How to get it right: floor joists

The next generation of engineered first floor joists offers greater spans and easier fitting of services. While hours can be saved removing the need for notching of pipework and drilling for cables, the floor joists still need careful attention to ensure they work correctly and as they are designed.

The posi-joists in this image are designed so that the top chord transmits all the weight. That is why there’s a gap where the bottom chords to the left are not in contact with the trimmer beam.

The problem with this floor joist image

The gap is too big! It should be a maximum of 6mm and if a stud wall is to be built beneath, there should also be a plasterboard noggin to prevent cracking at the junction. There should also be noggins between the top chords to prevent movement.

The joists to the right of the trimmer correctly have a timber fillet at the point at which they sit on the wall to stiffen the joist through its depth. But they should really be seated on a timber batten rather than the blockwork.

The cantilevered ends should be fixed to the trimmer beam with joist hangers to remove any bounce in the floor to prevent any cracking of the first floor finishes.

This will also minimise creaking, which is one of the biggest frustrations for new house owners.

Further information

Found this interesting? Try How to build level floors and browse our 'How to get it right' articles.
 

Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.