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  • Sample report of a structural investigation - Metal Building Roof Leaks

    Introduction:

    At the request of the owner, Thomas R. Price P.E., of Structural Solutions LLC, a licensed professional engineer, inspected the Facility in the attempt to determine the cause of weather-tightness problems associated with the building roof.

    This report is a follow-up to the inspection.

    The inspection took place between noon and 3:00 PM.
    Weather was clear and sunny, with a temperature of approximately 85 to 90 deg F.
    Present on the site was the owner, a representative of the contractor, and Mr. Price.

    The building in question is a metal building manufactured by a major metal building manufacturer. It is 440’ wide and 400’ long. It has rigid frames at 40’ spacings, with steel bar joists at 5’ o.c. The roof is a clip attached
    vertical rib standing seam roof system, with 18” wide panels. The ridge of the building is at the center of the 440’ width. The roof panels are made non-continuous by an approximate 6” vertical step at 110’ from each eave. (See Figure 1). Each of the four 110’ panel runs is fixed by screws at its low end. The upper end of the panel run is free, presumably to allow for thermal movement due to ambient temperature variations. The roof slope is 1/4” on 12”.


    History:

    The roof was installed, and complaints of roof leaks began shortly thereafter. Structural Solutions was shown various correspondence between the owner, the pre-enginnered metal building manufacturer, the contractor, and others, in regaDr to attempts to repair the problem. It was immediately assumed that the roof expansion joint was the source of the leaks. We concur with this conclusion.

    Various repairs have been attempted, however none have sealed the leaks satisfactorily. The major repair attempts involved the application of a sprayed on rubber sealant covering the entire expansion joint, and the installation of a second metal flashing, (See Figure 2).


    Inspection:

    Prior to inspecting the site, we viewed a videotape of a leak test where water was being sprayed on the expansion joint. A hole was made in the lower panel cap, and water flowed freely through it. This was at a point on the interior side of the closure which presumably should be a water tight area. From the video, it was evident that water was flowing through at least one other location as well. (This was evidenced by drips occurring down the lower panel, about 6”).

    At the site, the building occupant provided us with locations of the leaks during a recent rain. Each of the leak locations was at or near a primary frame line.

    Our inspection began by walking along the roof steps, visually inspecting the area for any obvious problems. It was evident that a lot of activity has occurred in the area surrounding the expansion joints. There were regions of poor panel modularity, where the vertical ribs of the upper roof panel were not closed tight. There was caulk on many of the fasteners, and various other components of the expansion joint detail. The layer of sprayed on rubber sealant had been cut around numerous fasteners with a razor.

    Of particular concern was many fasteners whose neoprene washers had split. Some fasteners which were removed, revealed slotted holes in the panel. A clear, apparently silicone based caulk was used in various areas
    Some screw heads were “tipping” in various areas of the expansion joint.

    We then selected a location where leaking had occurred in the past, and removed an approximate 10’ section of the second metal flashing (See Figure 2) in order to get a good look at the original installation. We were able to lift the upper roof slightly, so as to view the seal between the flashing and upper roof. Inside the building, we utilized a moveable scaffold to view various leak areas from the inside. The presence of leaks was evidenced by staining and by saturated insulation adjacent to the expansion joint. We were able to view a standing seam roof clip, and verify that it was a slotted clip. The slot is presumed to have the purpose of allowing thermal movement of the roof panels.

    Conclusions:

    Of primary concern on a roof of this size is the allowance for thermal expansion and contraction of the roof panels. If a continuous steel roof sheet is 110’-0” long at 50 deg F, it will be 7/16” shorter at 0deg F, and 7/16” longer at 100deg F, assuming it is unrestrained against longitudinal movement. If it is restrained against movement by screws fastening the sheet into a rigid lateral support, large forces will develop in the panel, causing the screw holes to tear and elongate, and tend to bend the screw, which causes the screw head to tip. When this happens,
    water can leak through. Since it is very difficult to keep thermal stresses from developing in a metal roof, it is strongly advisable to utilize details where, if an exterior fastener does leak, it will drip outside the building envelope. This can be accomplished with Z shaped end dams and eave plates. (See Figure 3).

    The sliding clips utilized in the roof attachment are slotted to allow for thermal movement. All sliding clips will cause some amount of resistance to thermal movement. It is difficult to tell how much resistance these clips are
    causing, but based on the locations of past leaks (at frame lines), and logic, it is quite possible that the clips are causing excessive resistance to thermal movement at the frame lines, which causes screw leaks, and since the screws at the low end of the upper roof panels penetrate the building envelope, the moisture enters the building.

