Design new floor joists or strengthen a bouncy or sagging floor

Cutaway of a house floor showing timber joists in section, with three new deeper joists in teal and a person standing above

For owners planning a new floor, a loft floor, or a fix for a bouncy or sagging one. Describe the floor and compare quotes from structural engineers who check the loads and design the joists your builder will fit.

Floor joists have to be strong enough to carry furniture and people, and stiff enough that the floor doesn’t bounce, sag or crack the ceiling below. A structural engineer works out the loads, span, spacing and timber depth needed for a new floor, or checks an existing one and designs a fix: doubling joists, adding steel or timber beams, fixing plates to the sides or adding support underneath. They also check the bearings, and any notches or holes cut for pipes and cables. You receive calculations and drawings your builder can fit from.

How a professional can help

Sizing joists for a new floor: They work out joist depth, thickness and spacing for the span and what the floor will carry, so it is strong enough and stiff enough, and note whether solid timber or engineered joists suit the job.
Stiffening a bouncy floor: They look at why the floor moves and suggest remedies such as doubling or plating joists, adding strutting (small braces between joists) or a support beam underneath, rather than guessing at a fix that doesn’t work.
Floors for conversions and extensions: For an upper floor in an extension or a converted loft, they design joists, the framing around any opening, and bearings, and show how the floor ties to the walls so it carries the new load without overloading the ceilings below.
Notches, holes and past damage: They review joists weakened by notches, drilled holes or cut sections for pipes, wires or heating, and design repairs or replacement where a weakened joist can no longer do its job.
Walls and baths on the floor: They check what sits on the joists, such as a new stud wall or a large bath, and add support or doubled joists where a heavier load lands.
Bearings and fixings: They check how joists bear on walls or hang from beams, and specify joist hangers, straps and bearing lengths, so loads travel to the walls without crushing or twisting the timber.

Is this right for you?

Your floor bounces or sags and you want to know whether to strengthen it or replace it.
You’re changing a bathroom or kitchen layout and want the joists checked before the floor is opened up.
You’re building an upper floor in an extension and want joist sizes calculated for building control.
A plumber or electrician has cut notches or holes in joists and you want to know whether they are safe.

How the project usually runs

Look at the floor: The engineer inspects the floor from above and below where they can, noting joist size, spacing, direction and span, the walls they bear on and what the floor supports.
Opening up: Where needed, a section of floorboard or ceiling is lifted so the joists can be measured and checked for rot, notches or other damage before the design is finalised.
Joist design and calculations: The engineer sizes new joists or designs the strengthening, checking strength, how much the floor flexes and the bearings, and shows how any new beam or support is carried down.
Drawings for builder and building control: Marked-up drawings show joist sizes, spacing, fixings and details, with calculations for building control where the structure is being altered.
Rot and surprises under the boards: If boards come up to reveal rot, undersized timbers or an extra load, the engineer can review it and issue a revised detail before the floor is closed up again.

What affects the cost

Whether you need a new floor designed from scratch, or an existing floor assessed and strengthened.
The span, which is the main influence on joist depth and on whether an extra support is needed.
How much of the existing floor has to be exposed, and how much is hidden under fixed finishes.
Whether solid timber, engineered joists or steel are involved, and how they bear on the walls.
Whether the design needs beams or posts, and how their loads are carried further down the building.
Whether the floor is in a flat, where sound and fire requirements and the lease can add to the work.

Floors and building control: what to expect

Building control normally expects calculations when floor structure is altered, or when a floor must carry a new use, such as a loft room. In Scotland that means a building warrant.
Floors between flats also have sound and fire requirements, and your lease may need your landlord’s consent before the structure is altered.
Building regulations differ across England, Scotland, Wales and Northern Ireland. Structural engineers cover the whole UK and work to the rules where the property is.
If the joists bear into a wall shared with a neighbour, tell the engineer. In England and Wales the Party Wall etc. Act 1996 may then apply.

Questions people ask

Why does my floor bounce?

Bounce usually means the joists are strong enough to carry the load but not stiff enough to feel solid, often because they are shallow for the span or widely spaced. Notches, loose bearings or missing strutting can add to it. An engineer can find the cause and design a fix, which is sometimes much less work than replacing the floor.

What does sistering joists mean?

It means fixing a new timber alongside an existing joist to share the load. It can stiffen a floor without lifting the whole thing, but it has to be sized and fixed correctly, and it only works if the old joist is still sound and the new one can bear properly. The engineer specifies the timber and fixings.

Can a plumber cut notches in my joists?

There are limits. Notches and holes weaken a joist, and there is guidance on how large they can be and where they can go. Larger or badly placed cuts should be checked by an engineer, who may design a repair. Ask your plumber or electrician to tell you before they cut, not after.

Are engineered joists better than solid timber?

Engineered joists, such as I-joists, can span further with less timber and stay straighter. Solid timber is simple, easy to adapt on site and familiar to most builders. Engineered joists shouldn’t be cut or notched on site without the manufacturer’s guidance. The right choice depends on the span, access and how the floor will be used.

Should I add a steel beam under a bouncy floor?

Sometimes. A beam under the floor shortens the joist span and can stiffen it, but it needs its own supports, such as walls or posts with something solid beneath them. The engineer checks this before recommending it. Alternatives such as doubling joists or adding strutting don’t need new supports.

Available across the UK

Professionals cover the whole of the UK: England, Scotland, Wales and Northern Ireland. Planning and Building Regulations differ between the four nations, and your professional will work to the rules that apply where your property is.