Floor framing consists of a system of sills, beams, girders, joists, and subflooring, all properly sized and connected together. Floor framing provides support for floor loads, and gives lateral support to exterior walls.

Fasteners and Connections

Proper design, specification, and installation of fasteners and connections is crucial to the long-term performance and structural integrity of any structure. Nails, used alone or in combination with metal framing anchors and construction adhesives, are the most common method of fastening framing lumber and sheathing panels. Nail joints provide best performance when loads are applied at right angles to the nails. Nailed joints with the load applied parallel to the nail (in withdrawal) should be avoided. Metal products in contact with pressure-treated wood must be corrosion resistant.

Sill Plates on Foundation Walls

Sill plates resting on continuous foundation walls (stem walls) are generally of nominal 2×4 or 2×6 pressure-treated lumber. They are anchored to concrete, masonry, or wood walls with steel anchor bolts or proprietary metal anchor straps. The required size and spacing of the bolts or straps is dependent upon the forces acting on the building.

Typically, 1/2″-diameter anchor bolts are placed within 12″ from each end of the sill plate and then spaced a maximum of 6′ on center. These bolts are usually embedded at least 7″ in concrete or masonry (15″ in masonry for uplift loads). Proprietary metal anchor straps providing equivalent anchorage may also be used in lieu of anchor bolts. Closer anchor bolt spacing and/or a larger bolt diameter may be required in seismic design categories D1 and D2 and where the design wind speed exceeds 110 mph. Consult the ANSI/AF&PA Wood Frame Construction Manual from the American Wood Council or your local building code official for specific anchorage requirements.

Sill Beams on Piers or Piles

Sill beams supported by freestanding piers or piles must be of adequate size to support imposed loads between piers. They must also be adequately attached to the supporting piers. In addition, sill beams must be preservative treated if bearing on concrete or masonry, or if closer than 12″ to exposed soil.

Sill beams are generally of solid-sawn lumber (typically 4×6) or timbers (typically 6×6 or 6×8), or glued-laminated timber.

Beams and Girders

Beams and girders are generally of solid-sawn lumber or timbers, glued-laminated timber, or structural composite lumber. They can also be built-up (nail-laminated) with multiple pieces of nominal 2″ lumber nailed together with the wide faces vertical. These multiple pieces should be nailed together with two rows of 20d nails — one row near the top edge, and the other near the bottom edge. Nails in each row are spaced 32″ apart. End joints of the nailed lumber should occur over the supporting column or pier.

Beams and girders must be adequately attached to supports, and should be tied together across supports if they are not continuous members. Beams and girders must be preservative treated if entering exterior masonry or concrete walls without a minimum 1/2″ air space on top, sides and end, or if closer than 12″ to exposed soil.

SFPA’s allowable load tables provide maximum pounds per lineal foot (plf) and required bearing lengths for the following Southern Pine beam and girder options:

  • Solid-sawn, heavy dimension lumber or timbers (Table 12)
  • Glued-laminated timber (Table 13)

For additional information, refer to the SFPA publication, Southern Pine Headers & Beams.

Floor Joists

Floors are commonly framed with solid-sawn lumber, floor trusses, or wood I-joists. Joist end-bearing should not be less than 1-1/2″ on wood or metal, or 3″ on masonry. Joists are usually attached to sills by toe-nails or by metal framing anchors. Table 10 provides the typical nailing schedule for floor framing, while Figure 17 illustrates floor framing connections. Floor joists must be preservative treated if closer than 18″ to exposed soil.

figure17

 

Table 10 Nailing Schedule for Floor Framing
Joint Description Number and Size
of Common Nails
Nail Spacing
Joist to Sill, Top Plate or Girder (toe-nailed)
4- 8d
per joist
Bridging to Joist (toe-nailed)
2- 8d
each end
Blocking to Joist (toe-nailed)
2- 8d
each end
Blocking to Sill or Top Plate (toe-nailed)
3- 16d
each block
Ledger Strip to Beam (face-nailed)
3- 16d
each joist
Joist on Ledger to Beam (toe-nailed)
3- 8d
per joist
Band Joist to Joist (end-nailed)
3- 16d
per joist
Band Joist to Sill or Top Plate (toe-nailed)
2- 16d¹
per joist

Source: Wood Frame Construction Manual, 2001 Edition, American Wood Council, Table 3.1.
1 Nailing requirements are based on wall sheathing nailed 6″ on-center at the panel edge. If wall sheathing is nailed 3″ on-center at the panel edge to obtain higher shear capacities, nailing requirements for structural members shall be doubled, or alternate connectors, such as shear plates, shall be used to maintain the load path.