    Based on current information, it is unclear how the slotted clips can slide, since the screw anchoring the clip to the joist clamps the mating surfaces around the slot. They will slide, but only when a large amount of force is applied. It is the experience of this office that a sliding standing seam clip should resist no more
    than 2 lbs. of lateral force before static friction is broken, and the clip slides. This way, even if there are 20 or more clips attaching a roof seam all along the slope, the total resistance (which is equal to the force on the screws anchoring the low end of the panel) does not become so great as to cause screw and screw hole deformation, which will leak.

    If the roof clips are not sliding freely, leak problems will tend to be much more pronounced at the frame lines. The bar joists can move laterally in between frames, as they are pushed by the non-sliding clip. However, at the
    frames, the joists are essentially immovable, because the rigid joist seats are welded to the frame.

    The second flashing which was installed at the expansion joint is screwed to the upper roof sheet, and the lower roof sheet. Combined with the first (original) flashing, there is significant additional rigidity created. This will tend to inhibit thermal movement all along the joint. It is desirable to remove this second flashing, especially since it has apparently not solved the leak problems.

    Figure 4 shows a possible remedy for the roof leaks, without requiring the removal or cutting of roof sheets. Note, however, that installation of an eave plate (See Figure 3) is the better way to prevent leaking screws at the low end of the upper roof panel. Eave plates may be difficult or impossible to install without removing or cutting roof sheets. However, the idea should be explored with the manufacturer and erector. Figure 4 is depending on roofing tape to seal screws, which may be effective, but will also cause a need for periodic inspection and maintenance. It must be noted that the roofing tape will cause a small amount of water pooling just upslope from the tape. This is not a desirable scenario, but the water shouldn’t enter through the screws if the panels are well cleaned, and the roofing tape is installed with care.

    The clear, apparently silicone based caulk which was prevalent in many locations at the expansion joint should not be used in the future. It did not adhere to the metal components of the roof. Caulking individual screws generally will not provide a good seal. Over time, the caulk will lose bond with the panel due to thermal strains. A good
    quality roofing tape is a better material to use. Care must be taken in tourquing down screws with rubber or neoprene washers. If they are too tight, the washer splits and leaks. The washer may not split immediately. Tighten to manufacturers recommendations and no more.

    The triangularization of the vertical ribs is apparently caused by poor panel modularity during erection. These should be caulked, and covered with roofing tape. (See Figure 4).

    Structural Solutions LLC and its agents are acting strictly as consultants in this matter. Structural Solutions LLC does not warrant that any action taken using the information in this report will resolve troubles associated with this building. Furthermore, Structural Solutions LLC does not recommend that any action be taken which would void any warranties of the product suppliers and sub contractors involved.

    If the primary reason for the roof leaks is due to excessive clip resistance, it’s quite possible no amount of work at the roof step will completely solve the leak problems. At this point two items that should be considered by the parties involved are:

    1. Removal of all anchorage screws at the low end of the upper roof. This would create a situation where the entire upper roof is free to float, theoretically preventing thermal forces from occurring. (Assuming the clips are
    capable of sliding). This step should only be done at the recommendation of the manufacturer.

    2. Consider the effects of the insulation. If the insulation is packed too tightly between the roof sheets, it may be causing thermal stresses to build up similar to, or in conjunction with the clips not sliding. Removal of thermal
    blocks at the roof panels adjacent to the building’s primary frames may relieve this problem


    Summary:

    It is our opinion that this roof has problems beyond which can be solved by caulk, or other quick fixes. The roof system design by the pre-engineered metal building manufacturer does not adequately allow for thermal movement of the panels. This is causing screw hole slotting, screw tipping, and the subsequent leaks. The details used may be effective for cold-formed purlin roofs, but not for bar joist roofs. Bar joists are very stiff at their supports, whereas cold-formed purlins are much more flexible.

    The manufacturer details show fasteners which penetrate the building envelope, and roof panel clips that have excessive resistance to sliding. Using these details on a bar joist roof with 110 ft. panel runs is inadvisable.

    Of additional concern is the quality of installation. Over tightened screws were noted in some locations. This must be avoided, as split rubber washers will leak, and if the screw is penetrating the building envelope, water will enter the building. Fastener manufacturers should be consulted for an appropriate screw and installation procedure.

    Installation of the roof panels should have stopped at the first sign of panel rib triangularization. It was inappropriate to continue the installation after it was apparent that something was wrong with the panel modularity.



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Grand Rapids, MI 49546
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