Joist

A joist is one of the horizontal beams that sit between walls or other supports to form the structural framework of a floor or ceiling. In most homes, joists are made from timber, though steel joists are used where longer spans or heavier loads are needed. When you walk across an upper floor, the joists underneath are what carry your weight down to the walls and foundations.

Types of joist

• Timber joists — the most common in residential buildings, typically solid softwood
• Engineered joists (I-joists) — manufactured from timber and oriented strand board (OSB), lighter and capable of spanning greater distances without sagging
• Steel joists — used where heavy loads need to be carried or where timber cannot span the required distance
• Concrete joists — occasionally found in older apartment blocks and commercial conversions

Why joists matter in a renovation

Joists come up in many renovation scenarios:

• Bathroom relocations — moving a bathroom means running new waste pipes and soil pipes between or through joists, which must be done carefully to avoid weakening the structure
• Removing load-bearing walls — when a wall is removed, a steel beam (or lintel) replaces it, and the joists that sat on that wall must be properly supported
• Adding weight — stone tiles, freestanding baths, and heavy kitchen appliances all add load; your contractor may need to sister (double up) joists or add extra support
• Levelling uneven floors — in older homes, joists can bow or settle over time, requiring packing or replacement to create a flat surface for new flooring

Notching and drilling rules

Building regulations set strict rules about where and how much you can cut into a joist to run pipes or cables. Cutting in the wrong place can weaken the joist and compromise the floor. This is something your contractor and any subcontractors should be well aware of.

Practical tip

If your renovation involves plumbing or electrical work on an upper floor, ask your contractor how the joists will be affected. Understanding joist direction and spacing early on can influence room layouts and bathroom positioning.