Can I put a washing machine on a suspended wooden floor upstairs?

A washing machine on a ground-floor concrete slab is a solved problem. On a suspended timber floor, two things change. First, the floor can flex, and a floor that flexes at the frequency the drum is spinning at will feed energy back into the machine instead of absorbing it, which is how a machine that is perfectly level still walks. Second, a leak no longer soaks a floor — it soaks a ceiling, and often the electrics in it.

Safety

What you'll need

• A sheet of 18mm or thicker plywood
• A spirit level and a spanner for the feet
• A drip tray sized for the machine (optional)
• Anti-vibration pads (optional)
• A torch, to look at the joists from below if possible (optional)

Step by step

1. Assess the floor honestly before anything else.Walk across it and watch a glass of water on the floor. Visible bounce, rattling doors or a floor that moves under normal walking means the floor needs attention first, and a spinning appliance will make it much worse.
2. Find out which way the joists run.Usually perpendicular to the floorboards. From below if there is access, or by the line of nails in the boards. You want the machine's weight spread across several joists rather than concentrated between two.
3. Position it near a wall rather than mid-span if you can.Joists are stiffest at their ends, where they bear on a wall. The middle of a span is where deflection is greatest and where a machine will bounce most.
4. Spread the load with plywood.A sheet of 18mm or thicker ply, extending well beyond the machine's footprint in both directions and ideally crossing three or more joists. Screwed down into the boards, not loose. This is the single most effective thing you can do.
5. Level the machine on all four feet and lock them.Spirit level front to back and side to side, then push each top corner in turn — any rock at all means a foot is not down. Tighten the locknuts hard against the body afterwards. On a flexible floor, a machine that rocks even slightly will walk.
6. Consider anti-vibration pads as a second measure.They help genuinely on a springy floor by damping the transmission into the boards. They are not a substitute for level and locked feet, and putting a machine on pads without levelling it usually makes things worse.
7. Drop the spin speed if it still moves.Going from 1400 to 1000 makes a substantial difference to the forces involved, at the cost of wetter washing. On a problem floor it is often the pragmatic answer.
8. Treat the water side as the more serious risk.Upstairs, a burst fill hose empties at mains pressure into a ceiling. Fit braided hoses with an anti-burst device, keep the isolating tap accessible, and close it when you go away.
9. Fit a drip tray or a waterproof floor.A moulded appliance tray sized for the machine catches a slow leak and gives you the chance to notice it. Failing that, sheet vinyl taken up the wall as an upstand at least contains it rather than letting it run straight between the boards.
10. Make sure the socket is not in the splash zone.A socket at skirting level directly behind or beside an appliance on a floor that could flood is a bad arrangement. A switched fused spur higher up, and accessible, is better on both counts.
11. Check the waste arrangement carefully.The standpipe still needs to be about 600mm high with its own trap and an air gap. Running the waste into a bath or basin trap upstairs is common but must not create a siphon or a route for waste smells.
12. Run it and watch the first full spin.Stand and watch a 1400 spin on a normal load. Any walking, any drumming of the floor, any rattling of doors elsewhere in the room, and something above needs revisiting.

Tips

• Chipboard flooring is more prone to flexing locally than tongue-and-groove timber boards, and it swells and disintegrates if it gets wet. The plywood spreader matters more, and so does the drip tray.
• If the machine will go in a fitted cupboard or an airing cupboard, remember that it still needs a gap all round so it cannot drum against the carcass.
• Leak detection is cheap. A battery water alarm on the floor behind the machine costs very little and will wake the house before the ceiling comes down.

Common mistakes

• Standing the machine on the floorboards without spreading the load — Four small feet concentrate 60 to 80kg plus a spinning load onto a very small area of board. Spread across a ply sheet crossing several joists, the same weight is unremarkable.
• Using pads or mats instead of levelling — An unlevel machine on soft pads has more freedom to move, not less. Level and locked first; pads only afterwards, if still needed.
• Treating the vibration as the whole problem — Upstairs, water is the expensive failure. Braided hoses, an accessible isolating tap and a drip tray cost little and prevent a ceiling replacement.

If it doesn't work

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