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How to Build a Simple
Wood Truss Trusses are often used to build a structurally sound roof
system in a variety of buildings. Often, in simple projects like
small sheds, storage buildings, or other structures not
governed by specific engineering requirements or building
codes, it's possible to build your own trusses from ordinary
lumber. See Step 1 below to get started.
1 Determine if homemade trusses are suitable for the
project you're building.Since your truss will support
the roof of your building, it should not violate local building
codes and must be designed to meet wind load, snow load,
and other structural requirements.
2 Design your truss. You will need to know at least the basic
following design elements for your project.
3 Select the appropriate lumber for the truss you'll
construct. High density softwoods like southern
yellow pine or fir are preferred over lumbers like spruce, lodge
pole pine, and white pine. Other things to look at when
selecting individual boards include the following:
Size of the lumber. 2X4 inch (5cm x 10cm) nominal boards
are suitable for lightweight trusses if the bracing and member
connections are well designed and fabricated.
Quality of the lumber. Straight grained, dense boards with few
knots are preferred for this project. Try to select lumber with
no obvious splitting or barked edges.
Straightness of lumber. Twisted, warped, or crooked boards
do not lend themselves well for building trusses. Lumber with
a slight curve, or crown may be used, as long as the crown is
facing up when it is installed.
Lumber should be kiln dried or seasoned, as green lumber
may shrink and/or warp after the truss is assembled and
installed.
4 Set up sawhorses or another flat, stable work place to
build your trusses. If possible, draw out a full size truss on a
floor to help you establish the lengths of each member and
the angles of the various cuts required. If you cannot do this,
use sawhorses and set the bottom chord board on them to
begin the process of marking pattern pieces.
5 Place the bottom chord board with the crown up, then
place the boards the top chords will be cut from over it,
so each member laps where they will intersect when the
truss is built. Scribe lines for the cuts at these overlapping
places, cutting the bottom chord ends first if required.
6 Mark the top cuts on the top chords where they will meet
at the roof peak. The angle of this cut can be determined
with a speed square if you know the roof pitch. Mark
a common rafter pitch cut on each top chord, or use
a string to make a mark from the center point of the bottom
chord perpendicular to that member across the two top chords
where they overlap each other.
7 Determine whether the top chord or bottom chord will
overhang the eave of your building, and then choose the
appropriate bottom cut of the top chord. For a plumb eave
line, you should be able to use the same angle you
established to the top cut of the top chord. If you want the
fascia board (if you anticipate installing one) to slope inward,
use a sharper angle.
8 Locate the position of the intermediate diagonal bracing
members you'll need to complete the truss. There are
specific engineering guidelines and even online resources you
can use to determine load bearing and span applications for
your project, but this article only applies to simple trusses
where the general rule of thumb is one brace from the one
third points of the bottom chords to the center of the top
chords.
Divide the length of the bottom chord by three, then measure
the resulting distance across the chord, marking the two
locations where these braces will intersect the chord.
Mark the center point of each top chord, then place a board
long enough to overlap each chord so the angles of the
intersection can be scribed on each end.
9 Use one of each unique member you have marked and
cut as a pattern to mark subsequent members for each
truss you plan to build. Be careful to make sure top and
bottom chords are placed so their crowns are up when the
trusses are built, and take care to be as precise as possible
when marking and cutting each piece, as fitting them in the
truss will be difficult if the lengths or angles are not correct.
10 Determine if toe-nailing the members will create a strong
enough truss for you project. Likely, you will want to use
reinforcing plates, gang-nailers, or gussets for all the
connecting points of the frame members. The illustrations
included here show 16 gauge galvanized steel plates, cut
from salvages scrap metal being used, but if weathering isn't
a consideration, 1/2 inch (12.7mm) plywood plates can be
used.
11
Nail each member of the first truss together, supporting it
at intersections with additional boards or posts to keep
the assembly flat while it is laying on it's side. Use clamps
to hold the fitted pieces tightly together while fastening them,
and to work out any twists that are present in the lumber
you're using.
For metal or plywood reinforcing plates, use plenty of nails of
a length that will not penetrate through the member you're
fastening, or you will have to bend the protruding ends of
these nails over before putting the plates on the opposite side
of the truss.
12 Use the first truss you assembled as a template to
assemble the remaining trusses. Make sure all the joints
are supported by temporary legs if you're working on a
sawhorse or a table that is not large enough for the complete
truss to set upon.
13 Stack the completed trusses on dunnage so they lay as
flat as possible, and in the best alignment you can
achieve to prevent them from warping prior to
installation. Placing the trusses stacked in this manner will
also allow you to observe how well the trusses match each
other. Keep in mind that unless you have perfectly sized and
straight material, you can expect minor variations from one
truss to the next.
14
Install your trusses on the project you're building. Attach
them to the top plate securely, and in accordance with
applicable building codes.
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