2. Unit 50 — Roof Trusses
Roof trusses are
commonly used in
residential construction.
Metal plate connectors
fasten the truss
components together.
3. Unit 50 — Roof Trusses
Roof trusses serve
the same purpose
as rafters but
provide a larger
unsupported span.
4. Unit 50 — Roof Trusses
Various roof truss
designs can be
combined to form
intersecting roofs.
5. Unit 50 — Roof Trusses
A variety of standard roof
truss designs are available.
An endless number of
variations of the standard
designs are possible.
6. Unit 50 — Roof Trusses
Galvanized metal
connector plates tie
together chords and web
members and distribute
and transfer loads
between adjacent
members. The teeth are
very sharp and should be
handled carefully.
7. Unit 50 — Roof Trusses
Hinge plates may be used as an
alternative to piggyback trusses.
The top chords are aligned at
the peak and nails are driven
through the prepunched holes.
8. Unit 50 — Roof Trusses
The balance of tension
and compression gives
a truss its ability to carry
heavy loads and cover
wide spans.
9. Unit 50 — Roof Trusses
A tight fit between truss members is essential to the
structural integrity of the truss.
10. Unit 50 — Roof Trusses
During manufactured
truss assembly, metal
plate connectors are
pressed into truss
components.
11. Unit 50 — Roof Trusses
When framing
intersecting roofs,
valley trusses are
installed over the
roof sheathing. Hip
roofs can be
framed using a
Dutch hip system
where truss jack
rafters are attached
to a hip truss.
12. Unit 50 — Roof Trusses
Light trusses can be positioned
by hand after they are placed
upside down between the
exterior walls of a building.
13. Unit 50 — Roof Trusses
Longer and heavier
trusses should
always be hoisted
by crane in an
upright position.
14. Unit 50 — Roof Trusses
Metal spacers such as these are used to properly
space trusses and are not designed to be used as
bracing. Sheathing is applied directly over the spacers.
15. Unit 50 — Roof Trusses
Truss anchors and
hangers are used to
attach trusses to other
structural members.
16. Unit 50 — Roof Trusses
For one-story buildings, the first
truss is placed along the double
top plate and is secured with
vertical, horizontal, and lateral
ground braces. For multistory
buildings, ground braces are
attached to the subfloor.
17. Unit 50 — Roof Trusses
Temporary diagonal
braces should be
installed across the
top chords of the
trusses at least every
30′ starting from one
end of the building.
Temporary lateral
braces are installed
at 8′-0″ intervals.
18. Unit 50 — Roof Trusses
Permanent metal
braces may be
installed between
roof trusses as
lateral bracing.
19. Unit 50 — Roof Trusses
Roof anchors are
installed to provide an
anchorage point for
lifelines of personal
fall-arrest systems.
Editor's Notes
A roof truss is an engineered combination of structural members arranged and fastened in triangular units to form a rigid framework for support of loads over a long span. Ends of the trusses bear directly on the opposing exterior walls. See Figure 50-1.
The basic components of a roof truss are the top and bottom chords and the web members. Web members extend between the top and bottom chords and are tied together with metal plate connectors. The top chords serve as the roof rafters and the bottom chords act as ceiling joists. See Figure 50-2.
Trusses may be placed over most types of walls, including wood-framed, precast concrete, and masonry walls. Trusses can be constructed to form various roof types including gable, hip, mansard, and gambrel. Trusses are also available for intersecting roofs. See Figure 50-3.
A variety of truss designs are available. Common truss designs include the king post, W-type (fink), queen post (fan), K-type (howe), room-in-attic, scissors, piggyback, hip (girder), and vault. See Figure 50-4.
Metal connector plates tie together the chords and web members and distribute and transfer loads between adjacent members. Metal connector plates are manufactured from 16-, 18-, and 20-ga galvanized structural steel that has been machine-stamped to produce small teeth that protrude from the face of the plate. Tooth length ranges from 1/4″ to 1″. SeeFigure 50-5.
Hinge plates are hinged, two-piece metal connectors used as an alternative to piggyback trusses. Hinge plates can be attached to the lower ends of top chords, allowing the chords to fold flat for shipping. At the job site, the top chords are raised into position and another hinge plate is fastened at the peak to join the adjoining top chords together. See Figure 50-6.
