Using the Roof Live Load Reduction

Consider a roof with a live load of 20 psf (pounds per square foot). This load is intended to account for construction loading (workers, materials, and equipment placed on the roof during construction or maintenance). If the roof has a large surface area, like a post-frame building, is it really likely that the entire roof area would be loaded with 20 psf of load at the same time? If we could design with a smaller roof live load, we could build more efficient and cost-effective trusses.

Because a large roof area will likely never experience the full design load across its entire area, the roof live load may be reduced using the ASCE live load reduction. ASCE 7, Minimum Design Loads for Buildings and Structures, is a standard published by the American Society of Civil Engineers. This document, representing a consensus of engineering judgment, provides the minimum load requirements that Truss Designers, Truss Engineers, and Building Designers must use when designing truss components. One of the provisions of ASCE 7 is the uniform roof live load reduction, which allows a Truss Designer to use a smaller roof live load when designing a roof truss with a large tributary area.

Tributary area is the area that a particular truss (or other structural component) is responsible for supporting. A common roof truss, spaced at 2 ft. on-center, with a 40 ft. span, has a tributary area of 2 * 40 = 80 ft², while a 7 ft. jack truss has a tributary area of only 14 ft². A 60 ft. pole barn truss, spaced at 6 ft. on-center, has a tributary area of 360 ft².

Calculating the ASCE Roof Live Load Reduction

The formula for finding the reduced roof live load is fairly simple. The nominal roof live load, L_o, is multiplied by two reduction factors, R₁ (based on the tributary area of the truss) and R₂ (based on the roof slope). The result is L_r, the reduced roof live load.

L_r = L_oR₁R₂                 where 12 ≤ L_r ≤ 20 psf

L_r = reduced roof live load per ft² of horizontal projection supported by the truss.

L_o = unreduced design roof live load per ft² of horizontal projection supported by the truss.

            1                                  for A_T ≤ 200 ft²

R1 =    1.2 – 0.001 A_T             for 200 ft² < A_T < 600 ft²

0.6                               for A_T ≥ 600 ft²

A_T = tributary area in ft² supported by the truss.

1                      for F ≤ 4

R2 =    1.2 – 0.05 F     for 4 < F < 12

0.6                   for F ≥ 12

where F is the slope.

Let’s look at an example. Consider a 50 ft. roof truss for a post-frame building. The truss is spaced 6 ft. on-center, and the slope is 4/12. First, the tributary area A_T is calculated as:

A_T = 50 ft. * 6 ft. = 300 ft².

Since this is greater than 200 ft² but less than 600 ft², we use the second equation for R₁:

R₁ = 1.2 – 0.001 A_T = 1.2 – 0.001 * 300 = 0.90

Now we calculate R₂, the reduction factor for roof slope. Since the slope (F) equals 4, R₂ is equal to 1. The code-required roof live load (L_o) is 20 psf. Our reduced live load (L_r) would then be:

L_r = L_oR₁R₂ = 20 psf * R₁ * R₂ = 20 * 0.90 * 1 = 18 psf

So, we could design this truss for 18 psf instead of 20 psf.

Now let’s consider a residential roof truss with a 30-ft. span, 2-ft. on-center spacing, and a roof slope of 8/12. The tributary area is less than 200 ft² (30 * 2 = 60 ft²), so R₁ is 1. Because the slope is between 4 and 12, the reduction for slope, R₂, is:

R₂ = 1.2 – 0.05 F = 1.2 – 0.05 * 8 = 0.8

The reduced roof live load (L_r) is:

L_r = L_oR₁R₂ = 20 psf * 1 * 0.8 = 16 psf

So, this residential roof truss could be designed for 16 psf roof live load instead of 20 psf.

There are a few important points to remember. First, this reduction can’t be used if the specified roof live load is greater than 20 psf. Some building departments may require a roof live load of 30 psf, so you couldn’t use this reduction in those jurisdictions. Second, the slope reduction factor only applies to the sloped section of a truss. So, if you have a mansard truss (like a fast-food restaurant) with sloped sections at the ends of the truss but flat in the middle, you couldn’t use the slope reduction on the entire span. If you have a hip truss, the flat section isn’t really flat; it’s sloped in the transverse direction (perpendicular to the plane of the truss). For this section of the truss, you must use the slope of the hip to calculate the slope reduction factor.

Another important point is that roof live load is not the same as snow load. Roof live load is intended to account for construction loading (including maintenance like re-roofing). The building codes generally require a roof live load of 20 psf. Snow load is intended to account for snow on the roof and can be as low as 5 psf or higher than 50 psf, depending on the local climate. If snow loading is required in your area, follow the load combinations in the applicable building code.

IBC & IRC Roof Live Load Reduction

If you are designing using the IBC (International Building Code), not the IRC (International Residential Code), you must use the formulas given above to calculate the reduced roof live load. If you’re designing residential buildings with the IRC, you must use Table R301.6, Minimum Roof Live Loads in Pounds-Force per Square Foot of Horizontal Projection, shown below:

IRC Table R301.6   Minimum Roof Live Loads in Pounds-Force per Square Foot of Horizontal Projection.

For additional information, or if you have questions, please refer to ASCE 7, Minimum Design Loads for Buildings and Structures, or contact the MiTek Engineering department.