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?

 

[ToadJones]

so you have an inverted hip?

Roof pitch would be taken along a "common rafter" not the valley IMO

 

[SteelPE]

Inverted hip... I guess you could call it that.

This is theoretical but common in large steel framed structures. You may have a building that might be 14-40' bays long and 10-40' bays wide for a building that is 400'x640' with internal drains.

 

[msquared48]

I agree.

The minimum pitch is for the sheet flow condition of the common rafters. By the time it gets to a valley, it's stream flow.

Mike McCann
MMC Engineering

 

[ToadJones]

....not to mention, steeper the better I would say.
for a typical even pitched hip/valley 1/4":12 common gives a 1/4:17 hip and valley
this is a significant change

(i hate flat roofs unless they are in a desert)

 

[SteveGregory]

I would not measure the slope along a hip or valley line. Think of measuring the slope perpendicular to the "ridge line".

 

[msquared48]

I hate anything that is flat. It's just not interesting...

Mike McCann
MMC Engineering

 

[RHTPE]

IMHO pitch is measured perpendicular to a level line.


Ralph
Structures Consulting
Northeast USA

 

[msquared48]

Pitch:

What a salesman gives you;
The sticky stuff on a joist;
Rise over the Run;

And the answer is...

Mike McCann
MMC Engineering

 

[ToadJones]

definitely conservative to use the 1/4":12 pitch in the valley.
Your common pitch would be ~1/4:8.5

 

[Teguci]

So ASCE says to use a 1/4"/ft min (or else) but shows something less than 3/16"/ft in their diagram/sample problems. However, with a slope less than 1/4" you are required to check for deflections that may cause ponding. I say go ahead and do a quick check.

By the way, ASCE doesn't exclude a flat roof, it just says to check for additional loading due to long term deflections.

 

[cvg]

I live in the desert. I have a flat roof. it stinks. by the way it does rain here occasionally and it is during these infrequent rains that the roof leaks...

 

[ToadJones]

cvg-
from now on i will say " i hate flat roofs, even in deserts"

 

[ToadJones]

There is precisely two types of flat roofs:
1). those that are presently leaking
2). those that will be leaking soon

 

[cvg]

thanks
I guess my point is that 1/4 inch/foot may be a code minimum, but even in the desert it is questionable if that is sufficient to prevent leaking. remember you can specify anything you want on a set of plans, but what will the actual constructed slope end up being? and what about several years after construction when things begin to sag?

drain pipes are pitched at that minimum slope. If it was up to me, I would slope your valleys the same. Unless the code is very specific that the slope is to be measured along a rafter line, than interpret it liberally, protect your client from future roof repairs and exceed the code minimum slope if possible.

 

[hokie66]

To Toad's two points about flat roofs, I would add that the same is true of internal roof drains, no matter what the pitch of the roof.

 

[Ron]

The slope of a roof surface is just that...the roof surface. That's what the codes address. They do not require that the slope come from structural framing or the roofing system....just that the final slope is 1/4" per foot.

If you have a requirement of 1/4" per foot for slope, and you have 50 feet of length for which to apply the slope, you have to have a "top of structure" elevation difference of 12-1/2 inches. Why would you want to worry about that when you can design your structure as "flat" and require that drainage slopes be achieved by the roofing system?

Then you can use multiple internal roof drains to keep the surface elevation differential to something reasonable ( 3 to 4 inches or so). Design the roof structural framing to accommodate the loads from deeper ponding (overflow condition) and move on.

 

[JAE]

The 1/4"/ft slope is required in the IBC - not in the AISC specification that I'm aware of.

My flat roof story (sorry to hijack the thread):

My son (who is an architect) recently bought a “mid-century modern” house - a 3 bedroom modest house which had a flat roof which was a feature of these MCM homes. The roof was constructed of flat (no slope at all) 2x6’s @ 16” o.c. with a 1/2” plywood roof. The roof was irregular in plan and had a step-down lower roof at the garage.

He decided he needed to re-roof it because it leaked a bit and found a roofer who agreed to roof it if my son would do the demolition of the old built-up roof. He started on a Saturday morning with a bunch of tarps ready and a 30% chance of rain. 20 friends (including me) helped tear it off and he discovered that the plywood roof (from 1959) was pretty bad - delamination and many soft spots. We talked about covering the plywood with a 3/8” layer but eventually decided he might just patch the bad areas with new 1/2”. We finished tarping the roof at midnight.

At 2:30am the storm sirens went off and in comes 60 mph winds, driving sheets of rain, lightning and thunder. We peeked outside and saw the brick pavers we had used to hold down the tarps flying off into the front yard. He and I climbed up on the roof (lighting and thunder, wind and rain, oh my) and tried desperately to hold down the flapping tarps which were ballooning up about 6 feet above the roof.

We laid down on the tarps trying to hold them in place but our bodies were flopping around - It was then that I realized that I might die. heh.

Eventually we just nailed the tarp down and then spent the rest of the rainy night mopping up water and watching the ceiling light fixtures fill with water.

Moral of the story - flat roofs leak. Old flat roofs really leak. If you find yourself totally wet, flopping around on a tarp on a roof at 3am, with driving rain, lighting, thunder, flying bricks, and 60 mph winds, start praying hard.

 

[cvg]

the problem I have experienced is as described above, soft spots or sags eventually develop and even with minimum designed slopes, eventually you get bird baths forming. common problem is that the scupper does not sag and the roof does. water can't get into the scupper and then leaks through the flashing. adding a multitude of roof drains may not solve this problem. I would highly recommend the maximum pitch possible on a "flat" roof. But it seems a bit risky to leave this important design up to a roofer.

 

[woodman88]

Actually, the IBC allows roof coverings to have slopes down to 1/8:12. See section 1507.10.1.
Also see Section 1611.2 which states “For roofs with a slope less than 1/4:12 the design calculations shall include verification of adequate stiffness...”
You can also design the roof with a 0:12 slope by designing it to support all the possible water as a wading/swimming pool.
Hey, if they have the money I will design it for them.



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.