Are you concerned about 1/8th of an inch?







By Robert J. Boessen, RRC, CDT, Senior Roof Consultant/Project Manager, Terracon Consultants, Inc.





In the National Roofing Contractors Association’s (NRCA) Roofing and Waterproofing Manual, Fifth Edition, it states, “In the case of metal components, the expansion and contraction that takes place with changes in temperature can cause severe problems if it is not accommodated for in the design and construction. Expansion and contraction factors are often overlooked or regarded as insignificant. The lack of or improper placement of expansion joints can result in ruptured or loosened flashings, or even a failed roof system. Metal roof systems can be the least forgiving of all roof systems if provisions for expansion and contraction are omitted during the design of the system.”

The statement, “Expansion and contraction factors are often overlooked or regarded as insignificant,” is just that: overlooked or regarded as insignificant. Yet, these factors are the cause of a myriad of roof failures, as well as the cause of moisture intrusion within roof systems. Designers and metal manufacturers all have statements that read something like, “…provide for expansion and contraction; show details that provide for expansion and contraction under thermal loads; etc.” Yet when asked, “what do you mean ‘provide for expansion and contraction,’” mostly you get something like, “Well, you know, metal moves so you need to install it to work.”

The majority of roof failure investigations I get involved with typically take place five years after installation. It’s been my experience that sheet metal issues contribute to those failures 70- to 80-percent of the time. When first arriving at a building, walking around the perimeter, observing gutter end lap joints leaking, exposed fasteners in metal fascia and bowed or buckled metal drip edges, it provides a good preview of what is to be found on the field of the roof.  

It is common knowledge that metal moves, yet it seems very few designers, contractors, or installers understand how metal functions in the elements and how to install it to provide a leak-free environment.

The most common sheet metals used in roofing are galvanized steel, pre-finished steel, copper, stainless steel and aluminum.

Movement of metal is calculated by the coefficient of thermal expansion (α). The following table provides the calculated increase in 10-foot lengths per 100ºF temperature change.

Metal Type             Coefficient of Thermal        Increase in 10-Foot Lengths 
                             Expansion (α)                   per 100° F Temperature Change
Galvanized Steel     0.0000067 in./in./F            .080 in. ~¹/16  inch
Steel                      0.0000067 in./in./F            .080 in.
Copper                   0.0000094 in./in./F            .113 in.
Stainless Steel       0.0000096 in./in./F            .115 in. ~ ⅛ inch
Aluminum               0.0000129 in./in./F            .155 in.

Table 1, above, indicates a 10-foot piece of steel moves about one-sixteenth of an inch and aluminum moves more than one-eighth of an inch (almost twice that of steel). When designing/installing sheet metal, it is imperative that the material does not grow into lengths that will cause severe problems. My dad used to say, “Pay Attention Son!” Let’s take a look at some examples as they relate to some common sheet metal details.

Gravel Stop, Perimeter Edge Fascia, Drip Edge,Photo-1-Split-in-stripping-ply-resize.jpg Rake Edge Fascia:

Generally, the configuration of these details consists of a horizontal flange of three to four inches broke at a 90-degree angle with a face of two to eight inches with a kick-out (drip edge). Let’s suppose that when we fasten the horizontal flange, we put a fastener in the end laps. At some point in the run (assume 50 feet) we have accumulated approximately three-eighths of an inch of expansion and contraction [five ten-foot pieces multiplied by 0.08 inch = 0.4 (⅜) inch]. Where does that three-eighths of an inch go? Those in the roofing business have all seen a split in the stripping plies that cover the metal deck flange.

To eliminate the split in the stripping ply membrane, here are a few simple steps I use:

  • Horizontal flange (assume 24 gauge steel at a minimum of four inches), with roof side edge hemmed/folded tight at three-eighths of an inch. I had not specified this hemmed detail until recently. I was watching a sheet metal worker install a gravel stop using a pneumatic nail gun. He had his regulator set too high and was overdriving the nails. Having the soft substrate of the roofing plies under the flange caused the nail heads to make the outer flange bow, thus raising and exposing the raw edge of the horizontal flange. Obviously, we discussed the proper setting of the gun. The fascia piece is sheared from flat stock, and the raw edges have a sharp serrated edge. Over time, thermal movement can attack the flashing membrane. A hemmed edge eliminates the sharp edge of the horizontal flange as well as protects the installation from an overzealous installer.


