Floor Framing

The floor is the first thing a framing crew builds and the platform for everything after it. If the floor is out of level or out of square, every wall inherits the error. A well-built floor is flat, stiff and quiet, and it stays that way for the life of the house.

This guide covers platform framing, where each floor is built and decked before the walls go on it. Numbers are from the International Residential Code (IRC). Your plans and the code your town has adopted control the work. Joist size and span come from the plan or from span tables, not from your eye.

What You Will Be Able to Do

• Install a sill plate on a foundation with the right anchors and barriers.
• Lay out, crown, set and fasten floor joists and rim boards.
• Frame an opening in a floor.
• Say where a joist can be drilled or notched, and how big.
• Lay and fasten a subfloor that will not squeak.

Parts of a Floor

Setting the Sill Plate

The sill plate sits right on the concrete. Concrete wicks moisture, so the plate must be pressure-treated or a naturally durable species. Pressure-treated lumber is by far the most common. A foam sill seal usually goes under it to block air and moisture.

1. Snap a line on the foundation for the inside edge of the plate. Check the foundation for square and level first. If it is off, find out now.
2. Mark the anchor bolt holes by setting the plate on the bolts and marking each one, or by measuring from the end. Drill holes slightly larger than the bolts.
3. Lay the sill seal, set the plate over the bolts, and add washers and nuts.
4. Tighten the nuts snug without crushing the plate.

Under the IRC, a typical wall has 1/2-inch anchor bolts no more than 6 feet apart. Each plate section needs at least two, with one within 12 inches of each end. Bolts must be embedded at least 7 inches in the concrete. Seismic and high-wind areas call for more, including larger square plate washers.

Use only hot-dip galvanized or stainless fasteners against treated wood.

Bearing

Bearing is the length of the joist end that rests on its support. The IRC requires at least:

• 1-1/2 inches on wood or metal.
• 3 inches on masonry or concrete.

Joists that lap over a beam must overlap at least 3 inches and be nailed together. Joists framing into the side of a beam need joist hangers or a ledger.

Setting Joists

Floor joists are usually at 16 inches on center. Some systems use 12, 19.2 or 24 inches. Follow the plan.

1. Lay out the rim or the sill with the same on-center marks used for walls above, so loads line up. Start from the same corner as the plan.
2. Mark any doubled joists, which go under walls running parallel to the joists, and under tubs or other heavy loads shown on the plan.
3. Sight each joist and mark the crown. Crowning means installing each joist with its natural upward bow facing up, so that loads flatten it toward level. Cull joists with large crowns or twists.
4. Roll joists into place, crown up, on the layout marks.
5. Install the rim joist at the ends. The rim joist caps the joist ends, stops them from rolling and carries wall loads down to the sill around the perimeter.
6. Nail joists to the sill and rim per the nailing schedule. Common patterns are three 8d toenails from joist to sill, and 16d nails through the rim into each joist end.
7. Check that joist tops line up. A high joist will make a hump. Plane it down or replace it.

Blocking and Bridging

Blocking is pieces of joist material fit tightly between joists. It keeps joists from twisting, stiffens the floor, and gives bearing support at beams and walls.

• Solid blocking is required at the ends of joists unless a rim or other member already holds them.
• Joists deeper than a 2x12 need bridging or blocking at intervals of no more than 8 feet.
• Add blocking under walls and under any heavy point load.

Install blocks in a straight line, offset by a nail's width, so you can face nail through the joist into the end of each block.

Framing an Opening

Stair openings cut through joists. The cut joists are tail joists, and they hang on a header. The header hangs on doubled trimmer joists alongside the opening.

1. Install the outer trimmers first, on layout.
2. Install the first (outer) header.
3. Nail on the tail joists.
4. Add the second header, then the inner trimmers.

Under the IRC, a header spanning more than 4 feet must be doubled, along with the trimmers carrying it, and a header more than 6 feet long must be supported by hangers or framing anchors at its ends.

