Floor Joists: All You Need to Know
Since we would like to begin at the beginning, allow us first to tell you what a joist is. A term related to architecture and engineering, a joist is a supporting component that runs from wall to wall, wall to beam or beam to beam, to support a ceiling, roof, or floor and may be made of wood, steel or concrete. It is usually smaller than a beam and will often be supported by one. In the following paragraphs, we intend to tell you all you need to know about floor joists.
Floor Joist Layout
Floor joists are most commonly installed parallel to each other for ease of construction and distribute weight evenly to ensure structural integrity. In some framing designs the floor joist layout may be parallel to the house.
KEEP IN MIND
The layout term for floor joists is “On Center” which is the center measurement from one joist to the next. Or the right side of one joist to the right side of the next, which is easier for layout in the building process using a tape measure
Floor joist sizes
Obviously, the size of your joist will depend on your project and your spacing on center. However, we will give you an idea of the most common floor joist sizes. Typically, lumber is cut and distributed in length divisions of 24” (8, 10, 12, 16). This makes for an easy flooring layout if your floor joists are 16” on center, which they usually are.
Installing floor joists
When installing your floor joists you should carefully read the board for defects. So here’s a checklist:
- Look for a crown. This is a slight bending of the board in one direction. The crown of a floor joist should always be upwards
- Another defect is a large knot on one side of a floor joist. If you continue with a joist bearing this defect, make sure that it is at the topside after installation, since the topside of a joist is always under compression and the bottom is always under tension. So a knot on the bottom of a joist will cause it to fail
- If your floor joist has a twist in it after attaching it to the ledger, you should straighten it before nailing on the header because you want your joists as perpendicular as possible
- Watch out for different width boards. Floor joist tops should always be installed flush with the ledger and header. With a girder beam, some joists will sit on the beam and some will not. To support the gap, you can use a pressure-treated shim under the joist to transfer the load to the beam.
- But above all, and before you start, remember that contrary to popular belief, one does not begin floor joist layouts from front to rear, marking 16” on center. Rather, start at the plumbing wall(s). Estimate where the toilet(s) will be and place your first two floor joists in such a way as to leave a 13” clear space for the toilet drain. After marking these two critical floor joists, proceed 16” on center in each direction till you come to the end
Moisture content and floor joist shrinkage
The moisture content of floor joists impacts noise and movement in floors. Joists that start out with a moisture content that is higher than that which they will reach in service will dry out and shrink. So remember:
- Intermediate floor joists are likely to attain a moisture content of 10-12% while those at ground floor level are likely to have about 14%. Naturally, therefore, moisture content at the time of installation should be similar to these values. Since this is not always possible, an upper moisture content limit of 20% is your best bet. Even so, be prepared for some shrinkage
- Joists installed at 30% moisture content may dry down to 10% in service. On average, a floor joist will shrink by 8-10 mm but individual members may even shrink by 15 mm. Even allowing for oversize floor joists to make up for the excess moisture, they will be undersized for the given span and deflection of the floor, which may develop ‘bounce’
- Shrinkage across the joist width will loosen the solid strutting, which results in noise through movement on nails. In fact, this is one of the commonest causes of noise in timber floors
- A floor joist drying down from 30% moisture content may warp or twist and this could spring the floor deck or ceiling, once again causing noise from movement on nails. Using “dry” timber is a solution, in that warps or twists will be evident before installation and damaged pieces can be rejected. However, even with relatively dry timber, different moisture contents in different joists can result in differential shrinkage
- If you use trimming joists, chances are their source is different from that of the other floor joists. So make sure their moisture content is similar to the other joists. As you can imagine, shrinkage in trimming joists may loosen their fixings and they will then cease to act together
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