The Retrofit Roofing Game:
Framing System Anchorage
Mark James ,
Posted
07/30/2012
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.