If you’re planning to convert your loft and use the space as habitable rooms then simply boarding out your loft with loft boards simple won’t do. Laying boards on your existing ceiling joists may be fine if you want to create some storage room up there but is wholly insufficient if you want to use the extra room as living space.
Doing it correctly is the only way to go, and while you can do some of it yourself, you will need to have some plans drawn which will take into account your existing layout and how your roof has been constructed.
Photos shows Steel RSJ with timber bolted to it to support the new floor
You simply can’t lay a new floor onto your existing ceiling. The correct way to do it is by having steel RSJ lintels installed at the front and rear of your house to support the new floor with another placed underneath the ridge to support the roof before the original timber supports are removed inside the loft area.
The most common sized RSJ used is 203 x 203 x 46kg UC (universal column) which is typically priced by the foot / metre. Thanks to T Good and Sons
Important: Size, placement and quantity of supporting steels is dependent on the size of your loft, always seek professional advice for your specific project