Publications Article

Safe Scaffolds

Thousands of builders are injured every year as a result of scaffold failures. Typical failures prove that safety is an active choice.

by Paul Fisette - © 1997

I've been lucky. Through 20 years of stretching from ladders, climbing on roofs and bouncing across over-extended staging planks, I stand unscathed. Like most conscientious builders, I always put safety first - usually.

Looking back, I could have easily avoided the few staging failures that occurred on my projects. All failures were caused either by using the wrong material correctly or the right material incorrectly. It's no surprise when I hear experts argue that improper material selection and misuse cause most scaffold failures.

PLANKS
Wooden planks seem to lie at the heart of most residential scaffold failures. In fact, some experts claim plank failures cause injury in 90% of the cases they investigate. Knots and cross grain show up like a bad penny in most failures. But the blame goes to workers unfamiliar with safe and proper use of equipment.

Misuse
Contractors often overload wooden staging planks. Think of the times you have seen a mason standing in the middle of a plank with bricks stacked up to his waist, only to have his tender sling a tub of mortar onto the pile. Or consider side-wallers who use pump staging: how often are the poles spaced less than 10 feet apart(OSHA regulation)? Overloading stresses wooden planks beyond their elastic range, causing them to fail at a later time under lighter loads. The fact that you have used staging planks successfully in the past doesn't assure safe use in the future if the planks have been overstressed.

One builder unsuccessfully sued a material supplier for providing off-grade scaffold planks to his job. A crew member fell from a roof when a plank snapped at mid-span. The worker was permanently injured. As it turned out, the supplier had provided "scaffold grade" planks, yet a plank failed. Particles of gravel were found imbedded in the end grain of the plank during the investigation. It seems the workers had developed a short-cut for dismantling staging: they dropped the planks to the ground. When the planks hit the ground they whipped and caused a partial failure in the plank.

Other cases implicating legitimate scaffold-grade planks have pointed to misuse as the cause of injury. One failure resulted even when OSHA regulations were followed to the letter of the law. Sure, proper material was used, but decay was found when the fracture was microscopically analyzed. Later it was learned that the general contractor stored the planks outdoors under a plastic tarp.

Wooden planks should be stored indoors. They should be stickered so air can circulate around the planks. Wood will rot when relative humidity reaches 80%. Stacking planks outdoors and wrapping them in plastic provides fungi with a cozy environment. Avoid this practice. It is suicidal.

Staging planks often perform secondary tasks when the need calls. I often see planks used as ramps for wheelbarrows loaded with concrete. Later the same planks are used to support workers on a roof or sidewall. A friend told me about a case he investigated where planks served as a runway for trucks driving through a muddy lot. Partial failure was induced in the planks and they collapsed when they were later stressed as a staging element. The injured builders sued their plank supplier, but didn't collect a cent. Negligence caused the injuries. Heavy concentrated loads like these can overstress planks.

Material Selection
Call the lumberyard and order scaffold planks for your next job. If you live in the Northeast, full-sized, rough-milled 2x10 spruce planks are likely to arrive at the jobsite. But don't use them. Planks are dangerous when they are not stamped "Scaffold Grade". The Northeastern Lumber Manufacturers Association (NeLMA), the primary lumber grading agency in the Northeast, does not publish a scaffold grade. Therefore, many builders in this region use #2 spruce as a substitute. Number 2 of any species is not good enough! Other grading agencies like: Western Wood Products Association(WWPA) and Southern Pine Inspection Bureau(SPIB) include scaffold grades in their rules. Insist on "Scaffold Grade" material when you order planks. They should be stamped indicating OSHA's recognition as an acceptable plank.

OSHA regulates the use of scaffold material, but the regulation seems flawed. The regulation says you must use scaffold grade or equivalent. And it is the "equivalent" that might kill you. Technically you can get by with #2 spruce in some applications. Run the design calculations for specific loading applications and #2 spruce might pass. But there are certain characteristics of #2 spruce (or #2 material of any species for that matter) that can prove catastrophic when used as a staging plank.

Scaffold grades restrict critical characteristics in wooden staging planks. For example: Knot size is limited to 1 7/8" in 10-inch wide Scaffold No.2 grade. However, 4 1/4" spike knots are permitted in the #2 Structural Joists and Planks grade. It's not only the knot that provides low strength. Knots are surrounded by weak cross-grain. The next time you look at a plank, notice how the wood's grain bulges in the area around a knot.

If you place large spike knots on the underside (tension side) of a plank, you can bet the plank will break right at one of the knots when it's overloaded. Workers typically walk out to a point on the plank where it creates the maximum bending moment at the defect and bang! The failure is sudden and catastrophic.

All things being equal, the strongest wood is straight-grained wood. Top scaffold grades restrict "slope of grain" to 1" in 20", while #2 grades permit grain angles as steep as 1" in 8". "Slope of grain" relates the wood-fiber direction to the edges of a piece of wood. The deviation from the edge is expressed as a ratio. Slope is based on how long it takes the grain to run inward 1-inch from the edge of a board. (* see figure) Planks with steep grain angles are weak.

Pith and juvenile wood are also allowed in #2 grades. This wood is very light and weak. Scaffold grades prohibit the presence of these features.

Non-graded, rough-milled 2x10 planks should never be used. Why take the chance? Sixteen-foot-long scaffold-grade planks are readily available from local suppliers for about $24.

Trus-Joist Corporation(TJI) provides builders with an attractive alternative. TJI manufactures Micro=Lam, laminated veneer lumber (LVL) scaffold planks. They are stronger and much more predictable than solid-sawn material. Every plank is proof-loaded and OSHA-recognized. There are no knots, cross-grain, juvenile wood or other strength-reducing characteristics to worry about. Be sure to specify scaffold-grade, since TJI also makes less rigid Micro=Lam header stock. A 1 1/2" x 9 1/2" x 16' TJI plank goes for around $32. Aluminum planks are another option, but they are pricey. A 16-foot aluminum plank will set you back $300.

