One of the most common claims are squeaky timber floors. Deflection in suspended floors is often caused by inadequate joists or joists at incorrect centres. In this article, we will take a look at what engineered joists are, their disadvantages and problems as well as what you should look out for on-site and alternative solutions.
What are ‘Engineered Joists’ and why use them?
Composite joists are made up of top and bottom flanges held apart by an infill web.
Why use composite joints? They’re Low noise – originally introduced as ‘silent joists’, due to their low shrinkage when compared to traditional joists.
What are the disadvantages?
- Very flimsy as individual components
- Only work as a composite panel with deck and ceiling in place
- Work at their maximum capacity
- Ready made for specific plots and therefore not adaptable on site
- All noggins/dwangs, deck, fixings, hangers are needed to provide the designed performance
- Design information
- Each floor should be a bespoke design and come with a fully detailed layout drawing
- There are often different ‘performance types’ that each supplier will sell
- Each floor pack comes with every joist, noggin, hanger and fixing. There should be nothing left over at the end!
In basic terms the joist sections are tall and thin and do not stand up. By adding decking, the joists are still not restrained. Restraint is provided by restraint hangers, solid blockings, building in to walls, etc. The basic rule is that there should be no free ends of joists, that have no restraint.
So what can go wrong?
Missing elements, poor fixings, or services!
To be more precise, tradesmen who do not understand that Engineered Joists are designed to tight limits, where nothing can be omitted or altered, put safety at risk.
The Technical Manual states ‘under no circumstances should the flanges of the I-Joist be cut, notched or drilled’.
Different manufacturers have different methods for backer block fixing positions, and particular care must be taken when trimming, and in particular trimmer joists where doubled, which may require backer blocks between joists and these, on occasion, have to be site assembled which can lead to omissions.
The centres of the joists are critical, as per traditional joists, and we have seen claims due to creep of centres leading to excessive deflection and therefore noise.
The deck is an integral part of the floor cassette. The Design, Material and Workmanship sections of the Functional Requirements in our own Technical Manual have specific guidance in 8.2, that says ‘particle boards should be either screwed or nailed to the joists at 250mm centres’.
In addition, reference should be made to BS8103 part 3 (timber floors for housing), that requires mechanical fixings and glued joints between boards.
The manual does not actually require decks to be glued down but in practice this seems to always be done as part of gluing the board joints.
What creates the problems?
- Loose deck – not fixed securely to joists or not fixed together at joints
- Deck flexing onto fixings
- Deck flexing due to missing supports
- Straining of deck due to no edge gap for expansion
What to look for on-site;
1. Exposed nails below the deck (ends poking through the deck)
2. Gaps between joists and deck
3. Lack of edge gap between floor deck and solid walls
4. Unsupported edges (missing noggins)
5. Lack of fixings (unless using a BBA certificated system such as described below)
We are currently aware of other floor deck systems that have third party product approval certificates to show compliance with the Functional Requirements that differ from clause 8.2.
If such deck boards are used and they have no fixings visible, then the system can only be accepted if it has been used in compliance with a valid third party product approval that is acceptable to our Warranty.
- Decks must be fixed at 250mm centres (and glued to joists as a preference), and glued together at joints
- Decks with no visible mechanical fixings must have a valid third party product approval accepted by our Warranty
- Floor structures must be checked to ensure that all designed components have been fitted
- All fixings and backer blocks between components must be in place
- Check joist centres