Thursday, 23 September 2010

How to Build a Simple Wood Truss

How to Build a Simple Wood Truss

One completed truss awaiting installation. Trusses are
often used to build a structurally sound roof system for
various types of buildings. This article provides the
instructions for building a basic truss suitable for a small
shed, storage building, or other structure not governed by
specific engineering requirements or building codes.

!! Steps !!

* 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.
Design your truss. You will need to know at least the basic
following design elements for your project.

A simple sketch showing the basic elements of a truss
Southern yellow pine 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, lodgepole 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.

Here is the truss table used to build this set of trusses. 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.

* _ Here, the wider board, a 2X6 (5cm x 15.2cm), is the
bottom chord, the narrower (2X4/5cm x 10cm) is the top chord,
showing how they fit together. 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.

* 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.

* 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. _ This illustration
shows a configuration where the bottom chord overhangs the wall
for eave framing.
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.

* 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.

* A sheet metal plate reinforces this connection. A large
nail is used to punch holes for the fasteners. Note, a C clamp
keeps everything aligned during this step. 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 ga.
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.
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.

* 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.

* 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.

Install your trusses on the project you're building. Attach them
to the top plate securely, and in accordance with applicable
building codes.

!! Video !!

!! Tips !!

* Choose the best available lumber for your truss building project.
Kiln-dried number 2 grade lumber of sufficient strength will help
insure your project is successful.

* Take care to saw cuts accurately, and to square cuts so that the
joints will fit tightly.

!! Warnings !!

* Follow all local building code requirements, which should include
factors such as wind loads, snow loads, or other environmental
conditions.

* Safety glasses are suggested when using power saws and hammers.

* Wearing work gloves will protect your hands while handling the
lumber.

!! Things You'll Need !!

* Lumber suitable for building a truss that meets your needs

* Nails or other fasteners of your choice

* Plywood or sheet metal for reinforcing plates

* Saw, hammer, square, and measuring tape

* Table, sawhorses, or other work surface

!! Related WikiHows !!

* How to Build a Shed

* How to Build a Saw Horse

* How to Repair a Pitched Roof

* How to Build a Brick Wall

* How to Repair a Tear in the Roof of a Metal Storage Shed

!! Article Tools !!

* Read on wikiHow

*

0 comments:

Post a Comment