After the foundation and deck framing of a building are completed, the
floor is built.
Joists are the wooden members, usually 2 or 3 inches thick, that make
up the body of the floor frame (Figure 6-18, page 6-16). The flooring
or subflooring is nailed to the joists. Joists as small as 2 x 6 are sometimes
used in light frame buildings. These are too small for floors with spans
over 10 feet, but are frequently used for ceiling joists.
Joists usually carry a uniform load
of materials and personnel; these
are live loads. The weight of joists
and floors is a dead load. The joists
carry the flooring load directly on
ends nearest the sills, girders,
bearing partitions, or bearing
walls. Joists are spaced 16 or 24
inches apart, center to center.
Sometimes the spacing is 12
inches, but where such spacing is
made necessary by the load,
heavier joists should be used.
To support heavily concentrated
loads or a partition wall, you may
need to double the joist or place
two joists together. Two typical
reinforced joists are shown in
In joining joists to sills, be sure
that the connection can hold the
load that the joist will carry. The joist-connecting method in Figure
6-20, A, is used most often
because it provides the strongest joint. The methods shown in Figure 6-20,
B and C, are used when it is not desirable to use joists on top of the
sill. The ledger plate should be securely fastened. If the joist must
be notched, it should not be notched to the sill and girder over onethird
of its depth to prevent splitting (Figure 6-20, D).
Joists must be level when framed to girders. If the girder is not the
same height as the sill, the joist must be notched as shown in Figure
6-20, C. If the girder and sill are the same height, the joists must be
framed to keep the joist level. The joist is always placed crown up. This
counteracts the weight on the joists. In most cases there will be no sag
below a straight line.
The simplest way to carry joists on steel girders is to
rest them on top (as shown in Figure 6-20, E), provided
headroom is not restricted. If there is a lack of
headroom, use straps or hangers (iron stirrups) as
shown in Figure 6-20, F. These art among the strongest
In connecting joists to girders and sills where posts are
used, a 2 x 4 is nailed to the face of the sill or girder,
flush with the bottom edge. This is called a ledger.
These pieces should be nailed securely with 20d nails
spaced 12 inches apart. When 2 x 6 or 2 x 8 joists are
used, it is better to use 2 x 4 ledgers. This prevents joists from splitting
at the notch.
When joists are 10 inches or more deep, 2 x 4s may be
used as ledgers without reducing the strength of the
joists. If a notch is used, joist ties may be used to
overcome this loss of strength. These ties are short 1 x 4
boards nailed across the joists. Board ends are flush
with the top and bottom edges of the joists.
Overhead joists are joined to plates as shown in Figure
6-21, A and B. The inner end of the joist rests on the partition wall
plates. If a joist is to rest on plates or girders, the joist is cut long
enough to extend the full width of the plate or girder. Alternatively,
the joists are cut to meet in the center of the plate or girder and connected
a scab. When the ends of two
joists lie side by side on the
plate, they should be nailed
together. Joists may also be
joined to girders with ledgers
(Figure 6-21, C and D).
FLOOR JOISTS FOR
Check the plans to determine the
size and direction of the joists. If
the sizes for joists are not
specified on the plans, consult
Tables 6-2 and 6-3 to determine
the appropriate size.
Joists tend to twist from side to
side, especially when used over a
long span. Floor frames are
bridged for stiffening and to prevent unequal deflection of the joists.
This stiffening also enables an overloaded joist to receive some help
from the joists on either side of it. A pattern for the bridging stock
is obtained by placing a piece of material between the joists, then marking
and sawing it. When sawed, the cut will form the correct angle.
The three kinds of bridging are:
solid (horizontal) bridging,
cross bridging, and compression
bridging (Figure 6-22, page 6-
20). Cross bridging is used most
often. It is very effective and
requires less time than
horizontal bridging. Cross
bridging looks like a cross and
is made of pieces of lumber,
usually diagonally cut 1 x 3 or 2
x 3 between the floor joists.
Each piece is nailed to the top
of each joist and forms a cross
(x) between the joists. Cross
bridging should be made so that
the two pieces of the cross are
against each other.
