Sloping and sagging floors: what causes them
An old house with a level floor is the exception. The question is not whether a floor slopes but whether it is still moving, and whether the slope is telling you something about the structure underneath.
Is a sloping floor in an old house a problem?
Often not. Many older floors have sloped for decades and are stable. The concern is a slope that is increasing, or one accompanied by new cracking, binding doors or gaps opening at the skirting.
Old slope or active movement
A floor that has sloped gently for a century and carries no cracking around it is usually a settled condition. Timber creeps under sustained load, joists take a permanent set, and the building found its position long ago. Levelling it is a finishes decision rather than a structural one.
A slope that has appeared or worsened recently is different, and the giveaways are usually elsewhere in the room. Fresh cracking where the wall meets the ceiling, doors that have started to bind at one corner, gaps opening between the skirting and the floor, all suggest the movement is current.
A spirit level and a note of the fall across the room, dated, gives you the same evidence base as with cracks. Movement that has stopped and movement that is continuing need different responses, and the difference is measurable rather than a matter of opinion.
What is usually underneath it
Undersized or overspanned joists are the most common cause in older houses, where floors were built for lighter use than they now get. Deflection increases with the fourth power of span, so a joist that spans a little too far sags a great deal more than the extra distance suggests. Adding a bathroom, a stone floor or a wall of books to a room designed for none of them has the same effect.
Bearing failure at the ends is the cause people miss. A joist built into a solid wall in a damp corner can rot where it sits, and the floor drops locally at the perimeter rather than in the middle. The slope points to the wall rather than to the centre of the room, which is the diagnostic clue.
Notching and drilling by other trades does real damage. A joist notched deeply on its underside near a support, usually for a pipe or a cable run, loses a large part of its capacity. This is one of the most common causes of a floor that started to bounce or drop shortly after a bathroom or heating installation.
What can be done about it
Where the joists are sound but overspanned, the usual answers are to reduce the span with a new beam or intermediate support, to sister new timbers alongside the existing ones, or to replace with deeper sections. Which one is appropriate depends on what is above and below, on whether ceiling finishes can be disturbed, and on the calculations for the replacement members.
Where the ends have decayed, the repair is local. The affected timber is cut back to sound material and supported on new bearings, often with a hanger onto a new plate rather than rebuilding into the damp wall that caused the problem in the first place. Fixing the source of the damp matters more than the timber repair.
Levelling the finish without addressing the cause is a legitimate choice when the movement has stopped and the structure is adequate. It is not a legitimate choice when the floor is still moving. Establishing which situation you are in is the point of the assessment.
What we check.
The points our calculations resolve for a project like this.
- Whether the slope is historic and stable or has changed recently
- Joist size, span and spacing against the loading the room now carries
- Condition of joist ends where they bear into walls, particularly in damp corners
- Notches and drilled holes from later plumbing or electrical work
- Whether the ceiling below can be disturbed, which governs the practical repair options
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
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Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
Is a sloping floor in an old house a problem?
Often not. Many older floors have sloped for decades and are stable. The concern is a slope that is increasing, or one accompanied by new cracking, binding doors or gaps opening at the skirting.
How much slope is acceptable?
There is no single figure that applies to every building. What matters more is whether the floor is still moving and whether the members carrying it have adequate capacity for the loads they now take. A stable slope and an active one are different problems.
Can I just level the floor with self-levelling compound?
Only where the movement has stopped and the structure beneath is adequate. Levelling compound adds weight and hides evidence, so on a floor that is still deflecting it makes the assessment harder and the problem no better.
Why did my floor start bouncing after a bathroom was fitted?
Commonly because joists were notched or drilled to run pipes, or because the new floor finish is much heavier than what it replaced. Both reduce the margin the floor was working to, and both are assessable.
Related in structural problems & symptoms.
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