Lesson 2 of 6 · Chapter 2
The Floor System
The floor system is the platform everything else stands on: the treated sill bolted to the foundation, the joists that span across it, and the subfloor glued and nailed on top. This lesson teaches each layer, and the numbers a framer checks on each one.
Step one
Read the material
Six ideas: the sill, anchoring it, joist spacing, crowning and blocking the joists, subfloor adhesive and panels, and the nailing pattern.
Idea one
Where the house meets the concrete
The first piece of wood on the foundation is the sill plate, also called the mud sill. The two names mean the same pressure-treated board, laid flat on top of the foundation wall and bolted to it. It is the base for everything above and the barrier between the frame and the moisture rising from concrete.
Pressure-treated lumber here is often a building code requirement, not just a recommendation, because of its resistance to rot and insects.
Prepare the foundation first: the top should be level and free of debris or irregularities. Snap a chalk line to mark where the sill sits, aligned with the walls above.
Where the water table is high or moisture is frequent, put a moisture barrier between the foundation and the sill. Any misalignment here shows up later as shifting walls and uneven floors.
Idea two
Bolts, spacing, and the ends
The sill is held down with anchor bolts, set in the concrete while it is wet or drilled and installed into cured concrete. They come up through holes in the sill and are tightened with nuts and washers.
Spacing comes from local code, often no more than 6 feet apart and within 12 inches of the ends of each piece of sill. The sill carries lateral and vertical forces from the whole frame above, so the ends are never left loose.
Count it the way an inspector would: take off 12 inches at each end, divide what is left by 6 feet, round up, and add one. A 20-foot sill: 240 minus 24 is 216 inches, 216 ÷ 72 is 3, so 4 bolts.
Where earthquakes are a concern, codes may add hold-down anchors or straps to resist movement. Check your local requirements.
Idea three
16 or 24 inches on center
Joists are the horizontal members that span between walls or supports and carry the subfloor. They carry the flooring, furniture, appliances and people above.
Lumber joists, SPF or Hem-Fir, are typically spaced 16 or 24 inches on center. Engineered products such as I-joists and LVL can often span farther.
16 inches is the common residential choice: less deflection and movement, better under tile and hardwood, and easier for insulation, plumbing and wiring. 24 inches saves material for lighter loads, but wider spacing means more deflection and a bouncier floor.
Counting joists: divide the floor length in inches by the spacing, round up, and add one for the starting joist. A 16-foot floor at 16 inches: 192 ÷ 16 = 12, plus 1 is 13 joists.
Idea four
Crown up, and brace between
Just like studs, joists have a crown. Install every joist with the crown facing up. The load settles the crown toward flat, and the floor stays level instead of sagging.
Blocking or bridging between joists gives lateral support and keeps them from twisting, which is a source of squeaks. It matters most on longer spans, where deflection is higher.
Mark the joist positions on the sill with a chalk line or laser, and double-check the spacing all the way along before anything is fastened.
In damp areas, pressure-treated joists and a ventilated crawl space or basement keep the floor framing dry.
Idea five
Glue, then lay the panels staggered
Subfloor adhesive bonds the panels to the joists, spreads loads and stops the small movements that make floors squeak. Use a product made for wood subfloors, which stays flexible as wood expands and contracts.
Clean the joist tops and panel edges, then run a continuous bead along each joist, and along the edges where panels meet.
Lay the panels, plywood or OSB, with their joints staggered, so the ends of neighbouring panels never line up. Lined-up joints make a weak line across the floor.
Keep the crawl space or basement dry beneath the floor; moisture from below leads to mold, mildew and rot in the framing.
Idea six
6 to 8 inches on the edges, 12 in the field
The fasteners hold the panels tight to the joists. A common guideline: every 6 to 8 inches along the panel edges and every 12 inches in the field, the middle of the panel.
Ring-shank nails are an excellent choice for their holding power where there is movement and vibration. Screws add strength and come out for adjustments; pre-drill in dense material.
Drive every fastener straight and flush. Overdriven, it weakens the connection and leaves a dimple; underdriven, it sticks up and damages the finished floor.
Then walk the floor. Anywhere that flexes or gives underfoot, add fasteners before the finished flooring goes down.
The whole of chapter 2 is free to read or hear on the Residential Framing reader page.
Step two
Work it with your hands on it
The Sill and Joist Bench. Set the length of a sill and the joist spacing, then choose how many anchor bolts to use. The bench places the bolts 12 inches in from each end and spreads the rest evenly, and tells you whether any gap is over 6 feet. It also counts the joists that sit on that sill.
Sill length:
Joist spacing
Anchor bolts
Hands on it, off the screen. Find an unfinished basement, crawl space or garage and look at the sill: count the anchor bolts and measure the gaps between them and to the ends.
Look up at the joists. Measure their spacing, look for blocking or bridging, and see which way the subfloor panels run.
Step three
Prove you understood it
One question per idea, a new situation every time. Miss one and you are sent back to the idea it was testing.
All six solid.
You can set a sill, anchor it and count the bolts, choose and count joists, crown and block them, glue and stagger a subfloor, and nail it off. Next: wall framing.
Residential Framing
Lesson two of six, from chapter two. Written by our founder, Dr. Gene A Constant, and donated to the Foundation. Published by Global Sovereign University Press.
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