Each truss component is in a state of tension or compression. Components in a state of tension are subjected to a pulling force. Components in a state of compression are subjected to a pushing force. The balance of tension and compression gives the truss its ability to carry heavy loads and cover wide spans. See Figure 50-7.
Trusses are assembled on large tables at a manufacturing plant using various jigs, fixtures, and holddowns. As the truss components are precut, they are laid out on the assembly table and secured in position. A tight fit between truss members is required for structural integrity of the truss. To ensure tight fits, the cuts must be accurate. See Figure 50-8.
Metal plate connectors are then properly positioned and pressed into the truss chords and web members. The metal connector plates are carefully positioned. See Figure 50-9. A large gantry press runs along rails on the sides of the assembly table, pressing the plates into the truss components. The truss then passes through a set of rollers that compress the connector plates a final time before shipment.
Gable roofs require only one type of truss and are the easiest type of trussed roof to construct. Hip roofs are framed using a combination of trusses and conventional framing or by using a Dutch hip system. When framing intersecting roofs, the most efficient method is to place the valley trusses on top of the sheathing. See Figure 50-10. Before trusses are installed, all walls must be aligned and properly braced.
Smaller and lighter trusses can be placed by hand on one-story buildings. The trusses are placed upside down between the walls. Using lifting poles with a V-shape at one end, one or two carpenters upright the truss and position it. Other carpenters on ladders along the exterior walls toenail the ends of the bottom truss chord to the top plate. If two poles are used to upright the truss, the poles should be positioned close to the quarter points of the span. If one pole is used, it should be placed at the peak when raising trusses. See Figure 50-11.
Longer and heavier trusses are hoisted by crane into the upright position. See Figure 50-12. If a truss is laid flat while being placed, lateral bending and jarring can place a strain on the connections and cause plates to loosen and pull away from the wood members. Bracket scaffolds may be attached to the inside of exterior walls so carpenters can fasten the trusses to the double top plate. For large commercial trusses, an aerial lift may be used along the inside of opposite exterior walls.
Metal truss spacers are a fast and accurate method for spacing trusses and eliminate the need to mark the top plate before placing trusses. See Figure 50-13. Truss spacers can remain in place under the roof sheathing.
Metal anchors, hangers, and angles are used to attach trusses to the tops of framed walls and to one another. See Figure 50-14. Metal anchors are used to attach the heels of a truss to the top plate of a framed wall. Truss hangers are used to attach trusses to other trusses such as when attaching hip jack rafters to hip rafters.
Trusses are erected starting at one end of a building and moving toward the other end. When the first truss is placed in a one-story building, the truss must be securely braced to the ground. In multistory buildings, truss braces are fastened to the subfloor. Vertical and diagonal braces should be installed every 8′, with one end nailed to the top chord and the other end nailed to a horizontal tie or stake driven into the ground. See Figure 50-15. Lateral braces are installed for additional stability.
After the first truss is properly braced, the remaining trusses are placed and properly braced. A 2 × 4 cut to22 3/8″ long can be used to space the trusses at24″ OC. Starting at the heel end, 2 × 4 lateral braces are installed at 8′ intervals along the top chords. See Figure 50-16. Each lateral brace should extend over four or five trusses and be fastened to the top chord of each truss with two 16d nails. The ends of the lateral braces should overlap at least two trusses. Bottom chords should also be braced with 2 × 4s nailed to the chords at 15′ intervals. Diagonal braces can also be installed across several trusses every 30′ starting at one end of the building. Diagonal braces extending from each corner of the building and nailed to the bottom chords are also recommended.
Where permitted by the local building code, permanent metal lateral braces may be installed between roof trusses rather than lateral top chord braces. See Figure 50-17. Teeth in the flanges at both ends of the brace are driven into the sides of the truss using a hammer. Permanent metal braces must be supplemented with the same diagonal bracing as used with wood top chord braces. When the braces are in place, roof sheathing can be installed directly over the braces and top chords.
When a portion of a trussed roof is properly braced and sheathed, a roof anchor and personal fall-arrest system may be used when installing the remaining roof trusses. Several designs of roof anchors are available. See Figure 50-18. Some roof anchors are made of metal and are attached to braced trusses or framing members with nails or screws. A D-ring provides an attachment point for a lifeline.
