Roof pitch

[SteelPE]

We have a small discussion going around my office at the moment with regards to roof pitch. The code states that we need to achieve a minimum roof pitch of ¼”/foot. However we are at a disagreement as to where this pitch should be taken from.

Say I have a building that has 2 bays of 50’ in either direction (100’x100’ overall). If I would like to put a drain in the middle of the building where do I take the pitch from? Do I take the pitch from the point furthest from the drain (in the corner) making the change in elevation 1’-5 ¾”, or do I take the pitch from one of the sides making the change in elevation 1’-0 ½”?

I have always taken the pitch from the furthest corner, however my colleague refers to ASCE 7-05 page 339 where they show it from one of the sides. Which is correct?

 

[Ron]

JAE...I feel your pain...not just the roofing issue, but the fact that you have that wayward child who is an architect!

I know your sense of adventure is high, but trying to hold down a roofing tarp in 60mph wind? Did you get visions of the Wizard, Dorothy?

 

[JAE]

Visions worse than that - but I have to say - looking back on it - it was just too cool!

 

[ToadJones]

JAE-
I have been the roofing contractor in very similar situations.
AS i was flopping around, I saw visions of lawsuits and money flying out of my pockets.

Now you know why the roofing contractor wanted no part of that demo job.

 

[JAE]

I must say, my son's insurance co. came through for him though - he was essentially better off that the event happened.

 

[ToadJones]

To continue the hi-jacking....

I'd have to say over 10-12 yrs working as a contractor or for contractors, we were hired ~20 times to frame pitched roofs over flat roofs.
we got to the point where on new builds, we would not install roofing on anything less than a 3:12 pitch. We put the onus back on the homeowner to hire their own roofing contractor.

Of course this is in the eastern midwest region of the country.

 

[woodman88]

Back to the original post, I can find no clear definition of slope in the IBC. In looking back at my old UBC 1994 Section 2312.4.6 Ponding states “In no case shall the roof slope provide a positive vertical displacement less than that equivalent to 1/4 unit vertical in 12 units horizontal of horizontal distance between the level of the drain and the high point of the roof for drainage.”

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[csd72]

Why penny pinch on slope, I really cannot imagine there is much of a cost fifferential.

 

[SteelPE]

I realize that the pitch it determined in the IBC, my colleague was just referencing ASCE 7 on how they seem to show the pitch. In this particular instance however, we don't really end up with valleys. The roofing system seems to "roll" down towards the drain.

csd, I have been surprised how much roof pitch can screw with cost. Increasing building heights add money to the bottom line. On a large project there can be a significant savings to keep the pitch to a minimum.

JAE, love the story about throwing all engineering common sense aside when working on your own construction project. These are the same positions I find myself in from time to time.

 

[JAE]

SteelPE - yes
I also like to think it was also throwing all "architectural" common sense aside, too! But neither architects or engineers are weathermen....cough....meteorologists.

 

[dhengr]

JAE:

No offense intended to junior, but what common sense do most architects have these days? My Father was one of those too, but that’s long enough ago, that he was as much, or more, an engineer and master builder, than an artsie-fartsie volumes/space arranger and brick color selector. He even knew Frank Lloyd.., but wasn’t quite that wild either.

 

[JAE]

There are a number of architects I've known over the years that were very impressive in the breadth of knowledge that they possessed as well as having a pretty good feel for what they didn't know.

 

[csd72]

SteelPE,

There are some construction cost savings that are not really cost savings. 10 or 20 years down the track the people with the leaking roof will not be grateful for the <1% of project cost that you saved them.