The subfloor platform caps the foundation. It provides the base for framing the walls and roof. Before starting, it's important to check the squareness of the foundation perimeter by measuring diagonally from the corners. This way, you know what you're dealing with. If the distances aren't quite the same, repositioning the sills may be necessary.
For definitions throughout this project, see Framing Glossary.
Setting Sill Plates
You usually need to drill holes in the sill plates so they'll fit over the anchor bolts cemented into the foundation. Measure in and over from the foundation edges to find the bolt locations. Transpose each bolt location onto the sill plates by measuring in from the sill edge and over from the sill end.
Drill the holes and ream them just a bit larger than the anchor bolts. This way, getting the sill plate started over several bolts will be easier. Also, put a sill gasket under the plates to insulate and prevent air infiltration. You may even want to caulk around both sides of the plates.
Codes require using pressure-treated lumber sill plates. Pressure-treated sill plates resists wood rot that can be caused by close proximity to soil and moisture.
Depending on the project, local codes may require 2x6s or 2x8s. A wider sill plate gives more room for the joists to set on and provides a larger nailing surface.
When all the sills are in place, toenail them together at the corners. Then snug the anchor bolt nuts. After the floor joists are on, you can go back and cinch them down tight.
NOTE: When installing the sill plate, your plan may call for setting the exterior wall sheathing flush with the outer foundation wall. So set the sill plates back the thickness of the exterior wall sheathing.
Installing Rim Joists
Rim joists are next. If you're using joist hangers, nail those to the rim joists before setting them in place.
On top of the sill plates, set the rim joist boards on edge -- flush with the outside edge of the plates -- and toenail the rims inward into the sill plates.
Then check the work again. Measure the span between the side joists at both ends and in the middle. Make sure the distances are correct -- so the floor joists will fit. Once the rims are set, nail them together at the corners.
If the floor system is wider than can be accommodated with a single span of lumber, an intermediate beam or a bearing wall may be necessary. Beams are usually steel I-beams or laminate veneer lumber (LVLs).
NOTE: Some beams required renting a crane & operator.
The ends of some beams set in premade notches in the foundation block so the beam sets flush with the foundation top. Other beams may set on top of the foundation. If this is the case, you may be able to nail on joist hangers to the beam to accept floor joists.
After the beam is installed, you may need to install steel posts (on concrete footings) under the middle of the beam for additional support.
Installing Floor Joists
Floor joists are usually spaced on 16" center. Floor joists run perpendicular over a beam and overlap or butt together over it -- and sometimes joists fit in hangers fastened to the beam's sides, instead of setting on the top.
If you're using joist hangers, installing floor joists can simply mean dropping them in place.
Fit the floor joists in the hangers nailed to the rim joists (and beam if you have one). Hanger brackets can really save you time, but they can get nailed on crooked, so make sure the joists are level to each other across their tops.
If you're not using joist hangers, toenail them directly to the rim joists.
NOTE: Any floor joists that have a crown (bow) in them should be installed crown up. Over time, the crown will straighten out.
While there's still good access to the floor joists (before the decking goes on), install the bridging. Fasten bridging at the mid-point of the joist span, between the floor joists, to give them stability and to prevent the floor from squeaking.
The extra weight of the floor joists push the sill plate down and expose a few more threads on the anchor bolts. So once the joists are installed go back and tighten down the anchor bolt nuts.
Fastening Subfloor Decking
Next the actual floor "decking" can go on. It's also called the subfloor. It's usually 3/4" plywood.
Utilize the factory-cut edges at joints when possible. And carefully try to get straight cuts so when you do use a scrap piece with a cut edge the joints fit together nicely. When installing any plywood decking, stagger the joints and you may want to gap the sheets about 1/32" to allow for expansion due to moisture.
Use construction adhesive and subfloor nails or screws to secure the sheets to the floor joists.
TIP: For an even stronger, more stable floor, special subfloor materials are now available. These are slightly thicker than standard plywood and have tongue and groove joints.