Loft Conversion Floor Joists - what are the requirements

This article also covers Loft Conversion Joists, Loft Conversion Joist Size, Loft Conversion Joist Regulations and Loft Conversion Joist Thickness.

All loft conversions that have to comply with Building regulations will require new floor joists to be fitted. Using the existing ceiling joists will not be acceptable in most cases as they will not be sized or man enough to take the additional floor and roof loadings. Most new floor joists for a loft conversion are placed alongside the existing ceiling joists and screwed together.

The sizes of the new floor joists is normally dependent upon the clear structural span of the joists and their spacing or centres between each new floor joist. Other factors for sizing of the loft conversion floor joists is whether or not the new floor joists will be taking any roof loads. As a rule, most new floor joists for loft conversions will be taking roof loads as the existing purlins and struts will be removed and replaced with perimeter stud partition walling that sit upon the new floor joists.

In most cases, the sizing of the floor joists for a loft conversion will need to be assessed and proven by structural calculations where all the loads and stresses within the timbers can be calculated and this is usually completed by an experienced and qualified structural engineer.

Most floor joists within a loft conversion also require strut noggin bracing at around 2M max. centres. These are timber infill sections spanning between each new joists and help distribute or share the floor loads over several floor joists rather than just the one. The floor bracing also helps to stabilise and fix the joists from moving or twisting when they start to dry out.

As a rule of thumb, most new floor joists for a li9ft conversion will be 75 x 200mm at 400mm centres for a span of about 3.7M and taking some roof loads. However, you are advised to have your own proving calculations completed by your own loft conversion specialist designer, engineer or company.