How Thick Should a Concrete Slab Be for a Hot Tub?

✔ Minimum thickness of 100–150mm

For most domestic hot tubs, a 150mm concrete slab is ideal. Smaller tubs on stable ground may be okay with 100mm, but more depth adds stability.
✔ Reinforced with mesh or fibre

Steel mesh (A142 or A193) or fibre additives help prevent cracking under constant weight. Especially important if the hot tub holds 1,000+ litres of water.
✔ Laid on a compacted MOT Type 1 sub-base

The concrete slab must sit on 100–150mm of well-compacted hardcore to stop sinking and to spread weight evenly across the soil.
✔ Flat, level, and smooth

Hot tubs must sit flush — any slope or dip causes pressure on the shell or frame. Use shuttering and levelling to keep things precise during the pour.
✔ Well-drained and separated from soil

Use a damp-proof membrane if the slab is flush with ground level. Geotextile fabric is smart on soft or clay ground under the sub-base.

Hot Tub Size	Water Capacity (Approx.)	Slab Thickness	Notes
2-Person Hot Tub	500–800 litres	100mm (reinforced)	Lightweight models may suit paving, but concrete is safer
4-Person Hot Tub	900–1,200 litres	125–150mm (with mesh)	Heavier tubs benefit from 150mm to reduce movement risk
6+ Person Hot Tub	1,300–1,800+ litres	150mm minimum	Always use rebar mesh or fibre mix — weight adds up fast
Swim Spa / Large Built-In	2,000+ litres	175–200mm (engineered)	May require a structural engineer for ground prep + depth

🔹 Can I put a hot tub directly on paving slabs?

You can, but it’s not ideal. Paving slabs can shift or settle unevenly under the weight of a full hot tub, especially if the sub-base isn’t properly compacted. Over time, even small movements can stress the hot tub frame or cause water to spill unevenly. A reinforced concrete slab is the safest, most stable option for long-term use.

🔹 Do I need insulation or a membrane under the concrete slab?

A damp-proof membrane (DPM) is a good idea, especially if the slab is at or near ground level. It stops moisture from rising through the concrete, which helps preserve strength and avoids mildew buildup under the hot tub. Insulation is optional but can help reduce heat loss if you're planning to insulate the slab edge or use underfloor heating.

🔹 How flat does the base need to be?

Very flat. Even a small slope or dip can throw the water level off or put pressure on one side of the hot tub shell. The slab should be level across the full footprint, with a smooth finish and no pooling. Use shuttering and a straightedge during the pour to get it right from the start.

🔹 Will a 100mm slab be enough for a hot tub?

For small 2-person tubs, a 100mm slab with good reinforcement may be fine. But for most hot tubs, especially anything 4-person or bigger, 125–150mm is safer. Water is heavy — and combined with body weight, you’re looking at over a ton of pressure in one spot. Always err on the side of more depth and stronger reinforcement.

How Thick Should a Concrete Slab Be for a Hot Tub?

What a Hot Tub Concrete Base Needs

Concrete Slab Thickness for Different Hot Tub Sizes

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