Installing a Main Drain for a Hot Tub or Plunge Pool (Twin Drains, Step by Step)

Last updated: September 2026

For most people, the main drains are the first bit of plumbing that goes into a DIY hot tub or plunge pool. They also have to go in at the most unforgiving moment: before you pour the floor slab. Once the concrete’s in, there’s no moving them, no changing the pipe size and no fixing a leaky joint without a breaker.

In this guide and the video below, I’ll explain what main drains do, why you always fit two, how to set them at the right height, and how to plumb them so they’re safe and easy to live with.

What is a main drain, and why do you need one?

A main drain is a round sump set into the floor of the tub, with a cover on top and a pipe coming out of the side or bottom. It does three jobs:

• Circulation from the bottom. Along with the skimmer at the top, it’s where the pump draws water from, so the water at the bottom of the tub gets filtered and heated too.
• Draining down. It’s the lowest point in the tub, so it’s how you empty it for cleaning or winterising.
• A housing for the hydrostatic relief valve. On an in-ground build, one of the drains usually holds the valve that protects the shell from groundwater pressure.

Main drains are typically 5–8in (125–200mm) across and, on anything I design, they come in pairs. For how they fit into the rest of the system (skimmer, pump, filter, heater and jets), see my DIY hot tub plumbing guide.

Suction safety: why you always fit two

This is the bit I’d stress most. A single suction outlet that can be completely covered by a body or a hand can hold someone underwater, and hair can get pulled into a drain cover and tangled. In-ground spas with flat grates and a single main drain are exactly what the Pool Safely FAQ names among the highest entrapment risks.

The fix is simple and cheap at the build stage:

• Two drains, teed together into one suction line. If one is blocked, the pump keeps drawing through the other, so it can’t pull hard on the blocked one. That also protects the pump from running dry.
• At least 1m (3ft) apart. Far enough that one person can’t cover both.
• Certified anti-entrapment covers on every drain and suction fitting. They’re domed and designed not to seal against a body or tangle hair.

What the rules say

• USA: the Virginia Graeme Baker (VGB) Pool and Spa Safety Act requires public pools and spas to have drain covers that meet ANSI/APSP/ICC-16. A public pool with a single blockable drain, or drains on the same plane less than 3ft apart, also needs a secondary anti-entrapment system. The Act is aimed at public pools, but it’s exactly how I’d build a home tub, and many local codes and inspectors apply similar rules to residential pools and spas.
• UK: there’s no equivalent law for a home hot tub. For public pools, PWTAG says two outlets with a flow through them of no more than 0.5m/s is a necessary requirement for new pools, referencing BS EN 13451-3. I’d follow the same principle at home.

Choosing drains and covers

Waterway, CMP, Pentair and Hayward all make main drains and VGB-compliant covers, and in the UK and Europe Certikin’s concrete main drain is the one I supply, as it’s designed to take their hydrostatic relief valve. Whatever you choose:

• Check the cover’s flow rating. Every certified cover is rated for a maximum flow. Work out what happens if one drain is blocked and all the flow goes through the other. As Pool & Spa News puts it, the maximum flow rating on the drain cover should be cut in half to start, in case one of the drains becomes blocked. In practice, pick covers where one cover alone is rated for your full pump flow.
• Buy the drain body and cover as a matched set, and keep the cover’s paperwork. In the USA an inspector may want to see it.
• Get the right body for your build. Drains made for concrete have a flange that the render and membrane bond to. Vinyl-liner drains are different.

Pipe size and suction speed

Use at least 2in (50mm) pipe from the drains. It’s always easier to reduce the suction later with a ball valve than it is to increase it, and you can’t swap a pipe that’s buried under concrete.

Suction speed matters for safety as well as for the pump. US pool codes limit suction pipe velocity to 6 ft/s, which works out at about 63 GPM through a 2in pipe. If your pump moves more than that, split the suction between the drains and the skimmer, or step up the pipe size. The full velocity table is in my plumbing guide, and there’s more on choosing pipe in Hot tub pipe: the ultimate guide.

Main drains vs jet suction

Main drains aren’t designed for the high flow that jet pumps need, so don’t rely on them alone to feed your jets. Jet pumps normally draw from high-flow spa suction fittings set low in the tub walls, which are built for much more flow than a pool-style main drain. The main drains and skimmer then look after circulation through the filter and heater. The same safety principle applies to those suction fittings: certified covers rated for the flow, and ideally two per pump so one can’t become a single blockable outlet. How many you need depends on your pumps. See How many pumps do you need? and How to correctly size a hot tub pump.

How to install main drains, step by step

This fits into the base build-up in my in-ground build guide and concrete slab guide: compacted sub-base, sand blinding and a DPM, insulation, then a reinforced slab of about 150mm (6in).

