The good news is that most sagging floors are a stiffness problem rather than a collapse risk. The work is to find out which of the three usual causes you have before anyone lifts more boards than they need to.
The three causes we find most often
Notching and drilling is the most common. A 45mm notch cut into the top of a joist to take a waste pipe can remove a large part of its strength exactly where it is needed. Several services running through the same bay compounds it.
Undersized or over-spanned joists come next, usually where a room was subdivided or a wall taken out and the joists lost their intermediate support. Rot and beetle damage is third, and is usually local to a wall plate, a bearing end, or timber near an old leak.
- Notching and drilling in the wrong part of the span
- Joists spanning further than their depth allows
- Decayed bearing ends where the joist sits into damp masonry
What a proper repair involves
Sistering — bolting a new joist alongside the old one over its full span — restores stiffness without disturbing ceilings below where access allows. Where bearing ends have gone, we cut back to sound timber and splice in new with a resin or steel connection onto a new bearing.
Where a wall has been removed and the span is genuinely too long, the answer is usually a new steel or timber beam picking up the joists mid-span, with a structural engineer's calculation. Levelling screed or firrings on top only ever hides a movement problem — we don't recommend it as a standalone fix.
What it costs to put right
Localised work — a handful of sistered joists in one room with the boards lifted and relaid — is a short job of a few days. A full floor rebuild with new joists, insulation and boarding is a bigger scope and usually gets bundled into a wider renovation while ceilings are already coming down.
We'll always tell you if the honest answer is that the floor is safe and simply not perfectly flat, which is normal in a building of a certain age.
Common questions
Is a bouncy floor dangerous?
Usually not. Bounce is a stiffness issue rather than a strength issue. It becomes a concern when there is visible decay at the joist ends, cracking in the ceiling below, or a noticeable step in floor level, all of which should be looked at.
Can you fix a sagging floor without taking the ceiling down?
Often yes, by lifting floorboards from above and sistering new joists alongside the old. Taking the ceiling down gives better access and is the sensible route if the ceiling is lath and plaster and already failing.
Do I need a structural engineer?
For sistering and end repairs, generally no. If the fix involves a new beam, removing support, or altering how loads reach the walls, then yes — and we arrange that as part of the job.
Will levelling compound solve it?
It will flatten the surface but does nothing for the timber. If the cause is decay or an over-long span, the dip will return and you will have added weight on top.