# Plan Review of footing tributary loads for decks with roof



## jar546 (Jul 4, 2013)

When those of you that have ground snow loads do plan review for people that are building a deck with a roof, do you convert the ground snow load to a live load for purposes of checking the tributary load and footer size?

In my jurisdictions in PA we mostly have a 40#GSL and construction is usually a 10#Deal Load type construction.  I normally just add 50psf when checking footer sizes but know that it can be slightly reduced in most cases.  I leave it as is to be safe when checking.  What do you do in areas where you don't require a RDP for design?


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## steveray (Jul 5, 2013)

We do 30ll and 10dl....if they argue I would tell them to pretend like I am from Missouri and "show me"...and they can do the math and reduce per code.....


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## mtlogcabin (Jul 5, 2013)

We still use the old roof snow loads established by the state. They are higher then the current ground snow loads found in the code. So to answer your question decks without a roof have to meet the roof snow loads plus drifting. If the deck is protected by a roof then we use 2009 IRC Table R301.5 requiring a 40 lb live load for a deck.


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## jar546 (Jul 5, 2013)

I guess I am not clarifying myself enough.  What I should ask is this:

Do you change the ground snow load to a live load for purposes of being more accurate or do you just figure tributary based off of the ground snow load and dead load (in addition to the deck design and dead load of course)?


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## joetheinspector (Jul 5, 2013)

steveray said:
			
		

> We do 30ll and 10dl....if they argue I would tell them to pretend like I am from Missouri and "show me"...and they can do the math and reduce per code.....


We do the same


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## jar546 (Jul 5, 2013)

Let me use an example since this always seems to be an issue due to people thinking that a 12" sonotube works for all applications.

This is how I operate and how we do plan review for this part of a deck with a roof over it.  Your mileage may vary but this is what we do:

Example:

Applicant plans on building an attached 12' x 12' deck to their house with a 12' ledger board properly attached to the house and 2 piers to support the outward end of the deck, one on each corner.

We first take the deck of 12x12 and find the square footage which is 144.  Half of that load will be carried by the deck ledger and half split between the two deck piers.  Half of the load is 72 square feet and a deck must be designed to a 40# live load and a 10# dead load at a minimum for a grand total of 50psf.  If we take 50 x 72 (half the load) we get 3,600 pounds.  Since there are 2 piers, each pier will take 1,800# each.  Sounds simple, right?

In our area of PA 1,500psf soil bearing capacity is about the norm, sometimes more but without testing, we can safely assume 1,500psf based on the IRC prescriptive requirements.

If the soil bearing capacity is 1,500psf and each pier must be designed to handle 1,800psf then we can easily see that a 12" round pier simply will not work.  There are 2 problems with using a 12" round pier.  First, it is less than 1 square foot since it has rounded corners and second, due to that fact, it cannot properly transfer the 1,800psf load.

What is the actual bearing capacity of a round pier?  Glad you asked, great question.  We must first know how to calculate the area of a circle.  Pi times Radius Square will give us the area of a circle. (I'm not sure I have those fonts available here so I'll spell it out).  The radius is 6" and radius square is 36.  Pi=3.14 so 3.14 x 36 = 113 square inches which is not quite the 144 needed for a square foot.  It is approximately 78.5% of a square foot.  So, with a soil bearing capacity of 1,500# the maximum load that a 12" round pier can handle is about 1177#, way short of our 1,800# design requirement.  We would need much better soil bearing capacity in order for a 12" pier to work.

Now that we know we have an 1,800# design load on each pier, lets add a roof like we see all the time.

In our area we have a 40# ground snow load and most construction is a 10# dead load.  That happens to work out to a total load of 50psf, the same as the deck so we can safely just double the design load on that pier.  That means that each pier must be designed to handle 3,600# of downward pressure.

So this is where my original question comes in.  Any architect or engineer will tell you that there is a difference between a ground snow load and its equivalent live load.  Luckily the IBC addresses this in Chapter 16 and with the help of the ASCE-7 this can be easily calculated.  In our area with different factors applied, our 40#gsl is the equivalent to more like a 34# live load which may make a difference for the piers.  Maybe, maybe not.  I think on a large job this is extremely important.

I want to look at the difference between the 2.  Instead of using 40#, we will use the 34# load.  34+10= 44psf so the same tributary load of 72' x 44 = 3,168 and since the piers will be sharing that load, it is 1,584# each pier.

So for the deck and roof we have the following design load for each pier:

Deck:  1,800

Roof:  1,800 or  1,584

Total load for the piers can be either 3,600# or 3,384#

The question is, prescriptively, what size pier/footing do they need?


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## Francis Vineyard (Jul 5, 2013)

*R301.6 Roof load. *

The roof shall be designed for the live load indicated in Table R301.6 or the snow load indicated in Table R301.2(1), whichever is greater. 

"All prescriptive provisions in the IRC use the ground snow load for ease of reference, but that does not imply that the design roof snow load is equal to the ground snow load."

2003 Interpretation

Francis


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## mtlogcabin (Jul 5, 2013)

> In our area with different factors applied, our 40#gsl is the equivalent to more like a 34# live load which may make a difference for the piers.


That is why we do not use ground snow. Our historical snow loads are about 10 lbs higher then the codes have.

Total load would be the 3,600 lbs in my area Based on R301.6.


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## jar546 (Jul 5, 2013)

For ease, we don't change it since the math is easier with GSL.  Between the deck and the roof we are at 100#.  If it is close, we break it down or if there is a complaint, we just tell them to show us just like in another post above.

We recently had a contractor unable to comprehend this situation even though he was brought into the office and explained directly.  He had a reverse gable and was going to put 3 piers under the far side of the deck.  He did not comprehend that the 2 outside corners were going to take more than the center pier which was directly under the gable end of the roof.  Although the piers were equally sharing 1/3 of the deck tributary load, the outside piers were also sharing 1/2 of the roof tributary load.  He offered to put a post down to the pier from this clear span truss roof.  It was then that I realized he had no clue what he was doing.


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## STB (Jul 5, 2013)

And the answer is-(based on 1500psf soil bearing)

Round footings

8"=525

10"=810

12"=1185

14"=1590

16"=2055

18"=2655

20"=3270

22"=3960

24"=4710

Square footings

12"x12"=1500

14"x14"=2042

16"x16"=2667

18"x18"=3375

20"x20"=4167

22"x22"=5042

24"x24"=6000

Feel free to copy and paste as a reference card.


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## jar546 (Jul 5, 2013)

STB said:
			
		

> And the answer is-(based on 1500psf soil bearing)Round footings
> 
> 8"=525
> 
> ...


Hey, I got approx 1177 for a 12" round.  Why are we off by 8 pounds?


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## mtlogcabin (Jul 5, 2013)

1185 -1177 = 8 lbs.


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## jar546 (Jul 5, 2013)

mtlogcabin said:
			
		

> 1185 -1177 = 8 lbs.


yeah, I edited that right after I posted it and before I saw your post.


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