OHSA designates three different categories for load bearing calculations on scaffold boards. They are:
Light Duty – 25 pounds per square foot.
Medium Duty – 50 pounds per square foot.
Heavy Duty – 75 pounds per square foot.
These ratings are based on the intended load, which is calculated by dividing the total weight to be placed between two spans of scaffolding by the total area between the two spans. Most of our customers fall in the medium-duty category so this is what will be addressed in this post. In most masonry applications, 5′ wide frames are spaced 7′ apart and use nominal thickness 2X10 lumber for the planks (nominal thickness 2X10 lumber actually measures 1.5” X 9.25′). In this setup, there is approximately 35 sq. ft. of area between the frames.
If you apply the 50 pounds per square foot maximum rating limit to the standard 7′ fully-planked span, a maximum of 1,750 pounds may be applied to the platform. Using full thickness (solid sawn) lumber, this weight is not a problem, however, full thickness lumber is hard to find so most masons use nominal thickness lumber now. According to OSHA, using the typical 7’ span between the frames with the 2X10 nominal thickness lumber will not achieve the 50 pounds/square foot rating in terms of board capacity. The maximum allowable span between the frames with this type of plank is 6’. Here is the link to this information: http://www.osha.gov/SLTC/etools/scaffolding/planking.html.
This limitation makes a good case for using manufactured boards/platforms instead of the 2X10 nominal boards since the industry standard is using a 7’ span. We sell both steel and aluminum 7′ and 10′ platforms that are rated for both 50 lbs/sq. ft. and 75 lbs/sq ft. Another solution would be to double your planks across the spans that will be receiving cubes of brick or block. Also, consider that when you are using outriggers, the weight of your workers on the outrigger platforms will not be applied to the main platform.