Floor strengthening design
Design to strengthen an existing floor that is sagging, bouncy, damaged or needs to carry more load.
About this service
A sagging, bouncy or damaged floor is one of the most common things we are asked to look at in London houses and flats. Some floors were undersized from the start. Many more have been weakened over the decades by plumbers and electricians notching the tops of joists, by a removed wall that once gave support at midspan, or by rot where joist ends sit in damp external walls. Our floor strengthening design sets out exactly how to put that right.
We also design strengthening where a sound floor needs to carry more than it was built for. A cast iron or stone bath, a kitchen island with a stone worktop, a wall of full bookshelves or a new partition sitting on the floor can all exceed what a typical joist was sized for. If you only need to know whether a floor can take a particular load, a floor loading assessment may be enough on its own.
Typical solutions include sistering new timbers alongside the old, bolting in steel flitch plates, adding joists between existing ones, fitting a steel beam to halve the span, and adding strutting to stiffen a lively floor. Calculations and details go through our three-step quality control process, and our Head of Engineering signs them off before issue. Fees start from £450 + VAT, with a fixed fee confirmed in writing first.
When you need it
- A floor is sagging or springy
- You're adding heavy items such as a bath, kitchen island or bookshelves
- Joists have been notched or damaged
What's included
- Assessment of the existing floor
- Strengthening design such as sistering, steel or new joists
- Calculations and details
How it works
- Find the causeWe inspect the floor and the structure below, lifting boards or opening ceilings where needed. Knowing why the floor has moved, whether undersizing, cutting, rot or lost support, decides the right repair.
- Choose a methodWe pick a solution that suits access and finishes. Working from above with boards up is often easiest, but in a flat with a finished ceiling below the method may need to work entirely from the top.
- Design and detailWe size the new timbers, steel plates or beams with their fixings, bearings and connections. Drawings show where each piece goes so the carpenter can price and build without interpretation.
- Check, sign off and submitCalculations and drawings go through our quality control checks before issue. Where Building Control needs to approve the work, we send them the package and deal with their questions.
What you receive
Why notched and drilled joists lose so much strength
Joists carry load mainly through their top and bottom edges. A deep notch in the top edge near midspan, or a large hole close to either edge, removes material exactly where stresses are highest. Building Regulations guidance limits the size and position of notches and holes for this reason, and older services installations often ignore those limits. A single badly cut joist can make a whole floor feel soft, especially in bathrooms where pipes run in several directions.
Once a joist is damaged, packing the notch rarely restores its strength. A new timber sistered alongside, fixed with bolts or structural screws, takes over the load instead. If the floor also supports a wall above, or lost support when a wall below was taken out, the problem may be bigger than the floor itself. Our guide on removing walls explains how floors and walls often depend on each other.
Projects that used this service
Frequently asked questions
How do you fix a sagging floor?
First establish why it sags. If joists are undersized or cut, the usual fix is sistering new timbers alongside them or adding steel flitch plates. If the span is too long, a new beam or wall can shorten it. Some sags in Victorian houses are historic and stable, in which case levelling the surface may be enough. An engineer can tell the difference on site.
Can a bouncy floor be made stiffer?
Yes. Bounce is usually a stiffness problem rather than a strength one. Solid blocking or herringbone strutting between joists makes them share load, and sistering deeper timbers or adding a steel at midspan reduces deflection. Gluing and screwing a structural board deck to the joists also helps.
Do I need Building Control approval to strengthen a floor?
A like-for-like repair may not need an application, but strengthening that changes how the structure carries load, or forms part of wider alterations, usually does. Building Control will confirm for your project. Where approval is needed, we submit our calculations and drawings and liaise with them. Our Building Control guide explains the routes available.
Will my floor take the weight of a cast iron bath?
A full cast iron or stone bath with someone in it can weigh well over half a tonne, concentrated on a small area. Many older floors carry this without trouble if the bath sits near a supporting wall and spans several joists. Where joists are cut for waste pipes or the bath sits at midspan, strengthening is often needed, and we check this as part of the design.