7 m bathhouse :

•———•———•———•———• (7 m side, 5 piles)

| |

• • (2.7 m side, 3 piles on each side)

| |

•———•———•———•———— (total 12–16 piles)

Screw in the piles using special equipment or manually (with a 2–3 m lever):

1. • Immersion depth: ≥1.5 m (below freezing depth)
   • Verticality: check with a level (tolerance ±2°)
   • The top edge of the piles should be at the same level (height above the ground 40-50 cm)

Step 2.2: Installing the Heads

1. Cut the tops of the piles at the same level (use a grinder)
2. Weld or screw the 100×100 mm heads.
3. Treat welds with an anti-corrosion compound

Step 2.3: Binding (optional but recommended)

1. Use a 150x100 mm wooden beam or a 100x50 mm metal channel.
2. Secure to the heads with screws or welding
3. Treat the wood with an antiseptic

Advantages of screw piles :

• ✅ Installation speed: 1–2 days
• ✅ Does not require concreting
• ✅ Can be installed in winter
• ✅ Ideal drainage (the space under the bathhouse  is ventilated)
• ✅ Easy to dismantle and move

Option B: FBS blocks (budget method)

Materials :

• FBS blocks 20×20×40 cm (18–24 pcs.)
• Medium-sized sand (2–3 m³)
• Crushed stone of fraction 20–40 mm (1–2 m³)
• Roofing felt or hydroisol (1 roll)
• Antiseptic for wood

Step 2.1: Preparing the pillow

1. Dig holes for the blocks 30–40 cm deep:
   • Pit size: 50×50 cm
   • Quantity: 12–16 points (as for piles)
2. Add a 10 cm layer of sand and compact it.
3. Add a 10 cm layer of crushed stone and compact it.
4. Pour water over the area to seal it.

Step 2.2: Installing the Blocks

1. Place roofing felt on the pad (waterproofing)
2. Install FBS blocks strictly according to the level:
   • Check the horizontal level with a level
   • Height above ground: 20-30 cm
   • Height difference tolerance: ±1 cm

Step 2.3: Waterproofing

1. Cover the top of the blocks with roofing felt in 2 layers.
2. Treat the side surfaces with bitumen mastic (optional)

Advantages of FBS blocks :

• ✅ Low cost (12–22 thousand UAH)
• ✅ Quick installation (1–2 days)
• ✅ No specialized equipment required
• ✅ You can install it yourself

⚠️ Disadvantages :

• Less stability on weak soils
• It's harder to align perfectly
• Limited dismantling capability

Option B: Concrete slab (maximum reliability)

Materials :

• Sand (4–5 m³)
• Crushed stone (3–4 m³)
• Rebar Ø10–12 mm (150–200 m)
• Concrete M200–M300 (8–10 m³)
• Polyethylene film (50–70 m²)
• Formwork boards (0.5–1 m³)

Step 2.1: Preparing the base

1. Remove the fertile soil layer (15–20 cm)
2. Add a 10 cm layer of sand and compact it with a vibrating plate.
3. Add a 10 cm layer of crushed stone and compact it.
4. Water to achieve final compaction.

Step 2.2: Waterproofing and formwork

1. Cover the pillow with plastic wrap (overlap 15-20 cm)
2. Install formwork from boards around the perimeter:
   • Formwork height: 12-15 cm (slab thickness)
   • Secure with stakes from the outside
   • Check the diagonals

Step 2.3: Reinforcement

1. Tie the reinforcement mesh:
   • Cell: 20×20 cm
   • Two layers (bottom and top)
   • Distance between layers: 8–10 cm
   • Use tie wire, do not weld
2. Raise the bottom layer of reinforcement 3-4 cm above the film (use plastic spacers or bricks)

Step 2.4: Concreting.

1. Order M200–M300 concrete with delivery (mixer) or prepare it yourself:
   • Proportion: 1 part cement: 3 parts sand: 4 parts crushed stone
2. Pour concrete in a layer of 12–15 cm:
   • Align with a rule
   • Compact with a vibrator or pin (air removal)
   • Check the horizontal level with a level
3. Cover with film to prevent moisture evaporation.

