Penetrations Near Center of Valley

[aaronm]

Code Man:

Can you please provide me with the requisite industry standard(s) regarding the minimum distance from the center of a roof valley for placing a 4-inch vent for a composition tab shingle application?

Thanks,

[Jerry Peck - Codeman]

Aaron, do you have the shingle manufacturer by chance?

I'll check the code for anything specific in it, however, the shingle manufacturer's installation instructions would be one direct and specific source that the code will refer to.

[Jerry Peck - Codeman]

[aaronm]

Code Man

Thanks for the thoughtful response, and pardon my delayed attendance.

I like "3. For closed valleys (valley covered with shingles), valley lining of one ply of smooth roll roofing complying with ASTM D6380 and not less than 36 inches wide (914 mm) or valley lining as described in Item 1 or 2 shall be permitted. Self-adhering polymer modified bitumen underlayment complying with ASTM D1970 shall be permitted in lieu of the lining material.
- (Jerry's Comment: "shall be not less than xx inches (xxx mm) wide" does not allow for a penetration through that material as that would not meet the "shall be ... wide" requirement.)"

I contacted the shingle manufacturer (who shall remain unnamed to preserve his state of stupor) to no avail. Their engineer referred me back to the vent manufacturer, who, in turn, referred me back to the shingle manufacturer. Huh. So, I contacted all of the major shingle manufacturers and all of the pertinent vent manufacturing firms. I finally got one vent manufacturer who was sufficiently cognizant to commit to writing that only the absolute minimum number of nails required to hold down the valley underlayment and shingles can be no closer than 8 inches to the center of a valley. No holes larger than a roofing nail are allowed within 18 inches of the center of a valley to preserve the integrity of the valley underlayment.

At times like these I often think I missed my calling and should have become a dentist.

Best regards,

[Jerry Peck - Codeman]

[Jerry Peck - Codeman]

Aaron, let's change the discussion from leakage oriented from penetrations through the valley to water flow oriented and the obstruction/disruption of water flow by penetrations through the valley.

Valleys are intended for, and designed to assist, drainage of water from two or more intersecting roof planes and to provide a channel to direct that water down the roof and runoff over the drip edge.

ANY obstruction in the valley will disrupt the intended flow of water down the valley and off the roof. Water flow in a valley meets in three directions (except for at the very top of the valley): cross-flow from the intersecting roof planes and downward flow from the valley area above any specific point in the valley below the top of the valley. Valleys have a tendency to become blocked by leaves, pine needles, and other debris if such debris accumulates more than the water flow down the valley can wash it down the valley.

I know that you already know the above, however, by changing our thinking toward the VTR (or any other penetration), adds another dimension and aspect of consideration to a discussion of installing any obstruction in the valley which impedes the flow of water down the valley.

I will look through the codes and other sources to see if there are any substantive sources of discussions on this aspect obstructions in a valley.

[Jerry Peck - Codeman]

Follow-up.

This document talks about the importance of water flow down the valley and includes "A clear, unobstructed drainage path is important for valleys to quickly carry away water."

8126.pdf

This document starts off talking about "Typically, not much thought is devoted to designing and constructing valleys for asphalt shingle roof assemblies. And occasionally, the result is a troublesome valley. It leaks, collects leaves and debris, supports moss growth or does not perform as well as the surrounding field of the roo"

5436.pdf

This document is about penetrations through valleys.

Roof-Valley-Penetrations.pdf