Sistering floor joists
A common problem in older buildings is that floors sag or feel ‘springy’ (that is they 'bounce' when walked on). This can be because the joists are undersized (particularly where older buildings are converted for modern uses where they may be subject to higher loads than was originally intended), or they have been weakened by decay or by having holes or notches cut through them incorrectly. The structural repair of damaged floor joists is known as ‘sistering’.
Floor joists can become damaged due to contact with water which may lead to rot setting in or infestation from termites, woodworm, and so on. These underlying problems should be rectified first before sistering, or the damage will recur.
The typical choice for sistering material is framing timer or engineered timber products such as plywood.
The first step to take is to remove any obstructions, such as electrical cables, pipes, and so on. If this is too difficult the new joists can be notched, although this may mean that are not as strong. The new joists are set alongside the existing ones. Joist material should be chosen that matches the height of what is already installed, as well as being, as closely as possible, the same span.
If necessary, a hydraulic jack and post can be used to jack up the joists until they are level. It is sometimes recommended that jacks are only raised by around 1/8 inch per day to avoid cracking in the walls and floors.
Adhesive should be applied along the length of a joist to strengthen the bond between the existing and its sister, as well as to prevent squeaking. Quick clamps are used to hold the sistered joist in place, and nails driven through the sister and into the existing joist. Shifting and settling can work to loosen and separate the boards over time, therefore, the nails should be used with lag or carriage bolts to lock them together.
The quick clamps can then be released. This process should then be repeated for each joist that needs strengthening or straightening.
[edit] Related articles on Designing Buildings Wiki
Featured articles and news
The UK's Modern Industrial Strategy: A 10 year plan
Previous consultation criticism, current key elements and general support with some persisting reservations.
Building Safety Regulator reforms
New roles, new staff and a new fast track service pave the way for a single construction regulator.
Architectural Technologist CPDs and Communications
CIAT CPD… and how you can do it!
Cooling centres and cool spaces
Managing extreme heat in cities by directing the public to places for heat stress relief and water sources.
Winter gardens: A brief history and warm variations
Extending the season with glass in different forms and terms.
Restoring Great Yarmouth's Winter Gardens
Transforming one of the least sustainable constructions imaginable.
Construction Skills Mission Board launch sector drive
Newly formed government and industry collaboration set strategy for recruiting an additional 100,000 construction workers a year.
New Architects Code comes into effect in September 2025
ARB Architects Code of Conduct and Practice available with ongoing consultation regarding guidance.
Welsh Skills Body (Medr) launches ambitious plan
The new skills body brings together funding and regulation of tertiary education and research for the devolved nation.
Paul Gandy FCIOB announced as next CIOB President
Former Tilbury Douglas CEO takes helm.
UK Infrastructure: A 10 Year Strategy. In brief with reactions
With the National Infrastructure and Service Transformation Authority (NISTA).
Ebenezer Howard: inventor of the garden city. Book review.
The Grenfell Tower fire, eight years on
A time to pause and reflect as Dubai tower block fire reported just before anniversary.
Airtightness Topic Guide BSRIA TG 27/2025
Explaining the basics of airtightness, what it is, why it's important, when it's required and how it's carried out.
Construction contract awards hit lowest point of 2025
Plummeting for second consecutive month, intensifying concerns for housing and infrastructure goals.
Understanding Mental Health in the Built Environment 2025
Examining the state of mental health in construction, shedding light on levels of stress, anxiety and depression.