Increased Thermal Values Affect an Existing Roof Edge

EXISTING-CONDITION KNOWLEDGE

When the need to raise the roof edge arises, you should start out by investigating the existing-building roof edge construction. Existing construction drawings should be reviewed if possible. If they are not available, you should disassemble the roof edge to determine the existing conditions. This should also be considered even when the drawings are located because construction often differs from that which is drawn.

The type of structural system should be determined. You also must determine the location of perimeter structural members, roof-deck type, wall-cladding construction and how the wall to roof deck juncture, as well as how the existing roof system terminates at the wall.

Roof edges that require additional wood blocking can be accomplished with steel “tees” and angles that support vertical wood blocking.

Photo 5: Roof edges that require additional wood blocking can be accomplished with steel “tees” and angles that support vertical wood blocking.

This knowledge will lead to design decisions in the roof edge design. Understanding how the wall to roof edge on an existing building is constructed is critical. On a recent project in which the roof sustained a wind event, investigation for the design of the new roof edge and system found multiple concerns: open metal stud cavities to the parapet, open metal panel joints, wood and substrate boards attached with drywall wall screws and moisture drive concerns. This information led to the design of one our most complicated roof edges (see Photo 1 and Figure 1).

DESIGN CONSIDERATION

A paramount consideration in design and construction of raising the roof edge is tying the new roof edge construction back to the structure of the building. This is required to prevent loss of the roof edge, roofing and components from the roof edge during a wind event. The method of anchorage to the structure needs to be considered, as well. Type of anchor, material type, size, spacing and installation method all must be considered.

The height of the new roof edge is often determined by the maximum height of the insulation, either by saddles or tapered insulation. The designer must not only calculate the total insulation height, but also consider any insulation adhesive. Spray or bead foam adhesive can add 3/16 inch per layer of height to the total insulation height. I have seen the top surface of insulation rise several inches above the roof edge as a result of this oversight.

Here, the vertical 2 by 10s have been bolted to the steel tees and a horizontal 2 by 4 is being installed along the top.

Photo 6: Here, the vertical 2 by 10s have been bolted to the steel tees and a horizontal 2 by 4 is being installed along the top.

You also must consider the roof edge width. Like a masonry wall’s slender ratio (height versus unbraced length), the taller the raised roof edge the wider the new roof edge should be. The days of stacked 2 by 4s are past. Six- and 8-inch widths allow for a better base width and resistance to bending. An additional concern to take into consideration, depending on new roof height, is an increased potential snow load along the roof edge, which can be a real concern when the roof edge is an overhang.

DESIGN

How does one raise the roof edge? Historically the roof edge perimeter was raised with stacked 2x lumber, nailed together. As the insulation height rose, the 2x number increased until they grew to ridiculous heights (see Figure 2 and photos 2, 3 and 4). The stacking of 2x lumber often resulted in a vertical edge that was not plumb but bent in- ward or outward—a condition that did the sheet-metal crews no favors.

When placing 2x material on steel tees or angles, it is recommended the joints be scarfed, alternated and located on the steel.

Photo 7: When placing 2x material on steel tees or angles, it is recommended the joints be scarfed, alternated and located on the steel.

Twenty-five years ago, I started using 12-inch-wide 1/4-inch bend plates with leg dimensions to the roof edge conditions to which vertical 2x members were bolted and capped with a 2 by 4 screwed to the verticals. There were challenges with this solution. For example, a 1/4-inch piece of plywood had to be inserted at the bend plates to make up the needed width of a 2 by 4; shimming the base of the bent plate often allowed rotation at the rolled corner making holding a vertically plumb exterior condition at the wall, to which sheet metal would be attached, difficult.
On this detail from an older project, the roof edge is being raised with multiple layers of 2 by 12s—a bit old school but easily performed. It is recommended to not specify preservative- treated wood, coated screws and off-set joints.

Figure 2: On this detail from an older project, the roof edge is being raised with multiple layers of 2 by 12s—a bit old school but easily performed. It is recommended to not specify preservative- treated wood, coated screws and off-set joints.

This detail was improved following in-field installation observation (an- other good reason to be out on the roof to observe installation) by changing the bend plate to a 1/2-inch steel angle so that the insertion of a 1/4-inch shim was not required, the square corner of the angle did not rotate as much and the angle was made continuous to eliminate concerns with the proper location of adjacent angles. This allowed for shimming ease and resulted in vertically plumb wood blocking (see Figure 3 and photos 5, 6, 7 and 8). The steel angle could be through-bolt- ed to steel, lag-bolted to wood and anchor-bolted to masonry. When the roof edge is thin, with a 2 by 4, a steel tee can be used (as in Photo 8).

Trying to improve on the stacking of wood that often resulted in snaking and bowing roof edges, the utilization of steel angle and tees reduced wood use and provided for a better chance of achieving a vertically plumb condition.

Figure 3: Trying to improve on the stacking of wood that often resulted in snaking and bowing roof edges, the utilization of steel angle and tees reduced wood use and provided for a better chance of achieving a vertically plumb condition.

Over time, the steel angle and vertical 2x solution appeared old school. Thinking in a new way: How can the roof edge be raised, installation time decreased, thermal breaks removed and quality improved? After some thought, the idea of a prefabricated roof edge curb rail was developed (see Figure 4). This concept improves on all the benefits of the previous concepts. Curbs have since been installed in a variety of anchorage configurations and seem to be the perfect solution.

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About Thomas W. Hutchinson, AIA, FRCI, RRC, CSI, RRP

Thomas W. Hutchinson, AIA,FRCI, RRC, is principal of Hutchinson Design Group Ltd., Barrington, Ill., and a member of Roofing’s editorial advisory board.

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