Compression is metal bridging
Bridging should be nailed at
the tops with 8d or 11 Id nails,
and the bottoms should be left
free until the subfloor is laid. This allows the joists to adjust to their
final position and keeps
the bridging from pushing up the joists and causing unevenness in the
floor. The bottom ends of the bridging may then be nailed, forming a continuous
truss across the floor. This prevents overloaded joists from sagging.
Cutting and fitting the bridging by hand is a slow process. A power
saw should be used if it is available. One line of bridging should be
placed on joists more than 8 feet long. On joists more than 16 feet long,
two lines should be used.
Table 6 3. Group classification—nonstress-graded
Floor openings for stairwells, ventilators, and chimneys are framed
by a combination of headers and trimmers. Headers run at right angles
to the direction of the joists and are doubled. Trimmers run parallel
to the joists and are actually doubled joists. The joists are framed at
right angles to the headers of the opening frame. These shorter joists,
framed to headers, are called tail beams, tail joists, or header joists.
The number of headers and trimmers
needed at any opening depends upon—
· The shape of the opening—whether it is a simple rectangle
or contains additional angles.
· The direction in which the opening runs, in relation to the joist
· The position of the opening, in relation to partitions or walls.
Figure 6-23, page 6-20, shows examples of openings. One runs parallel
to the joist and requires two headers and one trimmer. The other runs
at right angles to the joists and requires one header and two trimmers.
The openings shown in Figure 6-24, page 6-21, are constructed with corner
angles supported in different ways. The cantilever method (shown on the
right of Figure 6-24) requires that the angle be fairly close to a supporting
partition with joists from an adjacent span that run to the header.
To frame openings of the type shown in Figure 6-25—
Step 1. Headers 1 and 2 are nailed to trimmers A and C with three 20d
Step 2. Headers 1 and 2 are nailed to short joists X and Y with three
Step 3. Headers 3 and 4 are
nailed to headers 1 and 2 with
16d nails spaced 6 inches
Step 4. Trimmers A and C are
nailed to headers 3 and 4 with
three 20d nails.
Step 5. Trimmers B and D are
nailed to trimmers A and C
with 16d nails spaced 12
The subfloor (Figure 6-26, page 6-22), if included in the plans, is
laid diagonally on the joist framework and nailed with 8d to 10d nails.
Subflooring boards 8 inches wide or more should have at least three nails
per joist. Where the subfloor is more than 1 inch thick, larger nails
should be used. The subfloor is normally laid before the walls are framed
so that it can be walked on while walls are being framed.
A finish floor in the TO is normally of 3/4-inch material, square-edged
(Figure 6-27, page 6-22). Finish flooring varies from 3 1/2 to 7 1/2 inches
wide. It is laid directly
on floor joists or on a subfloor
and nailed with 8d common
nails in every joist. When a
subfloor exists, building paper
is used between it and the
finish floor to keep out
dampness and insects.
In warehouses, where heavy
loads are to be carried on the
floor, 2-inch material should be used for the finish floor. Such
flooring is also face-nailed with
16d or 20d nails. It is not
tongue-and-groove, and it
ranges in width from 4 to 12
inches. The joints are made on
the center of the joist.
Wood floors must be strong enough to carry the load. The type of building
and its intended use determine the arrangement of the floor system, the
thickness of the sheathing, and the approximate spacing of the joists.
Concrete floors may be
constructed for shops where
earthen or wood floors are not
suitable. These include aircraft
repair and assembly shops,
shops for heavy equipment, and
certain kinds of warehouses.
After the earth has been graded
and compacted, concrete is placed on the ground. The floor
should be reinforced with steel
or wire mesh. The foundation
wall may be placed first and the
concrete floor placed after the
building is completed. This gives
protection to the concrete floor
while it sets.
Drainage is provided for the
floor area around the footing and the area near the floor, to prevent
flooding after heavy rains. A concrete floor is likely to be damp unless
it is protected.
These types of floors include earth, adobe brick, duckboard, or rushes.
Miscellaneous flooring is used when conventional materials are unavailable
or where there is a need to save time or labor. Such floors may be used
if facilities are temporary or if required by the special nature of a
structure. Selection of material is usually determined by availability.
Duckboard is widely used for shower flooring. Earthen floors are common;
they conserve both materials and labor if the ground site is even without
extensive grading. Rush or thatch floors are primarily an insulating measure
and must be replaced frequently.