Drilling and Notching

Plumbers, electricians and HVAC crews will all want holes in your joists. These IRC limits apply to solid sawn lumber:

Worked example. A 2x10 is actually 9-1/4 inches deep.

• Largest hole: 9-1/4 / 3 = about 3 inches.
• Deepest notch away from the middle third: 9-1/4 / 6 = about 1-1/2 inches.
• Deepest notch at the end: 9-1/4 / 4 = about 2-1/4 inches.

A 3-inch pipe with a 3-1/2-inch hole does not fit in a 2x10 joist. You need a different route or an engineer's fix.

Engineered joists follow their own rules. I-joist flanges can never be cut, and holes go only where the manufacturer's chart allows them. Trusses cannot be cut at all.

Laying the Subfloor

Subfloor panels tie the joists together and make the walking surface. Most floors use 3/4-inch (23/32) tongue-and-groove OSB or plywood.

1. Snap a line 48 inches or so from the rim to start the first row straight.
2. Run the long side across the joists. Stagger end joints so they do not line up from row to row, and land every end joint on a joist.
3. Glue every joist with a subfloor adhesive rated for the panel. Lay only as much glue as you can cover before it skins over. Glue is what prevents most squeaks.
4. Set the panel, tapping the groove side with a block rather than hitting the tongue directly.
5. Leave a 1/8-inch gap at panel ends and edges, or follow the panel's stamp, so panels do not buckle when they swell.
6. Fasten at 6 inches along edges and 12 inches in the field unless the plan says otherwise. Ring-shank nails or screws hold better than smooth nails. Follow the plan's fastener size.

Keep the top of the subfloor clean. Do not leave panels exposed to rain longer than necessary, because swollen edges telegraph through finished floors.

Working Safely on an Open Floor

Warning: An open floor is a leading edge. Falls of 6 feet or more to a lower level require fall protection in residential construction. That can be guardrails, a personal fall arrest system or a safety net, and your employer must set it up before you step out.

Before you walk the joists or lay subfloor:

• Cover or guard any hole a person could step into. Covers must be secured and marked.
• Use planks or a temporary walkway. Joists roll and tops are narrow.
• Do not stack bundles of sheathing on a few joists. Spread the load or set it over a bearing wall.
• Cut on sawhorses or on a surface that lets the offcut fall safely. Do not cut with the saw above your shoulders or balanced on a joist.

Troubleshooting

Check Your Understanding

1. A joist on a concrete foundation wall has 2 inches of bearing. Is it enough?

No. On concrete or masonry, the IRC requires at least 3 inches. On wood or metal, 1-1/2 inches is enough.

2. An electrician needs a 2-1/2-inch hole in a 2x8 joist. Is that allowed? How far from the top?

A 2x8 is 7-1/4 inches deep. One-third is about 2-3/8 inches, so 2-1/2 inches is too big. A smaller hole must stay 2 inches from the top and bottom.

3. Why do you need rim joists?

They cap joist ends, stop them from rolling and carry perimeter wall loads to the sill.

4. A panel edge lands between joists. What do you do?

Trim the panel so the joint lands on a joist. Tongue-and-groove edges along the long side are fine, but end joints need to be supported.

5. Your foreman wants a sill plate on a slab of fresh concrete. Why must it be pressure-treated?

Concrete wicks moisture into the wood. Untreated lumber in contact with the concrete will rot.

Key Takeaways

• Sill plates are pressure-treated, sealed underneath and bolted down.
• Joists need 1-1/2 inches of bearing on wood or metal, 3 inches on concrete or masonry.
• Joists go at 16 inches on center unless the plan says otherwise, crown up.
• Holes up to one-third of the depth and at least 2 inches from each edge. No notches in the middle third.
• Glue and nail the subfloor, 6 inches on edges and 12 inches in the field.
• Guard every edge and hole before anyone works on the deck.