Calculating beam spans is a bit
more complicated than floor
joists. First, the tributary width
supported by the beam must be
determined. For simple spans
the tributary width is 1/2 the
joist length. For a center beam
the tributary width is the sum
of 1/2 the span from each side.
If there is a cantilever, we add
in the total length of the
cantilever. Note that 2x8 and
larger floor joists cannot
cantilever more than 3 feet.
Once the tributary area has
been determined, the beam
span can then be determined using
Figure 11
HOUSE HOUSE
a/2 b/2
ab a
Overhang
d
a/2 d
tt
Overhang
ad
ab
a/2 d
t
a/2 b/2
t
Joist (typ.)
Beam (typ.)
A. t = a/2 + b/2
B. t = a/2 + d
C. t = a/2 + b/2 D. t = a/2 + d
Tributary load width (t) for deck beams
Overhang, “d” shall
not exceed 3 feet
Tables 6 or 7. Decide what will be used for the beam and check the span based on the tributary
load width. If 2 – 2x10’s are going to be used as the beam and the tributary area of the beam is 7
feet, the maximum span for the beam is 9 feet, 2 inches.
4-34-54-95-15-76-27-07-88-72x10
3-43-63-94-04-34-105-56-27-22x8
2-62-82-103-13-43-84-24-95-72x6
7-68-08-58-109-510-010-1011-1012-10(3)2x8
9-29-710-010-711-211-1112-1014-115-7(3)2x10
10-911-211-912-413-113-1115-015-518-3(3)2x12
7-07-47-88-18-59-29-1010-912-0(2)2x10
8-28-79-09-510-010-811-612-714-0(2)2x12
5-5
4-1
12’
7-2
5-7
8’
6-9
5-2
9’
6-3
4-9
10’
5-10
4-5
11’
7-8
8-39-110-1(2)2x8
5-11
6-57-07-10(2)2x6
7’
6’5’4’
Tributary load width (ft)
Beam size
40 lb/ft
2
live load – 10 lb/ft
2
dead load
Maximum beam spans for Treated Southern Pine
Spans are distances in feet-inches between centers of posts or supports. Grade is No. 2 or Better.
Number in parentheses is number of full-length nailed laminations.
When multiple members are used, they
must be attached so they act as one. This
requires nailing the members together w
10d nails at 16 inches on-center stagge
ith
red.
a single Cedar 4x10 were going to be
of
If
used instead of a built-up treated beam,
then Table 7 would be used. In the case
our 7 foot tributary area, a 4x10, which by
the way, is not
the same as 2 – 2x10’s, has
a span capability of 8 feet, 11 inches.
Table 6
A beam should always be supported
directly by the columns beneath it. This is
usually accomplished by notching the
beam into the post so there is direct wood
to wood bearing. Note 6x6 posts are
recommended if the beams are to be
notched into the posts.
a
4-34-54-84-115-25-66-06-77-44x6
6-16-56-107-37-108-89-84x8
7-27-67-118-48-119-810-711-104x10
8-48-89-29-910-511-312-413-94x12
5-75-106-26-77-07-78-39-36x8
7-77-118-48-99-39-1110-911-913-26x10
12’8’ 9’ 10’ 11’7’6’5’4’
9-29-710-110-711-312-013-014-315-116x12
Spans are distances in feet-inches between centers of posts or supports. Grade is No. 2 or Better.
Table 7
5-75-10
6-10
7-11
5-4
Tributary load width (ft)
Beam size
40 lb/ft
2
live load – 10 lb/ft
2
dead load
Maximum beam spans for Redwood
nd Western Red Cedar
10