Parts of a Floor – Joists, Subfloor, Girder and Sill Plate

All 12 parts on the diagram

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Start here: 7 main parts 7

Joist One of a series of parallel beams that carry the floor.
Subfloor Boards or panels nailed over the joists, under the other layers.
Finish floor The final wearing surface of a floor.
Girder A large beam that carries the inner ends of the joists.
Post A wood or steel upright in the basement that supports the girder.
Sill plate A plate leveled on the foundation wall and anchored to it.
Trimmer joist A joist beside an opening; the header is framed into it.

Cut away in layers 3

Header joist (rim joist) Joist ends butt against it, and are nailed to it.
Bridging Small members set between joists to brace them and spread loads.
Underlayment A thin panel layer between the subfloor and the floor covering.

An opening in the floor 2

Header A beam set square to the joists at an opening.
Tail joist A short joist that runs from a header to a support.

How this list was compiled: Read 2026-09-30; a name is kept only where two of these use it. (1) US National Bureau of Standards, Building Materials and Structures Report BMS91, A Glossary of Housing Terms, https://archive.org/details/glossaryofhousin91subc : joist (one of a series of parallel beams used to support floor and ceiling loads), girder (a large or principal beam), bridging (small wood or metal members inserted in a diagonal position between the floor joists, bracing the joists and spreading the effect of loads), header (a beam placed perpendicular to joists and into which joists are framed in framing for a chimney, stairway or other opening), trimmer (a beam or joist into which a header is framed). (2) L. O. Anderson and O. C. Heyer, Wood-Frame House Construction, US Department of Agriculture, Agriculture Handbook No. 73 (1955), https://archive.org/details/WoodFrameHouseConstruction : its chapter Floor Framing (wood or steel posts in the basement support girders, which support the inner ends of the first-floor joists; built-up girders; the sill plate leveled on the foundation wall and anchored to it; the header joist nailed to each joist; cross-bridging at midspan to stiffen the joists and help transfer loads; subflooring; trimmers, headers and tail joists at floor openings) and its chapter Floor Coverings (finish flooring is the final wearing surface). (3) US Naval Education and Training Command, Builder 3 & 2 (1981), https://archive.org/details/ERIC_ED210471 : sill, girder, header joist flush with the sill's outer edge, cross bridging and solid bridging, trimmer joist and header on its floor framing plan, subflooring, floor underlayment, finish flooring. (4) Canada Mortgage and Housing Corporation, Canadian Wood-Frame House Construction, https://archive.org/details/CANADIANWOODFRAMEHOUSECONSTRUCTION : sill plates, floor joists, rim joists and joist headers, cross bridging, subfloor, underlay, finish flooring, trimmer joists, headers and tail joists. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work

How it works

A framed floor is a row of joists with layers of board on top. These are the names a carpenter or an inspector uses for each piece, from the floor you walk on down to the post in the basement.

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From the top down

The finish floor is the wearing surface. Under it a thin underlayment may smooth the way for it, and the subfloor is nailed to the joists. Bridging braces the joists between supports. Their outer ends butt against the header joist, which sits on the sill plate on the foundation wall. Their inner ends rest on a girder, held up by posts.

Round an opening

Where a stair or a chimney passes through, the joists are cut short. A header crosses each end of the opening and carries the tail joists. The headers are framed into a trimmer joist at each side.

Headers and trimmers are usually doubled, because they carry the load of the joists that were cut.

Questions and answers

What are the parts of a floor called?

From the top: finish floor, underlayment, subfloor and joists. The joists are braced by bridging, end at a header joist on the sill plate, and rest on a girder held up by posts. Round an opening there are trimmer joists, headers and tail joists.

What is the difference between a subfloor and an underlayment?

The subfloor is the structural layer of boards or panels nailed to the joists. An underlayment is a thin panel laid over it, under a floor covering such as vinyl or tile that needs a smooth base.

What holds a floor up?

The joists. Their outer ends sit on the sill plate on top of the foundation wall. In most houses the span is too long for one joist, so the inner ends rest on a girder, and posts in the basement carry the girder.

What is bridging between floor joists?

Short pieces fixed between neighboring joists, usually at midspan. Cross bridging is a pair of diagonal struts that cross; solid bridging is a block of joist stock. Both stiffen the joists and help spread a load to the joists alongside.