Joists should be placed so the top edges provide an even plane for the subfloor and finish floor. Preferably, joists should be attached to the sides of girders as shown in Figure 22Figure 23 and Figure 25. This will reduce the cumulative amount of shrinkage as lumber dries to its in-place moisture content.

Construction adhesive is applied to floor joists before subfloor sheathing is installed. Gluing improves floor stiffness, reduces floor vibration, and helps eliminate squeaks.
Construction adhesive is applied to floor joists before subfloor sheathing is installed. Gluing improves floor stiffness, reduces floor vibration, and helps eliminate squeaks.

Proper alignment of the upper edges of floor joists is maintained by adequately nailed subflooring. Nailing the ends of joists to band joists or headers provides additional joist support. These typical construction techniques usually eliminate the need for intermediate bridging. Where the nominal depth-to-thickness ratio of lumber joists exceeds six, intermediate bridging is installed at 8′ intervals. Bridging may be accomplished with solid 2″ blocking or 1×4 cross braces as illustrated in Figure 17, and helps reduce floor vibration. See Reducing Floor Vibration.

SFPA’s easy-to-use Span Tables – Floor Joists provide maximum allowable spans for Southern Pine floor joists. For additional information, refer to the SFPA publication Maximum Spans for Southern Pine Joists & Rafters.

Notching and Boring of Joists

Figure 18 illustrates joist notching and boring limits for solid-sawn joists. Do not cut notches in the top or bottom edges in the middle one-third of the joist span. Notches in the outer thirds of the span cannot exceed one-sixth the actual joist depth, and cannot be longer than one-third the depth. Notches made at a support, such as shown in Figure 23 for joists supported by ledgers, cannot exceed one-fourth the actual joist depth.

figure18

Bored holes are limited in diameter to one-third the actual joist depth, and the edge of the hole cannot be closer than 2″ to the top or bottom edges of the joist.

No boring, cutting or other modification of wood trusses is allowed. For rules on modification of I-joists, refer to I-Joists for Residential Floors at www.apawood.org or to the manufacturer’s requirements.

 

Workers install subfloor sheathing over floor trusses of a 5,000 sq. ft. assembly building. See Assembly Building case study.
Workers install subfloor sheathing over floor trusses of a 5,000 sq. ft. assembly building. See Assembly Building case study.

Subflooring

Subflooring, also commonly called “rough floor,” is the material applied over floor joists to provide a base for the finish floor. The subfloor also works as a horizontal diaphragm in high-wind and seismic areas, transferring lateral loads from the wall system to the foundation below.

Structural wood panels (e.g. plywood, OSB) commonly form the subfloor of a raised floor system. These panels are typically manufactured in 4×8 sheets. The panels are then applied to the floor joists with 1/8″ gaps left between the sheets to allow for expansion and to prevent buckling. Table 11 provides a nailing schedule for properly attaching wood structural panel subflooring to the top of the floor joists.

APA-rated Sturd-I-Floor is a combined subfloor-underlayment product designed specifically for single-layer floor construction beneath carpet and pad. When other subfloor products are used, builders often install a separate underlayment grade of plywood over the subfloor to provide a proper base for the finish flooring.

