Pitched Roofs

Roof trusses are most often pre-assem-bled, although they can be constructed on site. Pitched roofs can also be stick-built, although this is a

Types of prefabricated roof trusses.

Roof Trusses With Dwarf Wall
| mono-pitch f f parallel chord
Mono Pitch Truss

time-consuming process. Of the pitched roofs, the gable roof is the simplest to construct, especially with the use of lightweight roof trusses (Fig. 48). Other configurations, such as the hip roof and L-shape roof, though more complex, can also be framed with trusses (Fig. 49).

Pre-assembled Roof Trusses

Pre-assembled roof trusses offer many advantages in that they save material and speed up the process of enclosing the house. They provide, in one step, a surface for the roof sheathing, a surface for the ceiling finish material and a space for insulation. Ventilation

Temporary bracing of roof trusses

Note: Gable ends are normally sheathed prior to installation of end braces

of the attic space is easily accomplished through the eaves or gables or both, and at or along the ridge. In most cases, trusses are designed to span from exterior wall to exterior wall with no intermediate loadbearing walls to support the roof loads (Fig. 50). Thus, the entire house may be used as one large workroom during construction. This increases the flexibility of interior planning, because partitions can be placed without regard to structural requirements. Additional flexibility and speed is gained with pre-assembled components and add-on features such as garage roof trusses, porch roofs, simulated mansards and window canopies, which can also be supplied by the truss manufacturer.

Metal-plate-connected trusses can be delivered to the construction site and placed on a flat, clean portion of the site. Trusses under a 20 ft. (6 m) span are usually installed by hand.

Trusses longer than 20 ft. (6 m) require special lifting techniques to avoid damage.

The trusses must be lifted into position with care to prevent excessive lateral bending. The first to be put in place is the gable truss, which is braced to the ground and wall. Each additional truss is lifted into position, generally 24 in. (600 mm) on centre, toenailed to the top plates and temporarily braced (Fig. 51). When all trusses are plumbed and properly positioned, they are braced permanently (Fig. 52). The stiffness of the roof is increased after the sheathing has been applied.

Site Assembly of Pitched Roofs

For on-site construction, the simplest roof is the gable roof (Fig 53A). All rafters are cut to the same length and pattern, and erection is straightforward. A variation of the gable roof may include dormers for additional light, head room and ventilation (Fig. 53B and Fig. 53C). However, openable windows and fixed skylights that can be fitted on a slope between rafters will provide ventilation and light without the complexity and cost of framing a dormer. In the hip roof design, shown in Figure 53D, common rafters are fastened to the ridge board while hip rafters supply the support for the jack rafters.

Important considerations in framing a liveable attic space are insulation and proper air and vapour barrier sealing; these techniques are

Permanent bracing of roof trusses: (A) permanent bracing of top chord plane; (B) and (C) permanent lateral bracing to web member or bottom chord plane.

top chord ridge line diagonal brace nailed to web — repeat at approximate 20' (6 m) intervals as required by truss manufacturer

2" x 3" (38 x 64 mm) or 1" x 4" (19 x 89 mm) web bracing as required by truss manufacturer

2" x 3" (38 x 64 mm) or 1" x 4" (19 x 89 mm) web bracing as required by truss manufacturer

Diagonal Bracing Techniques

sheathing blocking lateral brace diagonal forms braced bay - repeat at both ends and at approximate 20' (6 m) intervals as required by truss manufacturer ceiling

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dealt with in the chapters on Thermal Insulation and Vapour and Air Barriers. The choice of framing members for structural strength, based on Tables 24 to 27, may not provide sufficient depth for insulation and necessary ventilation space. Larger size members or a modified framing technique will be needed to meet current standards.

Celling Joists are used to support the ceiling finish and to act as ties between exterior walls and, in some cases, opposing rafters. They may also provide support for roof loads transferred to them by dwarf walls (knee walls) used as intermediate support for rafters, in which case they need to be appropriately Increased in size. (See Table 28 for ceiling joist spans.) When the joists also support floor loads, their size should be determined by the floor joist tables. (See Tables 15 and 16.)

In pitched-roof framing, the ceiling joists are nailed in place after the interior and exterior wall framing Is complete but before the rafters are erected, because the thrust of the rafters will otherwise tend to push out the outside walls. Ceiling joists are generally used to tie the lower ends of the rafters in pitched roofs that slope 1:3 or more. To prevent the rafter ends from moving outward, the ceiling joist is nailed to the side of each pair of rafters (Fig. 54). The joists are lapped and nailed together or spliced at the centre loadbearing wall, thus providing a continuous tie across opposing rafters. The number of nails used in the connections depends upon the roof slope, rafter spacing, snow load and width of the house. (See Table 19 for nailing practice.)

The additional roof load imposed by dwarf walls that run at right angles to the ceiling joists (Fig. 55) should be taken into account when the size of the joists are determined. An increase in the depth of the joists to the next standard depth will usually provide the additional strength required where the roof slope is more than 1:4. When the slope is 1:4 or less, the size of the ceiling joists is determined from span tables for roof joists (Tables 24 and

Since hip rafters are about 2 in. (51 mm) deeper than the common or jack rafters, they reduce the space along the end walls to the extent that in low-slope roofs, there may not be enough room to install the outside ceiling joist at normal spacing from the wall. In this case, doubled joists are used and positioned to suit the available space (Fig. 56). Tail joists are then added and toenailed to the outside wall plate and end-nailed to the doubled joist. The spacing of these tail joists is usually the same as the spacing of the main ceiling joists.

Rafters are cut to length with the proper angle cut at the ridge and eaves and with notches (known as birdmouths) provided for the wall or rafter plates. The heel or lower part of the rafters should bear directly over the exterior wall. Depending on the plan of the roof and the shape of the outside walls, the rafters are placed:

• on a rafter plate nailed to the top of the ceiling joists (Fig. 55), or

• on a loadbearing wall supported on the exterior wall plate (Fig. 57).

Notch Rafters

The last method is used where a portion of the outside wall is set back. In this case, the ceiling joists extend beyond the exterior wall and are nailed to the side of the rafters. This provides lateral support for the bearing wall and resists outward and downward movement of the ends of the rafters.

A ridge board (Fig. 54) or a ridge beam (Fig. 55) is used to ensure a level ridge line and for ease in erecting and aligning the rafters. Rafters are erected in pairs and nailed to the ridge board or beam. The lower ends are toenailed to the wall plate. Each pair of rafters is usually located directly opposite each other. However, they may be offset at the ridge by their own thickness. This offsetting is

00 ceiling and roof framing with ridge board: 0) each rafter toenailed to ridge board with four 2 «/* in. (57 mm) nails or end-nailed with three 3 '/<, in. (82 mm) nails; (2) 1 x 4 in. (19 x 89 mm) strip nailed to top of collar braces at their centre with two 2 1/4 in. (57 mm) nails when the braces are more than 8 ft. (2.4 m) long; (3) ceiling joists butted with splice plate over centre bearing partition. Joists also nailed to each part of rafters (see Table 27 for nailing practice); (4) collar brace used as intermediate support for rafters nailed to each pair of rafters with three 3 in. (76 mm) nails at each end; (5) ceiling joists toenailed to top wall plate with two 3 1/4 in. (82 mm) nails, one each side; (6) rafter nailed to plate with three 3 >/4 in. (82 mm) nails;

(B) jack rafter nailed to hip rafter with two 3 '/4 in. (82 mm) nails.

Gable End Porch Pictures

ridge board rafter collar brace collar tie gable end stud ceiling joist top wall plate

Collar Brace ConstructionRafter Ridge Board Nailing
corner post

Rafter heel supported on a rafter plate nailed to top of each ceiling joist with two 4 in. (101 mm) nails.

2" x 4" (38 x 89 mm) rafter plate rafter exterior wall knee wall with 2" x 4" (38 x 89 mm) top and bottom plate and 2" x 4" (38 x 89 mm) studs in line with each ceiling joist and rafter

2" x 4" (38 x 89 mm) strut at 4' (1.2 m) o.c.

ceiling joists lapped directly above interior loadbearmg wal rafter

Ceiling Joist Span Table

ceiling joists lapped directly above interior loadbearmg wal

Doubled ceiling joists and tail joists used where hip rafter reduces clear space near the end wall.

Easy Hip Rafter Connection Patio

2" x 6" (38 x 140 mm) jack rafter 2" x 8" (38 x 184 mm) hip rafter ceiling joist

2" x 6" (38 x 140 mm) double ceiling joist normal location for ceiling joist if space is available

2" x 4" (38 x 89 mm) tail joist end wall framing top plate

2" x 6" (38 x 140 mm) jack rafter 2" x 8" (38 x 184 mm) hip rafter ceiling joist

2" x 6" (38 x 140 mm) double ceiling joist normal location for ceiling joist if space is available

2" x 4" (38 x 89 mm) tail joist end wall framing top plate required to maintain vertical alignment of the rafters when the lower ends are tied to ceiling joists that have been lapped (rather than butted together) at the centre loadbearing wall (Fig. 55).

