Concise Details and Coordination between Trades Will Lead to a Quality Long-term Solution for Roof Drains

PHOTO 1: Roof drains should be set into a sump receiver provided and installed by the plumbing contractor.

PHOTO 1: Roof drains should be set into a sump receiver provided and installed by the plumbing contractor.

The 2015 IECC roof thermal insulation codes have forced roof system designers to actually think through the roof system design rather than rely on generic manufacturers’ details or the old built-up roof detail that has been used in the office. Don’t laugh! I see it all the time. For the purpose of this article, I will deal with new construction so I can address the coordination of the interrelated disciplines: plumbing, steel and roof design. In roofing removal and replacement projects, the process and design elements would be similar but the existing roof deck and structural framing would be in place. The existing roof drain would need to be evaluated as to whether it could remain or needs to be replaced. My firm typically replaces 85 percent of all old roof drains for a variety of reasons.

The new 2015 IECC has made two distinctive changes to the 2012 IECC in regard to the thermal insulation requirements for low-slope roofs with the continuous insulation on the exterior side of the roof deck:

  • 1. It increased the minimum requirement of thermal R-value in each of the ASHRAE regions.
  • 2. It now requires that this minimum R-value be attained within 4 feet of the roof drain.

Item two is the game changer. If you consider that with tapered insulation you now need to meet the minimum near the drain, as opposed to an aver- age, the total insulation thickness can increase substantially.

PHOTO 2: Roof drains need to be secured to the roof deck with under-deck clamps so they cannot move.

PHOTO 2: Roof drains need to be secured to the roof deck with under-deck clamps so they cannot move.

THE ROOF DRAIN CHALLENGE

The challenge I see for designers is how to properly achieve a roof system design that will accommodate the new insulation thicknesses (without holding the drain off the roof deck, which I believe is below the designer’s standard of care), transition the roof membrane into the drain and coordinate with the related disciplines.

For the purpose of this tutorial, let’s make the following assumptions:

  • Steel roof deck, level, no slope
  • Internal roof drains
  • Vapor/air retarder required, placed on sheathing
  • Base layer and tapered insulation will be required
  • Cover board
  • Fully adhered 60-mil EPDM
  • ASHRAE Zone 5: Chicago area

FIGURE 1: Your detail should show the steel roof deck, steel angle framing coped to the structure, the metal sump receiver (manufactured by the roof drain manufacturer), roof drain and underdeck clamp to hold the roof drain to the roof deck.

FIGURE 1: Your detail should show the steel roof deck, steel angle framing coped to the structure, the metal sump receiver (manufactured by the roof drain manufacturer), roof drain and underdeck clamp to hold the roof drain to the roof deck.

Once the roof drain locations have been selected (for those new to this, the roof system designer should select the roof drain locations to best suit the tapered insulation layout), one should try to locate the roof drain in linear alignment to reduce tapered insulation offsets. The drain outlets should be of good size, 4-inch minimum, even if the plumbing engineer says they can be smaller. Don’t place them hundreds of feet apart. Once the roof drain location is selected, inform the plumbing and structural engineers.

STRUCTURAL ENGINEER COORDINATION
The first order of business would be to give the structural engineer a call and tell him the plumbing engineer will specify the roof drain sump pan and that the structural engineer should not specify an archaic, out-of-date sump pan for built-up roofs incorporating minimal insulation.

When located in the field of the roof, the roof drains should be at structural mid spans, not at columns. When a structural roof slope is used and sloped to an exterior roof edge, the roof drains should be located as close to walls as possible. Do not locate drains sever- al or more feet off the roof edge; it is just too difficult to back slope to them. Inform the structural engineer that the steel angles used to frame the opening need to be coped to the structure, not laid atop the structure. There’s no need to raise the roof deck right where all the water is to drain.

FIGURE 2: A threaded roof drain extension is required to make up the distance from deck up to the top of the insulation and must be screwed to a proper location (top of the insulation is recommended). To do so, the insulation below the drain will need to be slightly beveled. This is shown in the detail.

FIGURE 2: A threaded roof drain extension is required
to make up the distance from deck up to the top of the insulation and must be screwed to a proper location (top of the insulation is recommended). To do so, the insulation below the drain will need to be slightly beveled. This is shown in the detail.

PLUMBING COORDINATION
Now call the plumbing engineer and tell him you need a metal sump receiver (see Photo 1), underdeck clamp (see Photo 2), cast-iron roof drain with reversible collar, threaded extension ring capable of expanding upward 5 inches, and cast-iron roof drain clamping ring and dome.

Send the structural and plumbing engineer your schematic roof drain detail so they know exactly what you are thinking. Then suggest they place your detail on their drawings. Why? Because you cannot believe how much the plumbing roof-related details and architectural roof details often differ! Because details differ, the trade that works on the project first—plumbing— leaves the roofing contractor to deal with any inconsistencies.

Your detail at this point should show the steel roof deck, steel angle framing coped to the structure, the metal sump receiver (manufactured by the roof drain manufacturer), roof drain and underdeck clamp to hold the roof drain to the roof deck (see Figure 1).

PHOTOS AND ILLUSTRATIONS: HUTCHINSON DESIGN GROUP LLC

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