Consider a shallow slope roof subject to a Snow load of 50 pounds per square foot (psf) and applied Dead (weight) load of 5 psf. Let’s find, and specify, heavy timber decking to carry the required loads while spanning 6 feet (ft) rafter-to-rafter. By case of example let’s assume that Ponderosa Pine, `Commercial’ grade decking is available, and in lengths up to 12 or 16 ft. Further, the decking will be exposed on the underside and also be the `ceiling’ material. We will use the American Institute of Timber Construction AITC 112-93,

*Standard for Tongue-and-Groove Heavy Timber Decking,*to determine the required thickness.

According to
the Standard, a number of `layups’ are
typically used for heavy timber decking construction. Some layups are stronger and stiffer than
others; while others are more (or less) affordable. Often the `Controlled Random Layup’ is the
most affordable (per piece), as it allows the manufacturer to provide decking
pieces of various (convenient) lengths.
On the other hand, it may result in more trim waste than layups of
pieces ordered to `exact lengths’.

Typical
heavy timber thicknesses are 2 inch (in.) nominal (1-1/2 in. net), 3 in.
nominal (2-1/2 in. net), 4 in. nominal (3-1/2 in. net). And the typical width is 6 in. nominal (5-1/2
in. net), though other thicknesses and widths are available from some
manufacturers.

In this example
we will investigate using 2 in. nominal (nom.) Commercial Grade Ponderosa Pine
with a Controlled Random Layup.

Page 10 of
the Standard gives us weights of various decking species. For Ponderosa Pine we are given:

2 in.
nominal weighs ... 4.1 psf

3 in. nom.
weighs ... 6.9 psf, and

4 in. nom.
weighs ... 9.6 psf.

Total load
of ... 50 + 9 = 59 psf (Snow plus Dead plus `self’ weight).

(Wood is 15%
`stronger’ under snow load than `normal’.)

For
Ponderosa Pine, Commercial Grade,

F

_{b}(Design Bending stress) = 1250 psi (pounds per square in.), and
E (Modulus
of Elasticity) = 1,100,000 psi.

_{F}, applied to the Design Bending stress:

for 2 in. nominal ... 1.10.

(The way
decking is `graded’ the 2-in. and 3-in. thicknesses have a bit more unit
strength. The values in Table 3 on Page
13 are based on 4 in. thickness.)

Thus, for
our 2 in. `P. Pine’ subject to Snow load, the Bending Stress adjusted for size
and load duration,

*, is*__for our application___{b}= 1250 psi x 1.15 x 1.10 =

__1581 psi.__

The Modulus
of Elasticity, E, doesn’t get adjusted; it stays at 1,100,000 psi.

To deal with
`strong enough’ we look at Page 14, Table 4, ... ALLOWABLE ROOF LOAD LIMITED BY
BENDING.

Scrolling down the left hand side of the Table to a Bending Stress of 1581 psi ... wait! ... we have 1550 and 1600 ... let’s go to 1550 psi (and be a bit conservative) ... for Controlled Random Layup, Span 6 ft, we get an Allowable load of 108 psf.

*enough.*

__strong__
l/180 ... 66
psf

l/240 ... 50
psf.

Now let’s
check.

The l/180
deals with total load; our total load is 59, and the Allowable is 66: good!

The l/240
deals with (the effect of) Live (Snow) load; our Snow load is 50, and the
Allowable is 50. Whoa, good,
barely. Perfect! (Close!)

The 2 in.
decking works!

Here is our
specification: 2 in. Ponderosa Pine
Decking, Commercial Grade, Controlled Random Layup.

The Standard
goes on to dictate how the pieces must be laid, as well as fastening
requirements.

2. use a stiffer or stronger WOOD (different
species or grade of decking),

3. try THICKER decking,

4. change the rafter spacing (???), or

5. take into consideration actual roof slope
(Page 12 of the Standard).

References

*Standard for Tongue-and-Groove Heavy Timber Decking*

*,*American Institute of Timber Construction (now managed by the West Coast Lumber Inspection Bureau, Portland, Oregon).

See also ...
http://voices.yahoo.com/weight-board-foot-quantity-calculations-decking-12006629.html?cat=6

## 2 comments:

Great Stuff!! I am just wondering by seeing the huge difference after following this article. I do appreciate your post. Thanks a ton.

Timber Decking

Thank you for sharing and writing

for Deck

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