1. Plan the positions

Mark both drains on your plans before you dig. They go in the deepest part of the floor, at least 1m (3ft) apart, usually on the centre line. Plan the route of the pipe to the wall and on to the control room, so it’s as short and straight as you can make it. See Hot tub control room position.

2. Dig the trench and the valve pocket

Once the sub-base is down, dig a trench for the pipe run. If one drain is housing a hydrostatic relief valve, dig a pocket under it and fill it with clean, free-draining gravel for the valve’s collection tube. The full details of that are in How to install a hydrostatic relief valve, and why you need one is in Why do I need a hydrostatic relief valve? The short version: the valve screws in (PTFE tape, not glue), the collection tube goes into gravel and never into concrete, and one valve is enough for a typical DIY tub.

3. Set the height

This is the step people get wrong. The drain’s rim needs to finish flush with your finished floor, not with the concrete. So set it proud of the slab by the thickness of your render, membrane, adhesive and tiles, which is about 25mm (1in) on average for render and mosaic. Work it out from your own build-up.

The easiest way is to have the actual cover or grate in your hand when you set it. Too high and you get a lump that’s uncomfortable underfoot and awkward to tile around. Too low and the cover sits in a dip, which collects grit and looks untidy.

4. Fix it level and tie it down

Check each drain is level and square with the shell, then support it on blocks and wire it to the rebar so it can’t float up or shift when the concrete goes in. There’s more on the steel in my rebar guide.

5. Plumb and pressure test

Run 2in pipe from each drain to a tee roughly halfway between them, then one line to the control room. Solvent-weld every joint properly (see how to make a PVC solvent weld joint). Where the pipe runs through concrete, a puddle flange helps stop water tracking along the outside of the pipe. Then cap it off and pressure test the drain line before you pour. A leak you find now costs you a fitting. A leak you find after the pour costs you the floor.

6. Protect it and pour

Tape over the top of each drain, or leave the grille on, so no concrete or slurry gets inside. Grit on the hydrostatic valve seat is the most common reason those leak. Work the concrete around the drain bodies so there are no voids, and trowel it neatly to the flange.

7. Waterproof into the drain

Render up to the drain flange, then take your pool-rated membrane into the drain as the membrane manufacturer details it. Penetrations like drains are where a membrane system is most likely to fail, so take your time. See my waterproofing, grout and tile adhesive guide.

Common mistakes

• Fitting a single main drain.
• Drains so close together that one person could cover both.
• Uncertified or flat covers, or covers not rated for the pump’s flow.
• Setting the drain flush with the concrete, so it ends up sunk below the tiles.
• Undersized pipe, which means fast, noisy and less safe suction.
• Not fixing the drain to the rebar, so it moves in the pour.
• Not pressure testing before the concrete goes in.
• Forgetting the hydrostatic relief valve until the concrete’s ordered.

If you do end up with a drop in water level later, How to detect and fix a hot tub leak takes you through finding it. More pitfalls are in Avoid these 7 DIY hot tub mistakes.

Want it drawn up for you?

My DIY hot tub plans show the drain and hydrostatic relief valve positions, the full plumbing layout and a parts list, with the build guide and video course to walk you through it. Building bigger? There are plunge pool / cocktail pool plans and swim spa plans too. More on that in the ultimate guide to building DIY plunge pools.

For a custom shape, design it in the free Tub & Pool Designer Pro software and I can draw up the 3D plumbing design to match. Main drains, covers and hydrostatic relief valves are available from parts4tubs.co.uk in the UK and parts4tubs.com in the USA.

FAQs

Does a hot tub need a main drain?

A built-in or in-ground hot tub or plunge pool should have them. They circulate water from the bottom, let you drain the tub, and house the hydrostatic relief valve. Fit two, not one.

How far apart should main drains be?

At least 1m (3ft) apart, teed into one suction line, with anti-entrapment covers on both.

What size pipe should I use for a main drain?

At least 2in (50mm). Keep suction speeds at or below about 6 ft/s, which is roughly 63 GPM through a 2in pipe. Bigger systems need more suction lines or bigger pipe.

Do residential hot tubs have to follow the VGB Act?

The VGB Act applies to public pools and spas. Local codes may apply similar rules to home pools, so check with your inspector. Either way, twin drains and certified covers are how I’d build any tub.

When do main drains go in?

After the sub-base and before the slab is poured, at the same time as the hydrostatic relief valve, sleeves and rebar.

How high should the main drain sit?

Proud of the concrete by the thickness of your render, membrane, adhesive and tiles (about 25mm or 1in for render and mosaic), so the cover finishes flush with the tiled floor.

Got a question about your drains? Drop it in the comments below. I read them all.

Cheers,
Andi

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Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi