Rafter & Truss

By the Orillia Home Inspector

Certified Building Code Official


Roof Support

When roof systems are not designed or installed properly deflection can occur. Deflection is defined as the bending of wood due to forces, or loads, placed on them. There are five (5) different types of loads that any structure must be able to withstand: dead load, live load, shear load, point load, and spread load. Some of these forces are the natural result of gravity tugging at the building, dragging it down. Some are the result of wind gusts, snow loads, or ground movement.

When a roof is inadequately supported, the main ridge board at the peak of the roof, is pushed downward and the roof rafters push the exterior walls outward. Not only will the roof from the outside look unsightly, but also cracks will form on the walls and ceilings, and perhaps windows and doors will become out of square preventing them from operating correctly. Dealing with the various loads that are placed on a house actually means preventing deflection. Proper building and design practices can prevent the negative effects on the house by loads.

Truss Construction

The building's roof must be designed and constructed to fulfill the following functions:  the roof must be able to transfer wind and snow loads to the house walls;  the roof must be able to shed rain and snow away from the building and prevent water from penetrating into the roof area;  the roof must be also able to provide for the removal of moisture and heat contained in the insulated roof assemblies;  and accommodate the installation of materials designed to restrict the flow of the air and moisture from the house into the roof assembly.  

Roof trusses or roof rafters are used in many applications such as new construction or add on additions. Roof trusses can save up to 25% in construction time and labor costs.

Another advantage to roof trusses is that they typically span from exterior wall to exterior wall allowing removal or reloation of interior partition walls.


Rafter Construction

Rafters are found on steep roofs (slope of more than 2 in 12). Rafters are typically 2 by 4's, 2 by 6's, 2 by 8's or 2 by 10s. The rafters support the sheathing and typically run from the roof peak down to the eaves. The rafters carry the dead load of the sheathing and roofing material, and the live loads above. Rafters may also carry the ceiling loads below on cathedral ceilings.

Rafters are similar to trusses as they are designed to carry the live loads of wind, rain, snow and people. They also carry the dead loads of insulation, sheathing, shingles and ceiling finishes.

Always remember that trusses are engineered and if you intend to use them for any other use other than the one designed for you will require a stamped detail from engineer or designer.


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Inspection Definitions

1. Chimney - vent flue gases from fireplaces or heating equipment.
2.  Chimney flue top or chimney cap (if present)
3.  Chimney crown or chimney top seal
4.  Chimney Flashing seals the roof penetration to avoid leaks into the structure.
5. Masonry fireplace
6. Fireplace ash pit door. 
7.  Fireplace ash pit cleanout door.
8.  Fireplace mantel - horizontal trim attached to wall above fireplace opening.
9. Hearth - flat surface in front of the fireplace, protects flooring from fire.
10. Ridge cap or ridge vent (if present)
11. Ridge board
12. Cripple rafters or Jack rafters (between chimney and house eaves - rafters that do not extend the full distance between house eaves and the roof ridge board)
13. Rafter blocking or cross bridging, also found on floor joists and in some wall framing
14. Soffit or lookout or house eaves. The soffit is the enclosed portion of the roof that overhangs the house walls at the roof lower edges.
15. Roof sheathing or roof decking.
16. Roof shingles (asphalt shingles, clay tiles, slates, wood shingles, or shakes, similar materials)
17. Drip edge (used at lower roof edges or eaves). The drip edge is special metal flashing intended to divert water off of the roof lower edges into the roof gutter system. Drip edges should spill into the gutter, not behind it. 
18. Gutter (attached over or to fascia board) to collect roof drainage and prevent it from spilling down and along the building walls (leaks) and basement (wet basements
19. Downspouts (conduct roof drainage from the gutters to a destination away from the building or into a storm drain system).
20.  Downspout leader or downspout extension (hard to see, behind that front right entry porch column)
21. Gable end and gable-end attic vent. The gable end the house wall on a conventional simple gable roof such as shown in our sketch is the triangular end wall
22. Gable end fascia. The gable end fascia is the trim board attached to the roof edges, extending from ridge to lower roof edge, and where a rake overhang is present, covering the outermost rake rafter or barge rafter.
23. Gable end vent or attic vent at gable end