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. See the Advisory
on Fasteners and Connectors for Treated Wood.
Sill Plates on Foundation Walls
Sill plates resting on continuous foundation walls (stem walls)
are generally of nominal 2x4 or 2x6 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 4x6)
or timbers (typically 6x6 or 6x8), 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.
The 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)
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.
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. See Tolerances
and Techniques for Flat Floors. Preferably, joists should
be attached to the sides of girders as shown in Figure
22, Figure 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.
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 1x4 cross braces as illustrated in Figure
17, and helps reduce floor vibration. See Reducing
Floor Vibration.
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.
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.
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 4x8 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. See Tolerances
and Techniques for Flat Floors. 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 2x2 (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.
