Carpentry & Joinery

Is replacing rotten joist with new thinner joist ok?

Anonymous user 23/02/2024 - 2.52 PM

Hi My house (built 1930's) has a concrete floor in a utility room placed directly on to floorboards by the previous occupant.(Genius!) Long story short, that and some very questionable plumbing caused the joist and floorboards to become mush. I took a sledge hammer to it and and removed the concrete floor and (which was supported by nothing as the joist came away in my hand) and rotten flooring and am now looking to replace the joist and floorboards. Luckily the damage has been restricted to this area and not spilled in to any other room (theres a 5 foot high celler, if you can call it that I've been able to have a walk around to check) Now here's my issue, the three joist I removed were 70x130x3200mm spaced 420mm apart (centre to centre) and have a wall at about the half way mark along the length to support them. For the life of me I cant find 70mm thick joist but the building regs seem to say that the 45mm timber would be strong enough to hold it. I'm just hesitant to use the thinner timber, am I being over cautious here? Any advice would be welcome. Thanks