Roof Purlin And Truss Spacing

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.

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. Purlins are dimensional lumber that are fastened to the top chord of trusses connecting them together for bracing purposes. For the lighter material such as lightweight aluminum the common spacing length is 4 inches of clear gap. The typical spacing standards for residential and commercial buildings are about 1 to 2 foot.

The economic spacing of trusses means the spacing at which the overall cost of trusses purlins roof cover columns etc. The roof truss structure and design is integral to roof structural integrity and shape. For large spacing of trusses purlins will work out to be heavy and costly. Standard gage are generally used for roofing.

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. Installation of pole barn purlins. 4 12 slope and 2 6 rake purlins 1 13 16 to first rake purlin spacing only and draw marks on truss top accordingly. 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.

The spacing of purlins on roofs and girts on wall is usually 4 to 6 ft. Numbers 20 and 22 u s. Load tables specifying safe loads for different spans are available from metal deck manufacturers. It is understood that purlins are spaced where laps and splices of roof sheets that vary in sizes.

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. Using table 52 2 add length shown example. Also important to note here is that a two inch in a. The spacing of purlins usually call for careful arrangement in the sense that it should follow the nodal pattern of the supporting trusses.

The purlin are ultimately used to fasten the roof steel providing a diaphragm effect along with the siding when properly engineered and installed. Balance of rake purlins will be on standard purlin spacing from the first pair of marks. For heavier material such as gage steel the metal roof purlins spacing length could go up to 4 feet. Spacing is typically 24 on center in low snow loads and is reduced based upon truss span and snow load.

This article and series of illustrated diagrams shows you all the parts of a basic roof truss king truss and queen truss. Here is the formula for calculation of a roof purlin.