This is the first part in a series of blogs about designing low slope roofs for wind loads.
Roofing design encompasses many different factors. The assembly is dictated by the use of the building, the owner’s budget, the building’s location, local building codes, energy codes, and the forces of nature that are regularly, as well as occasionally unleashed upon it. In addition, a change in one part of the building envelope can adversely affect something else. As this implies, there are often many choices that the designer has to make. It is important to note that the installer also has a great effect upon the overall performance of the system. Communication between the designer and the installer is paramount to the success of the system. The designer needs to relay exactly what components should comprise the assembly, as well as how the system should be installed. Conversely, the installer should alert the designer of any conditions or potential changes that do not match the plans and specification, since a small change can affect the entire envelope.
Communication between the designer and the installer is paramount to the success of the system.
Let’s talk about golf for a moment. A 300 yard drive has exactly the same value on the score card as a 6 inch putt. The same is true on the roof. If the installer omits sealant and a clamp on a pipe flashing detail because the incorrect one was displayed in the plans, it has the same result as a cold-welded seam: Water in the building. So nearly every detail, no matter how small, can have the same effect. One such mishap may be small, but like strokes on the scorecard, they all add up.
There are certain details that often get overlooked. Sometimes specifications and plans don’t match. If that happens, which one prevails? Sometimes plans trump details, others the opposite is true. Very often, perhaps in the interest of conserving time or effort, a specification or plan detail will state to comply with an established standard, such as those published by FM (Factory Mutual, which does its own system testing for its member insurance companies), SMACNA (Sheet Metal and Air Conditioning Contractors National Association), or the International Plumbing Code without specifying which exact detail or practice. One very common mistake, for example, is specifying FM 1-105 on an OSB deck, but FM doesn’t test over combustible decks. So according to FM, a system over an OSB deck wouldn’t be rated to withstand 105 pounds per square foot, or PSF, of uplift pressure. Perhaps, in that case, it would be better to outline specific enhanced fastening patterns or fastener pull out values.
Sometimes specifications and plans don’t match. If that happens, which one prevails?
The designer of record can possibly open themselves up to liability if they leave details up to the installer’s interpretation. Quite often, trades will mix and match responsibility of interfacing details, such as components of drains, counterflashing roof edge termination, coping cap waterproofing, and HVAC transitions to name a few. Returning to the situation where FM 1-105 over an OSB deck has been specified, the designer should ideally have consulted with the membrane manufacturer to identify options that have been demonstrated to conform to established standards. Good and experienced suppliers do a lot of system testing to understand how to achieve required levels of performance with as many options as feasible.
Wind Uplift – The Basics
Wind uplift, in general, is the upward force pulling on the building components as a result of wind blowing around and over the building. The roof is naturally exposed to these forces due to its location. When the wind flow moves over the edge of the roof it creates negative pressure. In addition, positive pressure exerted from inside the building from HVAC and openings such as doors and windows can also contribute to these forces, depending upon the building’s construction.
Edges are Critical
Corners and perimeter zones are especially vulnerable to wind uplift forces due to their proximity to the edge. Vortices are created at corners, which can increase the upward pull. The next illustration is a top view of the roof, identifying perimeter and corner zones. As a rule of thumb, attachment (uplift resistance) is enhanced at a rate of 1.5x at the perimeter and 2x in the corner to combat these forces. Roof edge termination is especially critical, since it is at the leading edge holding the roof to the structure.
This fully adhered TPO roof was peeled back from the edge during a wind event, separating insulation layers.
Roof edge termination is instrumental in resilience to these forces. Remember the golf analogy? Well, nearly every detail counts the same on the score card. Imagine this: You are on the 8th tee just starting your backswing when a meteor the size of a 1966 Volkswagen Beetle crashes in the middle of the fairway leaving a huge smoking crater. This is not simply a stroke, but instead, it is a catastrophic ending to the game (and quite a story). The same is true with the roof edge. A few years ago, the National Roofing Contractors Association, NRCA, independently tested numerous roof edge terminations. Mark Graham, the Vice President of Technical Services for the NRCA stated in an article featured in Professional Roofing magazine, “…flexural failure during edge metal testing is much more common than fastener pull-out…” The act of just adding more fasteners will not suffice, because if the metal is an insufficient gauge for the application, it will flex, allowing wind to lift it. It is reasonable to assume that when the edge catches air, the rest of the system is likely to follow like dominoes.
Roof edge termination is especially critical, since it is at the leading edge holding the roof to the structure.
Attention to Detail
So, if edges are critical, what is to be done? Ideally two things are recommended; first, instead of a general reference to compliance with SMACNA standards, it would be prudent to call out the exact detail that should be applied in specific locations. Second, the specifier may want to designate which trade is the best to be responsible for each detail, as opposed to leaving the decision up to the trades to decide what to include or exclude within their respective scopes. If a SMACNA detail is to be applied, then perhaps a sheet metal contractor may be the better choice to be responsible for that scope.
TP-3 Courtesy of NRCA Guidelines for Single-ply Membrane Roof Systems
Take a moment to look at one common example from the NRCA, which is generally understood to be considered “good roofing practice.” Shown above is Detail TP-3 from the NRCA Guidelines for Single-ply Membrane Roof Systems.
The field membrane extends over the roof edge, and down the wood nailer, and is secured by the fastening of the anchoring cleat on the face. The thermoplastic (TPO or PVC) coated metal is then placed on top of the membrane and fastened based upon the Architectural Metal Flashing Securement options found within the NRCA Roofing manual. That detail is completed with a hot air welded flashing strip that ties the roof membrane to the Thermoplastic coated metal for a watertight assembly. That is a roofing detail to be installed by a roofer.
Imagine for a moment that the owner wanted to save some money; would you, as the designer, decide to do it here?
Keep in mind that even though the assembly may qualify for a standard warranty, the owner is still exposed to the inconvenience of dealing with replacement, as well as collateral damage such as lost wages due to clean up, lost merchandise due to damage, and lost use of space while waiting for repair. There are other ways for a designer to save money on the assembly that do not significantly increase the risk of the roof blowing off. Remember the meteor?
The diagram on the right is from ANSI/SPRI/FM 4435/ ES-1-11. This document establishes standards for roof edge details as they relate to wind uplift resistance based upon actual testing from collaboration with ANSI (American National Standards Institute), SPRI (Single Ply Roofing Industry), and FM (Factory Mutual). It illustrates one of the methods of testing the edge termination. This demonstrates a mechanically attached system with the same detail as above (NRCA TP-3). A load is applied to the field membrane at a 25 degree angle from the deck to simulate the stresses of the field sheet billowing.
How would the less expensive alternate detail fare in this test?
…even though the assembly may qualify for a standard warranty, the owner is still exposed to the inconvenience of dealing with replacement…
The table below is from ANSI/SPRI/FM 4435/ ES-1-11:
Courtesy of ANSI/SPRI/FM 4435/ES-1-11
Pay special attention to a few things; first it shows the recommended minimum gauge for each metal (a thicker gauge can be specified for added strength), second it is based upon the width of the exposed metal, so the wider it is, the thicker it should be. ES-1-11 outlines design criteria for wind uplift for edge details. This document is created as a guide to keep roofs where they belong.
Wrapping it Up
The designer of record, whether an Architect or a Consultant, should be decisive, and choose specific appropriate details. The owner is looking for a roof that is resilient, cost effective, and does not cause any problems. Keep ANSI/SPRI/FM 4435/ ES-1-11 close, and don’t risk your reputation in the hands of the lowest bidder. Ask any golf pro and they will tell you that putting is 40% of your game, so you had better make it 40% of your practice.