Question 1

What is a trimmer joist?

Accepted Answer

A trimmer joist is a doubled joist (two joists nailed together) placed on each side of an opening in a timber floor - typically for a staircase well, loft hatch, or floor-to-floor opening. The trimmer joists run parallel to the main floor joists. The trimmed joists are the shortened joists whose ends are cut off to create the opening; they cannot span to the wall as they are interrupted by the opening. Instead, their cut ends are supported by a trimming joist (also called a header joist) which runs perpendicular to the main span and connects between the two trimmer joists. The trimmer is doubled because it carries the extra load from the trimming joist and the trimmed joists hanging from it.

Question 2

What is the difference between a trimmer and a trimming joist?

Accepted Answer

The terminology is specific: the trimmer joists are the doubled joists running parallel to the main span on each side of the opening. The trimming joists (also called headers in American usage) run perpendicular to the main span, connecting between the two trimmers at the ends of the opening - they carry the ends of the trimmed joists. The trimmed joists are the shortened members whose ends are supported by the trimming joist. So around a staircase opening: two trimmer joists run the length of the opening; two trimming joists cross between them at the top and bottom of the opening; and the trimmed joists hang from the trimming joists on joist hangers (or formerly, tusk tenon joints) or are skew-nailed.

Question 3

Do trimmers need to be structurally designed?

Accepted Answer

For small openings (loft hatches up to approximately 900mm wide) in domestic floors, the doubled trimmer joist and trimming joist are typically sized to match the floor joists - this is accepted as adequate by Approved Document A for small openings. For larger openings (staircase wells, attic conversions, large floor penetrations), the trimmer and trimming joist should be structurally calculated. The trimmer must carry the tributary load from the trimming joist and the trimmed joists; if this load is significant, the trimmer may need to be a deeper section or an LVL (laminated veneer lumber) member rather than simply doubled solid timber. Engineered timber calculations are typically required for large or heavily loaded openings.

Question 4

What are trimmer rafters used for?

Accepted Answer

Trimmer rafters serve the same function in a roof as trimmer joists in a floor - they are doubled rafters placed on each side of an opening in the roof slope, typically for a dormer window, a roof light (Velux), or a chimney penetration. The trimmer rafters support a trimming rafter (header) that runs across the opening, and the trimmed rafters (the shortened rafters whose line is interrupted by the opening) hang from the trimming rafter. For a roof light opening, the trimmers and trimming rafter are usually the same size as the existing rafters if the opening is small. For a dormer, structural design is required as the loads are significantly higher.

Trimmer

Just so you know...