You might think anchoring a steel retrofit framing system to an existing roof is about as simple as it gets. Well, it’s not. It is probably the most important step of a retrofit roofing project.
In addition to fastening the retrofit framing to the existing roof, there are other very important topics that will ensure a properly anchored retrofit system. You might say that the following is the ABCs of retrofit anchorage:
A. Satisfying Wind Uplift
If anything having to do with a retrofit roof system is important, this is it. The vast majority of catastrophic retrofit system failures are related to extreme wind storms, but the truth is if the anchorage system isn’t engineered correctly, even a 50-mph wind can wreak havoc. The first step of designing a retrofit anchorage system is to test for pullout resistance. This is done on the existing roof using a pullout testing device. A pullout tester simply applies upward pressure (pulls) on an installed anchor until it strips out of its hole. The tester’s calibrated gauge records the total pounds exerted at the moment of pullout. This is known as the real-world job-site pullout value, which is much more realistic than using a manufacturer’s published laboratory test value.
Each test is conducted by installing anchors that are compatible with the building’s construction type, such as wood, concrete or steel. You should perform multiple tests at different areas of the roof to establish an average value to be used in anchor design calculations.
Once the tests are completed, an engineer calculates the number of anchors to be used at each attachment location. Let’s say that pullout test shows the average value of a 1/4-inch 14 x #3 point self-drilling screw penetrating a 1/8-inch steel bar joist chord is 2,100 pounds. To calculate the quantity of fasteners at each attachment location, divide this value (2,100 pounds) by the maximum uplift load including an applied safety factor. (The engineer of the overall retrofit system can provide the uplift load.) For our example, the maximum uplift load is 1,100 pounds and specifies a 2.5 safety factor, so the total pullout value at each attachment location is 1,100 x 2.5 or 2,750 pounds. Divide the total pullout value by the pullout test results (2,750/2,100) to get the number of anchors, which is 1.31. That indicates, because the value is greater than 1 and less than 2, that a minimum of two anchors is required.
B. Protection During Framing Installation
Anchors must be compatible with the material they are attaching to, of course, but the installation must also provide you protection during and after construction. Here are three very important items to remember:
• Each anchor should have a compatible sealant applied during installation to prevent rainwater entering the building prior to the retrofit being dried in.
• Chances are your existing roof may have trapped moisture since it has been leaking. Because of this, all anchors may be subjected to premature corrosion. Therefore, all anchors should have a corrosion-resistant coating.
• If continuous base framing members are used, they should be shimmed at each anchorage location to prevent obstructing roof drainage during construction. Sizing of the shim is a factor of the compressive strength of the existing roof assembly.
C. Distribution of Gravity Load
Understand that at each anchorage location, the existing roof substrate (decking, insulation and membrane) will be subjected to a concentrated downward load. Because of this, you will need to determine the roof substrate’s compressive strength. If the existing assembly does not provide sufficient compressive strength, you may get a phone call from the building owner after the first snowfall melts claiming that his new roof is severely distorted, undulated and oil-canned. The snow weight can cause the framing base members to depress into the existing roof assembly causing major out-of-plane issues on the new metal roof.
To identifying the compressive strength, place a piece of 3/4-inch plywood directly beneath the pullout tester when conducting your pullout tests. When pulling the installed anchor, record the gauge’s value at the exact moment the existing roof beneath the plywood begins to depress.
This is not totally scientific, but it is as close to real world as it gets. For example, using an 8-inch square piece of plywood, the test yields a value of 1,150 pounds. Divide the 1,150 pounds by the 64 square inches of the plywood, which results in 17.97 pounds per square inch (PSI). This is the net compressive strength of the existing substrate. Use this PSI to size your base members or shims by dividing it into the total gravity load at each attachment point. For example, if the gravity load is 900 pounds, then the minimum base member or shim beneath continuous bases should have 50 square inches of surface area (900 pounds/17.97 psi=50 square inches).
Mark James has more than 40 years experience in the retrofit and metal construction markets, working for leading manufacturers and executing great projects. Currently, he is president of RetroSpec LLC, a consulting company offering direction and deep knowledge of the retrofit business for manufacturers and contractors. James can be reached at mark@retrospecllc.com.