  • For built-up roofing (BUR) systems, the underside of the horizontal flange should be primed and set in a one-eighth inch bed of compatible roof cement (comparable sealants as they relate to single-ply membranes apply). The horizontal flange should be fastefigure-1.jpgned into wood block which is at least one and one-half inches thick and wide enough to extend a minimum of one-half inch beyond the edge of the flange. Typically, 2x6’s suffice. NRCA recommends fasteners be spaced three to six inches. I like to standardize the spacing to 4” on center (see Figure 1).  Starting one inch from the roof side edge, install using one and one-quarter inch (minimum) annular ring shank nails in two rows spaced two inches apart staggered. The nails should not be installed in one row or in a straight line. My preference for the end lap/joinery of the pieces is using a lapped/bayonet detail with a minimum of a four inch overlap. This is a simple, effective joint as long as you remove the back of the hemmed edge so you can slip the two pieces together. Remember when you start nailing the next piece, fasteners start past the four inch lap. I cannot emphasize this enough; do not nail through the end lap.


  • Assuming fascia is used at the perimeter edge where you want to keep the water from flowing over the edge, a minimum of seven-eighths of an inch rise should be broke at a 45-degree angle in the outer edge of the deck flange. If using 24 gauge materials, you can get by without installing a wind clip when the face is a maximum of four inches (depending on the wind zone). A minimum of one-half of an inch at the bottom edge of the fascia should be broke out at a 30-degree angle and then three-eighths of an inch hemmed or folded back tight.

Additional Points:
1. End lap joints at inside/outside miters should not exceed 24 inches.
2. Fascia sits on the finished plies of the roof. At least one ply should extend over the edge at least one and one-half inches below the face of the wood nailer(s).
3. Both sides of the horizontal flange must be primed.
4. Mastic or sealant must be applied between the metal at the end lap.
5. The face of the fascia should not exceed eight inches without provisions for some type of metal extender or stiffener to prevent oil-canning.
6. A wind clip might be required, but depends on the dimension of the face and wind zone of installation location.
7. An ‘L’ style wind clip seems to work best, understanding that fasteners for the clip are installed in the face of the wood nailer and not on top.

There are many variables associated with perimeter fascia. Keeping the above basics in mind will go a long way to minimizing water intrusion into the roof system as it relates to expansion and contraction under thermal loads.

How many gutter joints have we all observed that leak? No matter the gutter style, a few years after installation there are leaks in some, if not all, of the end lap joints. So a repair techinician is sent to strip in the joints. Whether we use mastic, exposed sealant, EPDM membrane, etc., it’s temporary. Typically, after one cycle of seasons it will need to be repaired all over again.

As with all rigid materials, expansion is a significant design consideration in gutters. The system of gutters, downspouts and their supports must have the flexibility and strength to accommodate expansion. According to NRCA, expansion joints are suggested for gutters to allow movement caused by thermal changes. Long, straight runs should not have expansion joints spacePhoto-2-alternative-resize.jpgd more than 50 feet apart and no more than 25 feet from any fixed corner. Some types of expansion joints act as a water dam in the gutter. Therefore, the number and placement of downspouts will be influenced by the type of expansion joint used. As stated by the NRCA, “Using metals with high coefficients of thermal expansion, such as aluminum and zinc, should be avoided”.

I am aware there are entities that love to specify and install gutters with a deck flange. However, in my opinion, unless the gutter runs are less than 50 feet, this type of gutter profile should be avoided. I personally have been involved with four separate issues relating to gutters with deck flanges that have resulted in thousands of dollars of corrections.

When I asked a metal building manufacturer representative where his expansion joint detail for the gutter system was, he responded, “Ah Robert, we don’t worry too much about that unless the run is over 150 feet.” Think about that: 15 x 0.080 inch =1.20 inch. Where does he think over one inch of expansion and contraction is going to go? I showed him where every third or fourth section of gutter was leaking. Pop rivets used to fasten the end lap joint were either severed and/or ripped from the face of the gutter.

The following points sum up gutters:

1. Gutter runs that exceed 50 feet must have an expansion joint detail and should not contain a deck flange.

2. End laps may be pop riveted and soldered, or lapped a minimum of three inches, placing two rows of sealant between the lap and installing appropriate type pop rivets and/or sheet metal stitch screws, in two rows, spaced one inch apart staggered.

In closing with this note, if you are a designer or consultant, “pay attention” as my dad says, and give some directions as to how you want the one-eighth inch of expansion and contraction handled in your sheet metal detail. If you are a contractor, “pay attention” when you submit shop drawings as to how you are going to handle the one-eight inch in the shop and the field. If you are a quality assurance representative, you better know how the one-eighth inch is being handled.

Robert J. Boessen, RRC, CDT is a Senior Roof Consultant/Project Manager with the Facilities Engineering Division of Terracon Consultants, Inc.  Robert has been in the roofing industry for 37 plus years. Robert has served as an expert witness in multiple roofing litigation issues, appointed/approved by US Federal District Court as an umpire in roofing issues. Robert has authored papers and presented seminars to his peers on the subject of roofing and sheet metal. Robert has been an active member of RCI for 20 years and serves on the RRC Exam Committee. He can be reached at 573.230.8224 or

Monica Keels
(713) 880-1432
Author: Monica Keels
Phone: (713) 880-1432