A roof which slopes less than 1:3 should be vertically supported at the peak. This can be accomplished by providing a 2 x 6 in. (38 x 140 mm) ridge beam, supported at 4 ft. (1.2 m) intervals by 2 x 4 in. (38 x 89 mm) vertical struts (Fig. 55). A loadbearing wall may be used instead of the ridge beam. Since these methods of support reduce the outward thrust of the roof, continuous ties between the lower ends of opposing rafters are not necessary. Ridge beams are also required for steeper pitched roofs when the outside ends of the rafters cannot be tied together to resist thrust.

Intermediate Support for Rafters is generally installed between the ridge and exterior walls to reduce the span of the rafters. This reduces the size of the rafters which are required as the span is taken from this intermediate point to the ridge or eave support.

For rafters in roofs which slope 1:3 or more, Intermediate support is generally provided by a 2 x 4 in. (38 x 89 mm) collar brace nailed to the side of each pair of rafters. Since these

Rafter heel supported on loadbearing wall. Ceiling joists project beyond the wall line and are nailed to the rafters (see Table 19 for nailing practice). Roof struts 2x4m. (38 x 89 mm) used as intermediate support for rafters. Struts are nailed to the side of the rafter with three 3 1/4 in. (82 mm) nails and toenailed to bearing wall with two 3 '/u in. (82 mm) nails.

3/4" (19 mm) thick ridge board intermediate support 2" x 4" (38 x 89 mm) struts to be set not less than 45° to the horizontal studs in loadbearing wall located directly under each rafter

2" (38 mm) thick splice plate interior bearing wall roof rafters notched and bearing directly on top plate of exterior wa:

2" x 4" (38 x 89 mm) continuous nailer and soffit supports ceiling joists bearing on double plate wall sheathing exterior wall

Rafter heel supported on loadbearing wall. Ceiling joists project beyond the wall line and are nailed to the rafters (see Table 19 for nailing practice). Roof struts 2x4m. (38 x 89 mm) used as intermediate support for rafters. Struts are nailed to the side of the rafter with three 3 1/4 in. (82 mm) nails and toenailed to bearing wall with two 3 '/u in. (82 mm) nails.

3/4" (19 mm) thick ridge board intermediate support 2" x 4" (38 x 89 mm) struts to be set not less than 45° to the horizontal

2" (38 mm) thick splice plate interior bearing wall roof rafters notched and bearing directly on top plate of exterior wa:

2" x 4" (38 x 89 mm) continuous nailer and soffit supports ceiling joists bearing on double plate wall sheathing exterior wall

Lapped Rafter

studs in loadbearing wall located directly under each rafter braces are in compression and subject to buckling, they should be supported against lateral deflection when more than 8 ft. (2.4 m) long. This can be done by nailing a 1 x 4 in. (19 x 89) mm continuous member at right angles to the collar braces near their centre with three 3 in. (76) mm nails at each end (Fig. 54).

Intermediate support for rafters in roofs which slope less than 1:3 is usually provided by a dwarf bearing wall (Fig. 55) built in the same way as a loadbearing partition, except that a single top plate may be used where the rafters are positioned directly over the studs.

Struts may also be used as intermediate supports for rafters in pitched roofs. A 2 x 4 in. (38 x 89 mm) strut (Fig. 57) is nailed to the side of each rafter and supported on a loadbearing partition. The angle of the struts should not be less than 45° to the horizontal.

Rafters which run at right angles to the ceiling joists may be supported at an intermediate point by a dwarf wall sitting on a beam placed between the ceiling joists. The underside of the beam is raised at least i in. (25 mm) above the ceiling finish by blocks inserted under the ends of the beam at the exterior walls and centre load-bearing partition. The space thus formed prevents the beam from damaging the ceiling finish when deflected at its centre by the roof loads.

A beam similarly installed may also be used as intermediate support for hip and valley rafters. In this case,

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Responses

  • giulia
    How to put a pitched roof on?
    8 years ago
  • milla
    How do you splice a ceiling joist?
    8 years ago
  • Alma
    How rafter ceiling joist connect?
    8 years ago
  • tiia
    How to assemble corner of gable porch?
    6 years ago
  • Phillipp
    Can i cut curve into a 230 x 45 mm rafter?
    6 years ago
  • annabella ferri
    What is a collar tie in construction?
    6 years ago
  • charlotte
    Is it necessary to put wall plate on every loadbearing wall in the constructon of roofs?
    6 years ago
  • medhanie
    What is rafter and brace due to pitched roof construction?
    2 years ago

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