Suspended timber floors explained

Learn to recognise a suspended timber floor, find its joists, and check safely underneath.

ST-01Learn what 'suspended timber floor' meansA suspended timber floor is made of floorboards nailed onto joists (thick timber beams that span the room), with an open gap underneath called a void. This is different from a solid floor, which is concrete poured straight onto the ground with no gap. The void under a timber floor needs airflow, which is why older houses have airbricks (bricks with holes) in the outside wall near ground level.

ST-02Spot the signs from outsideWalk round the outside of the house and look for airbricks low down in the wall, roughly in line with the floor inside. Also check if the ground floor sits noticeably higher than the garden or path outside. Both are strong clues there's a void underneath, meaning a suspended timber floor rather than solid concrete.

ST-03Do the bounce and sound test indoorsWalk slowly across the room and feel for a slight give or bounce underfoot, especially in the middle of the room away from walls. Stamp gently near the skirting (the timber board along the bottom of the wall) and in the centre of the floor, and listen: a hollow, drum-like sound means timber over a void, while a dull, solid thud usually means concrete.

ST-04Find the direction the joists runLook at the gaps between the floorboards, the timber joists always run underneath at right angles to them. Tap along the skirting with your knuckle: a solid, dull knock means you're over a joist, a hollower knock means you're between them. Joists are usually spaced around 400mm to 600mm apart, and beginners often assume every floorboard has a joist under it, which isn't true.

ST-05Confirm through an access point firstBefore touching any floorboards, check if there's an easier way to see the joists directly, such as a loft hatch above (if the room is a single storey, some houses have crawl space access), an under-stairs cupboard, or an airbrick you can peer through with a torch. Seeing bare timber joists and the void beneath confirms your identification without disturbing anything, and this is always the safest first step.

ST-06Lift a board edge only if you must check furtherIf you can't confirm any other way, gently prise up one board near the skirting using a bolster chisel (a wide flat chisel for prising), working slowly so you don't snap it. Before you do this, expect a cable or pipe to be running along or near the joists, not just where you think it should be. If there is any doubt at all, isolate the electrics at the consumer unit (the fuse box) and turn the water off at the stopcock before cutting or forcing anything, a cable detector helps but can miss deep cables, metal-wrapped ones, or amateur wiring that ignores normal safe zones, so treat the space as live and full until you've proven it isn't.

ST-07Check you've got it rightYou've correctly identified a suspended timber floor if you found a hollow sound when tapping, joists spaced fairly evenly around 400-600mm apart, and a void underneath with bare earth, hardcore, or an old dusty smell rather than solid concrete. If everything sounds solid throughout and there are no airbricks outside, you likely have a solid floor instead, and the advice in this guide won't apply.

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