Step 2.5: Concrete Curing

1. First 7 days: Water 2-3 times a day (to gain strength)
2. After 7 days: the formwork can be removed
3. After 14–21 days: the concrete gains 70–100% strength, the bathhouse can be installed.

Advantages of concrete slab :

• ✅ Maximum reliability and durability (50+ years)
• ✅ Perfectly flat surface
• ✅ Additional thermal insulation from the ground
• ✅ Possibility of installing underfloor heating

⚠️ Disadvantages :

• High cost (25–45 thousand UAH)
• Long installation time (3–5 days + 14–21 days for strength gain)
• Impossibility of dismantling

Stage 3: Connecting communications (1–2 days)

Step 3.1: Power Supply

Requirements :

• Power: minimum 9 kW (for electric oven + lighting + boiler)
• Voltage: 220V (single-phase) or 380V (three-phase, if available)
• Protection: RCD 30 mA, circuit breakers 16–32 A
• Grounding: resistance ≤4 ohms

Process :

1. Lay the VVGng-LS cable 3×6 mm² (for 220V) or 5×6 mm² (for 380V):
   • In the ground: in a protective corrugated pipe or HDPE pipe, depth 60-80 cm
   • By air: on a rope, height ≥3.5 m above the ground
2. Install a distribution board near the bathhouse :
   • Automatic circuit breaker 32A
   • RCD 30 mA
   • Switches for lighting and sockets
3. Connect the internal electrical wiring of the bathhouse  (already installed by the manufacturer):
   • Check the insulation resistance (≥1 MOhm)
   • Check the grounding

⚠️ Important : Electrical installation work must be performed by a certified electrician with a clearance group of up to 1000V!

Step 3.2: Water supply and sewerage

Option A: Centralized water supply

1. Connect the PPR pipe Ø20–25 mm from the water source:
   • Planting depth: 60–80 cm (below the freezing depth)
   • Use insulation (energoflex) for winter
2. Install a shut-off valve near the bathhouse
3. Connect to the terminals in the shower area (already prepared by the manufacturer)

Option B: Autonomous water supply (well/water well)

1. Install the pumping station:
   • In the bathhouse (  if there is heating)
   • Or in a caisson near the well
2. Connect to the sauna system

Sewerage:

1. Option A: Centralized sewerage
   • Bring the PP pipe Ø110 mm to the bathhouse
   • Slope: 2 cm per 1 m of length
   • Connect to the drain in the steam room and the outlet in the shower
2. Option B: Septic tank or cesspool
   • Dig a hole at a distance of ≥5 m from the bathhouse
   • Install a septic tank or make a pit from concrete rings
   • Bring in a Ø110 mm pipe with a 2 cm/m slope
3. Option B: Drainage well (for shower)
   • Dig a hole 1×1×1.5 m
   • Fill with crushed stone and sand in layers
   • Connect the pipe from the shower

💡 Expert advice : For winter use, be sure to insulate the water supply and sewerage system or install a heating cable.

Step 3.3: Ventilation

Although the bathhouse already  has F-100 ventilation valves, it is recommended:

1. Provide natural ventilation of the space under the bathhouse  (for screw piles and blocks):
   • Leave a gap of 40-50 cm between the ground and the floor
   • Make vents in the base (if any)
2. For concrete slab:
   • Provide ventilation ducts in the stove (optional)

Stage 4: Final preparation before installing the sauna  (0.5–1 day)

Step 4.1: Checking the Foundation

1. Check the horizontality:
   • Use a laser or water level
   • Tolerance: ±1 cm per 7 m length
2. Check the dimensions:
   • Length: 7000 ±20 mm
   • Width: 2700 ±20 mm
   • Diagonals: difference ≤20 mm

Step 4.2: Waterproofing the contact points

1. Place the following on the areas where the bathhouse contacts  the foundation (piles, blocks or slab):
   • Roofing felt in 2 layers
   • Or hydroisol
   • Or a special waterproofing membrane
2. This prevents capillary absorption of moisture by wooden structures.

