Flat valley

[ICE]

As soon as I was on the roof I said that valley is flat.

They insisted that since the bubble is not exactly centered, there is absolutely a slope.

I drove it home with a digital level.

The reality is that any slope is good enough. My level would not register any slope.

R908.1 General. Materials and methods of application used for re-covering or replacing an existing roof covering shall comply with the requirements of Chapter 9.
Exceptions:
1. Reroofing shall not be required to meet the minimum design slope requirement of one-quarter unit vertical in 12 units horizontal (2-percent slope) in Section R905 for roofs that provide positive roof drainage.

 

[Yankee Chronicler]

At my first job as an intern architect when I graduated, my boss told me more than once that 1% (1/8 inch per foot) is a "natural drain." Less than that and water generally can't overcome surface tension to flow -- except perhaps on extremely slick surfaces. I would hold out for 1% as the minimum under R908.1.

 

[Mr. Inspector]

I'm not allowed to go on pitched roofs so I would never know. I'm cool with that.

 

[Joe.B]

Time to build a cricket. In the time spent hawing over it you'd be done. Just do it!

 

[Yikes]

A 1/4":12" roof is just over 2% (2.083%).
A valley that runs at 45 degrees between two of these roof plans will be the hypotenuse of an isosceles triangle, basically 12" x 1.41 = 16.92
So the slope of the valley will be 1/4":16.92" = 1.47%

Roofing companies typically require 1/4" per foot minimum, but their sales reps say that is main slope, not valley slope.
However, even if a valley slope is OK at 1.47%, a small pebble, leaf or other component can dam up the valley.

For asphalt roofs, the long-term damage comes not from direct water pressure, but rather from standing water on the roof making the membrane cooler in the wet area than in the dry area, resulting in differential expansion and contraction that creates micro-tears in the membrane.

Our firm has been using 3/8" per foot main slope for all "flat" roofs.

 

[ICE]

The new roof was done just in time for a rain storm. When does a roof pond become an issue worth the effort to correct it? What depth? What size? these folks are sure to complain. I would not have known it without the rain.

I wrote a correction about the ponds.

 

[Paul Sweet]

As well as the open J box?

I think some manufacturers won't warranty a roof if puddles are too large to dry up within 48 hours of when the rain stops.

 

[ICE]

There is an example of the finished product in the background.