STRAWBALE ROOFS AND ROOFING

Roof Structures

Roofs of most houses are either built of wood rafters or wood trusses. Rafters are cut on site from dimensional lumber and run from the roof peak to the roof eaves. They are supported at the peak by a ridge beam, and at their lower edges by the house walls. Rafters allow the space between the first floor ceiling and roof to be used as a living or storage area. Dormers can be used to create additional useable upstairs area.

Trusses are usually built in a truss factory from dimensional lumber, and are triangulated internally with shorter members to provide strength. Trusses offer the ability to span large distances between walls without requiring supporting walls or posts beneath, within the structure, and are delivered assembled, meaning that installation in the structure is relatively fast. Trusses rest on the structure's walls, and require no ridge beam. The low cost of factory built trusses compared to site-built trusses and rafter/ridgebeam type roofs makes them the typical choice for most builders. The triangulation of trusses has one significant disadvantage: attic spaces cannot be used as living or storage spaces. Scissors trusses are used to create vaulted ceilings while maintaining a space between ceiling and roof that is large enough to contain insulation in high R value amounts.

A third category, being neither rafter nor truss, is the family of engineered wooden I-beams commonly known as TJI's (TJI is a brand name). While rafters and trusses are made from dimensional lumber, and thus impact forests heavily, TJI's are composed of wood fibers of varying lengths and sizes, bonded together with glue. TJI's present an efficient use of trees, and are relatively inexpensive for their strength, but they have the drawback that the glue used is somewhat unfriendly to humans.

Roofing Materials

Common roofing materials include composition (asphalt) shingles; wood shingles and shakes; tiles, and preformed steel roofing. Tiles are used in hot climates as they keep the underlying roofing cooler. Tiles are relatively expensive, come in variety of colors, have relatively high embodied energy (because they are fired), and are very long lived (barring mechanical breakage).

Composition shingles are available in a variety of colors, are common in the temperate zone, and are probably the least expensive roofing alternative. Their lifespan (20-40 years) is much dependent upon the roof pitch (a steeper pitch results in a longer lived roof), and their exposure to high wind.

Wood shingles and shakes are relatively expensive, but can last up to 100 years on moderate to steep pitched roofs. Wood shingles are favored for rustic designs, but are avoided where forest fires are possible, and where roof pitches are low (shallow).

Galvanized and painted steel roofs have high embodied energy, but are long lived, relatively inexpensive, and fast to install. Steel roofs are preferred in areas where the potential for forest and brush fires is high, and in areas of heavy snow fall, as steep pitched steel roofs shed snow loads very well.

Return to Lighthook's Main Page
Return to The Strawbale Structural Components Page

James Lux, January 12, 1996