Step 4.3: Preparing for unloading

1. Provide access for a crane or truck:
   • Width: ≥3 m
   • Height: ≥3.5 m
   • Soil strength: compact if necessary
2. Prepare the area for unloading:
   • Size: ≥10×5 m
   • Flat surface

 

❌ Common mistakes and how to avoid them

Mistake 1: Ignoring the groundwater level

Problem : High groundwater levels lead to flooding of the site, rotting of wood, and destruction of the foundation.

How to avoid :

• Check the groundwater level in spring (highest level)
• If water is close (<1 m from the surface):
  • Choose screw piles (highest support points)
  • Make drainage around the site
  • Raise the base 50-60 cm above the ground

Error 2: Insufficient compaction of the base

Problem : Soil subsidence under load leads to the bathhouse becoming uneven , cracks appearing, and damage to utilities.

How to avoid :

• Be sure to compact the sand and gravel bed.
• Use a vibrating plate (not just your foot)
• Water to achieve final compaction.
• Wait 1-2 days before installation

Error 3: Lack of waterproofing

Problem : Moisture from the soil rises through capillaries into wooden structures → rotting, mold, unpleasant odor.

How to avoid :

• Between the foundation (blocks, piles, slab) and the bathhouse,  be sure to lay roofing felt in 2 layers.
• For concrete slab: use polyethylene film under the concrete and waterproofing on top.
• Treat the bottom crown of the wooden frame with an antiseptic.

Error 4: Incorrect slope for sewerage

Problem : Stagnant water in pipes, unpleasant odor, blockage, backflow.

How to avoid :

• Slope of sewer pipe Ø110 mm: 2 cm per 1 m of length
• For a pipe Ø50 mm: 3 cm per 1 m
• Check the slope with a level during installation.
• Avoid right angles (use 45° knees instead of 90°)

Error 5: Insufficient electrical power

Problem : Tripping of circuit breakers, overheating of wiring, fire hazard.

How to avoid :

• Calculate the total power:
  • Electric oven: 6-9 kW
  • Boiler: 1.5–2 kW
  • Lighting: 0.5 kW
  • Sockets: 1–2 kW
  • Total: 9–14 kW
• Run a separate cable from the meter
• Use section:
  • For 9 kW: 3×6 mm² (copper)
  • For 14 kW: 5×6 mm² (three-phase)
• Install a separate 32–40A circuit breaker and a 30 mA RCD.

Mistake 6: Ignoring drainage

Problem : Stagnant water around the bathhouse , flooding, rotting, uncomfortable operation.

How to avoid :

• Make the site slope 2–3° away from the bathhouse  (water flows out)
• Install drainage channels around the perimeter (optional)
• For screw piles: the space under the bathhouse  provides natural drainage.
• For a concrete slab: make a 50-70 cm wide blind area with a slope away from the slab

Error 7: Lack of ventilation vents

Problem : Stagnant moist air under the floor → rotting, mold, unpleasant odor.

How to avoid :

• For screw piles and blocks: leave a space of 40–50 cm between the ground and the floor
• Make vents (holes) in the base: 1 hole per 2-3 m of perimeter
• Spirit size: 10×10 cm or Ø10–15 cm
• In winter, the vents can be closed, but not completely.

Mistake 8: Skimping on materials

Problem : Cheap materials deteriorate quickly, leading to costly repairs.

How to avoid :

• Don't skimp on:
  • Waterproofing (high-quality roofing felt or hydroisol)
  • Rebar (not rusty, correct diameter)
  • Cables (only copper VVGNG-LS)
  • Pipes (PPR for water, PP for sewerage)
• It's better to spend 2-3 thousand UAH more now than 20-30 thousand UAH on repairs in 2 years.

 

📖 Case Study: How We Prepared a  7x2.7m Bathhouse Site in Sokolovka

Initial data:

• Location : Sokolovka village, Zhytomyr region
• Soil type : loam, groundwater level 1.2 m (in spring)
• Terrain : flat area, slight slope of 2°
• Client budget : up to 25,000 UAH for site preparation
• Deadlines : The bathhouse had to be installed  within 10 days.