PUMP JACKS
Pump-jack scaffolding is a favorite among builders and remodelers. It is efficient, goes up quickly and adapts easily to out-of-level jobsites. But anyone who has "pumped up" has also wondered: Is the insurance policy paid? The shaky nature of the beast, especially when working 25-feet above grade, should command the ultimate respect from workers. But amazingly, this is not always true.

I've seen staging set-ups where the planks were cantilevered dangerously past the last pump jack and more than once watched as one stretching worker yelled to a partner, "Don't move off the end of the plank while I nail this end of the trim."

There are many jobs where pump poles are left unbraced - or run to heights well above the 30-foot OSHA limit. And workers often cleat planks together to create a wider walkway. Good idea. But why nail the cleats to the top surface of the planks where they can be tripped over? Careless behavior causes most pump-jack accidents.

OSHA insists that pump poles must be spaced less than 10 feet apart when wooden planks are used. Typically, builders purchase 16-foot planks and space the poles 14-feet apart. Seldom are the prescribed guardrails, midrails, endrails and toeboards installed on residential applications. Poles must be braced at the top, bottom and every 10-feet along the height of the pole. And the plank ends must be secured to the pump foot. Have you done this lately?

Poles are a weak link in pump staging. They should be made from doubled Douglas-fir 2x4's (or equivalent) that are straight-grained, clear, and free of cross-grain. OSHA regulations specify that spliced 2x4's must develop the full strength of the unjoined member. Good luck here. The only way to do that is by attaching a long plate to the outside surface of the pole - An unlikely feature on most pump poles.

Improper materials and inadequate joint design cause many pole failures. But lateral buckling of joints is easily avoided. In a typical case, two workers had loaded a metal scaffold plank with roof shingles and pumped up to the eaves of a split-level home. A third worker stepped from the roof onto the plank, rocked the staging and the double 2x4 pole snapped. Shorter lengths of 2x4 had been spliced to make the pole. A knot located in the adjoining 2x4, opposite the butt-joint splice, caused the failure. The knot and weak cross-grain associated with the knot provided little lateral resistance to the unbraced scaffold. Solid bracing absolves many sins. The sub-standard pole could probably have survived if proper crossbracing had been installed.

In a similar case, two painters fell from pump staging attached to the back side of a full-dormered cape. Again, the pole snapped at a joint. The 2x4's used to build the pole were relatively clear and straight-grained. However, the painters did not store the poles responsibly. The poles were left uncovered for extended periods of time. Moisture collected in the seam between the 2x4's and caused the poles to rot.

Contractor, Carl Anderson worried enough about the safety of wooden pump staging that he invented an entire aluminum pump-staging system. [Alum-A-Pole, Corp. 1011 Capouse Ave. Scranton, PA 18509. 800-421-2586] Alum-A-Pole is recognized by OSHA and can be ordered with: poles, joint splines, workbench(with telescopic arms), guardrails, toeboards, plank tie-downs, swivel braces, safety net and aluminum planks. The modular system comes in a variety of sizes. Planks can span up to 24 feet and poles can safely support scaffold platforms to a 50' shoulder working height. A typical 24'high x 24'wide Alum-A-Pole system retails for about $2,200 - complete. A 24' x 48' version costs $3740. It represents a solid investment when you factor in the added level of safety and life-cycle cost.

LADDERS
Driving home from work the other night I spied a construction site and stopped to take some photos. As I walked around the back of the house - there it was, the perfect picture: a carpenter standing on a 16-foot spruce plank supported by a step ladder at each end. He was friendly enough, but no photographs thank you.

Step ladders command little respect and as a result cause many injuries. Most people feel secure using step ladders since they are working at low heights. A friend who has investigated dozens of ladder accidents claims that he has yet to find a defective step ladder as the cause of injury. Rather he finds that step ladders are misused or used in a broken condition.

Step ladders are good products, but it is very easy to get into trouble with them. Workers stretch too far from them, climb too high on them and use them outdoors on soft ground where they can easily tip over. On the other hand, builders respect the potential harm inherent with the misuse of long ladders. So here they act prudently. Most long-ladder injuries are caused by defective material.

Cross grain is the most treacherous defect that a ladder can have. As with wooden planks, cross grain failure is sudden and total. I recently heard about a worker who was reduced to vegetable status when the rail of his ladder snapped. Cross grain caused the failure. His settlement will never compensate for the permanent brain damage he received.

Standards control how ladders are made, but given the critical importance of the tool, be on guard for defects. Inspect ladders carefully (rails and rungs): look for knots and cross grain (slope 1" in 12"); look for a bulge in the grain where it might have been located near a knot in the original piece of lumber; look for chipped grain; check for corrosion on the inside of hollow metal rungs(metal ladders); and discard checked material (weathering).

Wooden ladders should never be stored outdoors. No matter how well you seal or treat the ladder, moisture will collect in the mortice between the rung and rail. High-quality ash rungs will simply rot through at the tenons when a ladder is kept in a damp location.

Scaffold failures are all too common. But most can be avoided. Sensible use, careful maintenance and regular inspection is a must. The most dangerous element is the staging plank. Don't settle for planks that aren't recognized by OSHA. And get your hands on a copy of the scaffolding regulations from OSHA. Ask for Construction Industry Standards part 1926. It's free.

Contact Information: Dave Damery, Director Building Materials and Wood Technology 120 Holdsworth Natural Resources Center University of Massachusetts, Amherst, MA 01003 Tel: +1 (413) 545-1770