Carpentry & Joinery

Flat roof joist direction

Anonymous user 28/02/2024 - 4.01 PM

Hi, Can flat roof ceiling joists run 90º degrees to the slope of the roof or must they be parallel to the slope? For context, I am building a garden room that is 3m x 4m with french doors (2800 wide) on the shorter 3m side. The flat roof will slope back across the 4m length. The building height must be below 2.5m to meet planning rules. I would like to maximise the headroom by using shallower ceiling joists. Looking at span tables, if I run them down the 4m span it would require 38 × 170mm timbers. If I run the joists across the 3m span it would allow 39x145mm timbers, a saving of 30mm headroom. This will make a big difference as I am tall person. Does joist direction matter here? Thanks