07:19PM | 09/03/05
Member Since: 09/02/05
4 lifetime posts
I am building a cabin and have been told that I need to design with a 100 lb. snow load. Because of the location of this cabin I cannot get pre-engineered trusses and must build the roofing system on site. I am considering using TJI roof trusses. I am not sure which provide me with the 100 lb. live load capacity. I will be placing them in a vaulted room that is 24' wide. I am planning to install a roof beam in the center, probably using a 8/12 or 9/12 pitch. Wall construction will be 2x6, 16 o.c. Can anyone please tell me which TJI will meet this load and span criteria? Thanks for your help.


02:42PM | 11/19/12
Member Since: 11/19/12
1 lifetime posts
You should use open web rafters / joists, with an air space above the insulation or trusses (hand-built on site is not that difficult). This will allow transverse flow of air from rafter bay to rafter bay, and to the exit vents near the ridge.

It would be difficult to get exit ventilation into each rafter bay at the ridge, with a roof beam as you describe. So, closed web joists would create a problem for ventilating the individual rafter bays. The IRC (and IBC, too) require eave to ridge ventilation of each rafter bay in closed web, vaulted ceiling construction.

It is almost certain that moisture will condense and accumulate in the roof assembly. Ventilation is necessary to allow moisture to evaporate. Otherwise, your roof structure could be susceptible to wood decay (rot) and rust.

A local structural engineer should size the rafters or trusses according to the requirements for your area. I presume that you have done your research and determined that the critical factor is the 100 lb snow load. Are you certain that wind or other forces do not control? Sometimes combined (snow and wind) loads control the design. Structural engineers are generally not very expensive and know the requirements in their local areas well.

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