Construction Purlin Flat Roof

Metal roof purlins are installed horizontally under metal roofs.

Construction purlin flat roof.

Purlins are 2 by 4 feet and are installed much like metal roofing. Or less x 1 1 cfu the flat use factor for applying the load to the wide face of a member. They are installed on top of the roof rafters with a felt underlayment or vapor barrier installed on top. 8 x section modulus of a 4 2 of 1 3125 x fb fiberstress in bending x 1 15 duration of load x 1 15 cr repetitive member factor as the purlins are spaced 24 o c.

Purlin plates are beams which support the mid span of rafters and are supported by posts. Roof live load roof dead load x spacing x span of purlin 2. The higher the snow load the closer the purlin spacing required. The term plate means a major horizontal supporting timber.

Too high a snow load means the purlins fail regardless of how close together you put them because a 2 4 or 2 6 flat can only span a certain distance between trusses as snow loads increase. The common 4 on center design is suitable for snow loads up to 60 for most applications. Also known as sheeting rails purlins create the roofing members of the secondary framing of a steel building. These structural supports run perpendicular across the rafter beams.

The purpose of purlins. By supporting the rafters they allow longer spans than the rafters alone. Purlin plates in wooden construction. Metal building roof purlins provide three structural functions.