Selected solution: Screw piles Ø89 mm

Why did you choose this option :

• ✅ Installation speed (1–2 days)
• ✅ Suitable for loam and high groundwater levels
• ✅ Budget: 22,000 UAH (including materials and labor)
• ✅ Possibility of dismantling in the future (the client planned to move in 5-7 years)

Stages of work:

Day 1: Preparation

• Clearing the site of vegetation (10 cm of fertile soil layer removed)
• Marking: 16 points for piles (5x3 pattern + 1 central)
• Delivery of materials: 16 piles Ø89×2500 mm, 16 heads 100×100 mm

Day 2: Installation of piles

• Screwing piles using specialized equipment (hydraulic drive)
• Immersion depth: 1.8 m (below the freezing depth for the region)
• Horizontal alignment (laser level)
• Trim the tops at the same level (height above ground: 45 cm)
• Welding of heads, treatment of seams with anti-corrosion compound

Day 3: Harnessing and Communications

• Installation of a frame made of 150×100 mm timber (along the perimeter + 2 longitudinal joists)
• Treating wood with Vidaron antiseptic
• Laying VVGng-LS 3×6 mm² electrical cable in a HDPE pipe (depth 70 cm)
• Supply of PPR water pipeline Ø25 mm from the well (distance 15 m)
• Installation of a PP sewer pipe Ø110 mm to a septic tank (slope 2 cm/m, length 12 m)

Day 4: Final Preparations

• Checking the horizontal level (a difference of ±5 mm over 7 m is normal)
• Waterproofing of roof heads with roofing felt in 2 layers
• Installing a temporary canopy to protect from rain
• Final check of communications (water pressure, insulation resistance)

Result:

• ✅ Completion time : 4 days (instead of the planned 10)
• ✅ Budget : 21,500 UAH (savings 3,500 UAH)
• ✅ Quality : perfect horizontal alignment, all utilities connected
• ✅ Customer Satisfaction : ⭐⭐⭐⭐⭐

Customer review : "The Thermowood team did a lightning-fast job! I thought I'd have to wait a week, but they did it all in four days. The sauna  is perfectly level, with no wobble. I especially liked that I didn't have to wait for the concrete to set—they screwed the piles in and were ready to install it right away . "
- Andrey K., sauna owner

 

💡 Expert advice for perfect results

Tip 1: Plan Ahead

Avoid installing the system in the rain or after heavy precipitation. The soil must be dry for proper compaction. The best time is April-October (when there is no frost).

Tip 2: Engage specialists

Even if you're an experienced builder, surveying and electrical installation require specialized knowledge and equipment. Saving 2,000–3,000 UAH on specialists can cost 20,000–30,000 UAH in repairs.

Tip 3: Document the process

Take photos of each stage:

• Markup
• Laying reinforcement
• Laying communications to the backfill

This will help with upcoming renovations or selling real estate.

Tip 4: Don't Ignore the Rules

DBN B.2.2-15-2005 "Residential Buildings. Basic Provisions" requires:

• Distance from the site boundaries: ≥1 m
• Distance from other buildings: ≥3 m
• Fire gap: ≥6 m (for wooden buildings)

Compliance with regulations will avoid problems with neighbors and regulatory authorities.

Tip 5: Think about the future

If you're planning an expansion (a terrace, a carport, a second sauna ), leave some space in advance. Removing and relocating utilities will cost more than the initial design.

 

📞 Get a free consultation with an engineer

Preparing the site is just the first step to your perfect sauna . To avoid mistakes and save time and money, order a  7x2.7 m sauna from Thermowood Production and receive:

• ✅ Free site preparation consultation with an engineer (on-site or online)
• ✅ Customized utility layout plan for your site
• ✅ Technical data sheet with requirements for the foundation and connections
• ✅ 3D visualization of a bathhouse  on your site
• ✅ Help with choosing the type of foundation (piles, blocks, slab) taking into account your soil and budget

📞 Contact information for orders:

• 🔹 Phone / WhatsApp / Viber : +38 067 201 50 50
• 🔹 Email : info@thermowood-production.de
• 🔹 Website : lestorg.org.ua | thermowood-production.de

📩 Write "PLOT 7×2.7" in Viber/WhatsApp — and receive within 24 hours:

• ✅ Free consultation with an engineer
• ✅ Individual site preparation estimate
• ✅ Catalog of completed projects 2024–2026
• ✅ Technical data sheet and certificates