 

Table 11 Nailing Schedule for Wood Structural Panel Subflooring¹
Panel
Span Rating
Panel
Thickness

(inches)
Maximum Span
(inches)
Nail Size and Type4 Supported Paniel Edges6
(inches)
Intermediate Supports
(inches)
24/16 7/16 16 6d common 6 12
32/16 15/32, 1/2 16 8d common² 6 12
40/20 19/32, 5/8 20³ 8d common 6 12
48/24 23/32, 3/4 24 8d common 6 12
60/325 7/8 32 8d common 6 12

Source: APA Design Construction Guide — Residential & Commercial; APA, (www.apawood.org), Table 11.
1 APA Rated Sturd-I-Floor may be substituted when the Span Rating is equal to or greater than tabulated maximum span.
2 6d common nail permitted if panel is 1/2 inch or thinner.
3 Span may be 24″ if a minimum 1-1/2 inches of lightweight concrete is applied over panels.
4 Other code-approved fasteners may be used.
5 Check with supplier for availability.
6 Supported panel joints shall occur approximately along the centerline of framing with a minimum bearing of 1/2″. Fasteners shall be located 3/8 inch from panel edges.

The quality of the subfloor and underlayment layers is fundamental to having a floor that is plumb and stable. Regardless of the type of flooring used, subfloor and underlayment rules are basically the same. They should be stable, clean, smooth and level. If a subfloor becomes wet during construction, it should be allowed to dry prior to the installation of any finish flooring material.

For more information on subflooring, refer to the Engineered Wood Construction Guide available from APA at www.apawood.org.

Framing of Floor Openings

Headers, trimmers, and joists form the framing for floor openings. Trimmers and headers are doubled when the header span exceeds four feet. Headers more than six feet in length are supported at the ends by joist hangers or framing anchors unless they are bearing on a partition, beam, or wall. Tail joists that exceed 12′ in length are supported on framing anchors or on ledger strips not less than 2×2 (nominal).

Support of Partitions

Bearing partitions are normally placed over girders or walls that support the floor system. Where floor framing is adequate to support the added load, bearing partitions may be offset from supporting members by no more than the joist depth, unless floor joists are designed to carry the increased load. Where nonbearing partitions run parallel to floor joists, the joist under the partition is doubled to support increased loads frequently occurring adjacent to the partition.

Framing Details

Figure 19Figure 20, and Figure 21 provide floor framing details. Figure 22Figure 23Figure 24, and Figure 25 illustrate various methods for supporting floor joists. Figure 26 and Figure 27 provide stair framing details, with emphasis on floor framing support.

figure19

figure20

figure21

figure22

figure23

figure24

figure25

figure26

figure27

Reducing Floor Vibration

Floor vibration, or bounce, is not a safety issue; it is a performance issue, and one that is likely to be important to homeowners. “Acceptable” floor performance is highly subjective: What’s adequate for one homeowner may not be adequate for another. But here are some general rules-of-thumb for reducing bounce that should prevent the majority of complaints.

  1. Glue and screw the sheathing. Floor sheathing should always be glued down. Gluing increases floor stiffness and helps eliminate squeaks, floor vibration, bounce, and nail-popping. Screws work better than nails for long-term bounce control. See Steps to Construct a Solid, Squeak-Free Floor System at www.apawood.org.
  2. Shorten the span. In general, shorter spans make for stiffer floors. For example, if the ℓ/360 span table tells you a joist of a given size, grade, and species will just barely work for your span, shorten the span by adding a girder near the center of the original span. The resulting floor will vibrate less.
  3. Increase the joist depth one size. If the building code requires a 2×8 at 16″ on center, then use a 2×10 of the same grade and species. Or use a 14″-deep floor truss when a 12″-deep truss would meet code requirements. This may not be the most cost-effective solution in every case, but it is easy to remember and can save time and worry.
  4. Reduce the on-center spacing. For example, if the span table calls for 16″ spacing of the floor joists, but this arrangement does not provide the stiffness required, reduce the on-center spacing to 12″. This is probably the least efficient and more costly way to improve floor vibration performance, but it can help. The reason is that “bounce” occurs as a result of a foot impacting an individual joist. Joists spaced 16″ on center may not be close enough for the shock of a foot to be carried by two joists, so reducing joist spacing to 12″ on center can help.

1 Reducing Floor Vibration by Frank Woeste, P.E., and Dan Dolan, P.E., Virginia Tech.