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How can I tell which way the floor joists span?

Look in the basement or the attic, check which way the floorboards run, or find the rows of nails in the ceiling. The direction is a clue about load-bearing — not an answer.

What the joist direction tells you — and what it doesn't

A floor or a roof spans in one direction, like a row of joists laid across a room. The load from the floor above runs along the joists out to their ends, and there the joists rest on something — a wall, a beam, or a wall plate.

So: a wall standing perpendicular to the joists is oriented so that joists can rest on it. A wall running parallel to the joists, on the other hand, can stand between two joists without carrying anything at all.

But there are three reasons this isn't an answer on its own:

• A wall can stand perpendicular to the joists without bearing them — the joists can span straight over it and rest further out.
• A wall can run parallel to the joists and still bear load, if a joist, a plate or a beam sits directly on top of it, or if it carries something entirely different — a chimney, a bathroom, a roof structure.
• The joist direction can change within the same floor level, especially in houses with extensions or ones that have been rebuilt.

So joist direction is one of the best clues you can find yourself. It is not a method for deciding whether a wall is load-bearing. That question has to be answered by a building surveyor or a consulting structural engineer on site, and breaking through a load-bearing wall normally has to be approved by the authorities before work begins.

Method 1: Look where the joists are visible

The most reliable method needs no tools: find a spot in the house where the structure isn't boxed in.

• The basement. In many older houses the ground floor's joists are visible from below, in the basement. There you can see directly which way the joists run — and whether they rest on a masonry wall, a beam, or a post. Take a photo with your phone and note which room you're standing under.
• The unfinished attic. Climb up through the loft hatch. Here you can see both the rafters (the roof's load-bearing members) and often the top side of the floor joists over the uppermost living level. Note where the rafters rest: typically on the exterior walls' wall plate, and in houses with a wide span, on an internal load-bearing wall too.
• Under the stairs, in a cupboard, or behind an inspection hatch. Hidden spots are often left unlined.

While you're up there or down there, note the spacing between the joists too. An ordinary timber joist floor typically has 55–90 cm between joist centres, and rafters are often spaced 90–120 cm apart. That figure is useful when you later have to explain the house to a professional.

Method 2: Read the floorboards

Floorboards on a timber joist floor are laid across the joists — otherwise the boards would have nothing to rest on. If the boards in the living room run from north to south, the joists usually span from east to west.

This method works on older wood floors laid directly on the joist structure. It does not work if a new floor has been laid on top of the old one, if the floor is laid on battens running a different direction, or if it's a floating floor on a concrete slab. Look for a distinct step at the door thresholds — that's often a sign of a floor laid on top of a floor.

Method 3: Find the nail rows in the ceiling

If the ceiling is lined with plasterboard or panels, the screws or nails sit in the joists. The rows of small dimples under the paint therefore run along the joists, and the spacing between the rows matches the joist spacing.

They're easiest to see in raking light: turn off the ceiling light, hold a torch right up against the ceiling and look along the surface. Where the finish is well filled, they can be invisible — a simple stud finder (magnetic or electronic) often works better then. Magnetic models find the screws; electronic ones find the timber itself.

Be aware that a suspended ceiling has its own frame, which doesn't have to follow the joists. If there's more than 5–6 cm between the visible ceiling surface and the underside of a window or door frame above it, there is probably a suspended ceiling.

Method 4: The house's original drawings — the best source of all

The building authorities usually hold the house's original drawings on file. Structural drawings often show the joist direction and spans directly, and load-bearing walls may be marked. In many places the archive can be searched online; otherwise you'll need to ask the authority for access to the building case file for the address.

Two caveats: the drawings show the house as it was designed, not necessarily as it was built. And if the house has been rebuilt since, the joist direction may have changed locally. The paper is an excellent starting point for the engineer — not a substitute for the site visit.

Write down what you find — it's worth gold to the engineer

Once you've found the joist direction, write it down somewhere it belongs with the rest of the home's measurements. What a consulting structural engineer typically wants on the table is:

• A measured floor plan of each level with room dimensions and wall thicknesses.
• The joist direction per level, and where you saw it.
• Which walls stand in the same place across several levels (vertical alignment through the house).
• Room heights, and whether there are suspended ceilings.
• Photos from the basement and attic.

HouseSense can help gather the first four: the app measures the rooms — scanned with the iPhone's LiDAR or drawn on graph paper — and produces the floor plan with dimensions, areas, wall thicknesses and room heights as a PDF you can send along. If the home has several levels, you can view the levels stacked on top of each other and read off which walls align vertically. Joist direction is one thing the app cannot measure: it's hidden behind the lining, and LiDAR only sees surfaces. That you have to find yourself, or let the professional find it.

And the conclusion stands, however many clues you gather: an app can measure the wall. It cannot decide what the wall carries.