Roof Purlin And Truss Spacing

For small spacing of trusses while the purlins become light and less costly the trusses will work out to be costly.

Roof purlin and truss spacing.

The economic spacing of trusses means the spacing at which the overall cost of trusses purlins roof cover columns etc. For heavier material such as gage steel the metal roof purlins spacing length could go up to 4 feet. Here is the formula for calculation of a roof purlin. Typical roof truss spacing means that they are usually about 24 inches from centre which means at the centre of one truss there will be a two foot gap or twenty four inch gap until the centre of the next truss.

There are many types of roof trusses as wall as many types of roofs more types than there are parts. The typical spacing standards for residential and commercial buildings are about 1 to 2 foot. Purlins are dimensional lumber that are fastened to the top chord of trusses connecting them together for bracing purposes. What i mean in this regard is that purlins should be placed at the nodes of trusses and not on the members themselves so as not to induce secondary bending and shear forces in the members of the truss.

Numbers 20 and 22 u s. Installation of pole barn purlins. In addition each roofing truss will be made of a two by four piece of lumber or in areas with expected or usual heavy snow fall there will be trusses which are two inches by six inches. Standard gage are generally used for roofing.

The spacing of purlins on roofs and girts on wall is usually 4 to 6 ft. For large spacing of trusses purlins will work out to be heavy and costly. Further to be considered is the roof framing system as a whole be it truss or rafters wood or steel as the case may be is a factor how purlins are installed at calculated distances apart. Also important to note here is that a two inch in a.

For the lighter material such as lightweight aluminum the common spacing length is 4 inches of clear gap. Spacing is typically 24 on center in low snow loads and is reduced based upon truss span and snow load. The roof truss structure and design is integral to roof structural integrity and shape. The purlin are ultimately used to fasten the roof steel providing a diaphragm effect along with the siding when properly engineered and installed.

4 12 slope and 2 6 rake purlins 1 13 16 to first rake purlin spacing only and draw marks on truss top accordingly. Using table 52 2 add length shown example. It is understood that purlins are spaced where laps and splices of roof sheets that vary in sizes. These standards are as follows.

Cos of live load dead load x cos of the purlin spacing x purlin span squared 8 x sm section modulus of the member x duration of load 1 15 in areas with snowfall x cr repetitive member factor of 1 15 where members are 24 on center or less fb fiberstress required. The spacing of purlins usually call for careful arrangement in the sense that it should follow the nodal pattern of the supporting trusses.