Roof Structure

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Structure
A considerable amount of new construction utilizes manufactured trusses.  These trusses are held together with gusset plates pressed into the wood, and consist of various web chords that are arranged to adequately support the roof system. Typically, these web chords are made from 2x4s.  Some problems related to roof trusses are due to the way that they are stored and handled. Trusses should be stored in a vertical position. Typically, they are laid flat on-site.  Any broken or missing member of a truss renders it ineffective, and because trusses are dependent on the accumulated strength of the truss components, the integrity of the roof system, or at least that truss could be compromised.  Truss repairs are relatively easy to design, but for credibility and liability reasons, a structural engineer should be consulted to design any truss repairs or modifications.

Sheathing
The type of material used on the surface is a factor in determining the type of sheathing necessary. Wood shakes, clay, ceramic and cement tiles often have nailing boards upon which the roofing material is hung or fastened.  Older houses may have one-inch dimensional lumber for sheathing.  In the 1940s and 1950s, plywood became the material of choice.  The plywood should have spacers between the sheets.  Plywood should not be used as sheathing for wood shakes, as it may not allow them to breathe and could shorten their life.  However, solid sheathing is required in areas that receive considerable amounts of snow, for shake roofs.

FRT Plywood
Fire Retardant Plywood is a material used in townhouse construction starting in the early 1980s. The purpose of the Fire Retardant Plywood was to eliminate parapet walls required between units.  One indication that a building may have Fire Retardant Plywood is that the walls separating the unit are constructed with fire rated drywall.  If Fire Retardant Plywood turns a coffee or darker color, it may be delaminating/deteriorating.  Care should be taken when inspecting the roof. It is not recommended that you walk on a roof with Fire Retardant Plywood until you have determined that it is structurally adequate.

Flashings
Flashings nailed into a wall or chimney and covered with tar or asphalt roof cement are not adequate. Flashings should be mechanical and permanent. The use of asphalt roof cement or tar for flashing is considered a “temporary repair.”  If caulk or roof cement is used as flashing for a new roof, it should be considered unacceptable or not dependable. 

Roof valleys can be either metal or woven shingles.  The woven shingle valleys have a tendency to wear more rapidly than metal (aluminum).

 

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