# joist span/beam spans



## my5sons (Oct 29, 2008)

I originally intended on building a 32x10 porch addition on the front of my house with a gable roof. I now have lowered it to 32x8.

My plans originally involved a cantilever beam of 2x10 lumber at 4 feet from my house, supporting 8 foot 2x8 lumber and then another beam at the front of the porch. (photo below)










I have dug 8 holes for the porch. I think now that this might be overkill and am interested in hearing others.

I am using 2x8 joists running perpendicular to my house at 12 inches on center. My posts for the roof and the deck will be at '8 ft 9 in' apart. I am using the BF20 (20 inch round) Big Foot footing forms along with 8" quiktubes.

I have been using American Forest and Paper document "Prescriptive Residential Wood Deck Construction Guide" to plan this porch addition. You can read it... located here:

http://www.awc.org/Publications/dca/dca6/dca6.pdf

According to their document, if I used 2x8 beam, I can only go 7 ft 11 in. on my posts to beam. I am exceeding this by about 10 inches. This is why I intended on using 2 beams. Maybe my thinking is flawed.

If I look on decks.com, they are saying that I can go 10 feet between posts using a 2x8 double beam. (using 12" on center joist spacing)

So here are my questions:

1. Do I really need a beam at 4 feet and a beam at 8 feet? Will this be a benefit or overkill? AF&PA also says that if I place beam closer than 5 feet to existing house, I must pour footings at the same height as existing foundation.
2. If I go with a single double beam at the outer band, is it ok to use a double 2x10, even though my ledger and floor joists are 2x8? 
3. If I use a double 2x8 beam flush beam (like this photo)








, 

will it still be able to support my roof as well?

Thanks in advance for any help.


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## AtlanticWBConst. (May 12, 2006)

Cantilevered beams are no more than 2' from the edge of a structure. That means a cantilevered beam would be at the 6' point (not 4'), on an 8' span. 
For an 8' span, you do not need two supports. I'd suggest cantilevering using 2x12's for your beam. 

Also, I'd suggest using 6x6 posts, but: Notching out each side of the post 1-1/2", and leaving the post's middle 2" section. Then you can drill and carriage bolt the whole arrangement together, sandwich style (2x12 beam / remaining 2" section of 6x6 post / 2x12 beam). So the 2x12's would sit ontop of the notched outside edge, of the 6x6 (each side).

Make sure all the rest of the important structureal areas are done properly, including the footing sizes, footing locations, ledger board installation, etc.

Remember that all has to be laid out on a structural detail, to submit to the building dept for approval.


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## my5sons (Oct 29, 2008)

AtlanticWBConst. said:


> Cantilevered beams are no more than 2' from the edge of a structure. That means a cantilevered beam would be at the 6' point (not 4'), on an 8' span.
> For an 8' span, you do not need two supports. I'd suggest cantilevering using 2x12's for your beam.
> 
> Also, I'd suggest using 6x6 posts, but: Notching out each side of the post 1-1/2", and leaving the post's middle 2" section. Then you can drill and carriage bolt the whole arrangement together, sandwich style (2x12 beam / remaining 2" section of 6x6 post / 2x12 beam). So the 2x12's would sit ontop of the notched outside edge, of the 6x6 (each side).
> ...


I originally attempted the cantilever at 2 feet, but I ended up running into issues with the placement of the posts for the roof. I am using BF20 Big Foot footings, and the placement of my pier posts and the roof support posts were in the way. Does that make any sense? I tried to move the posts around to keep them staggered, but nothing I did worked, so I moved the holes to a new location.

Are you saying that I can use double 2x12's at the front as a flush beam?

Thanks again for the help!


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## my5sons (Oct 29, 2008)

Is this what you mean?


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## 12penny (Nov 21, 2008)

5sons,
AtlanticWB is correct. Find someone who has a little more stuctural knowledge to help you with your drawings before submitting them to your your building dept. Typical span for a 2x8 joist, 12" oc is about 13 1/2'. Therefore, you dont need the beam in the middle. Unless you want to re-drill your footers, you will have to use double 2x12 beam because the max span ( thats the maximum span between posts) for a double 2x10 is 8' and yours are a little farther apart. Good luck with your project.


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## my5sons (Oct 29, 2008)

According to AF&PA document, when using 8 foot joist spans, I can use a Southern Yellow Pine double 2x10 that has a max beam span of 10' 3". So because I don't really need a beam in the middle or even cantilevered, does that mean that I can use a double 2x10 on the outer band to support the deck?


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## my5sons (Oct 29, 2008)

AtlanticWBConst. said:


> Remember that all has to be laid out on a structural detail, to submit to the building dept for approval.





12penny said:


> 5sons,
> AtlanticWB is correct. Find someone who has a little more stuctural knowledge to help you with your drawings before submitting them to your your building dept.


In South Carolina, I do not need to submit drawings to the building department for residential work. I do have a permit, but my question is whether or not I can use the flush beam application for my deck?

Thanks.


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## jogr (Jul 24, 2007)

my5,

What you've drawn is not a cantilever. A cantilever would have no post and beam at the outside edge. Since you are putting a roof on this deck I think you should not use a cantilever. For an 8' covered deck you should be able to select a beam and post configuration that will allow you to use one beam at the outside edge supported by posts at appropriate intervals.

Do you know what loads you are planning for? Around here we would need to plan for 50psf for the deck plus 55 psf for the roof for a total of 105 psf. For example, assume an 8' deck supported by one beam at the end with posts 4' apart and the roof. The interior posts would support 16 square feet of deck and roof x 105 psf = 1680 lbs of load per post and pier.

According to the prescriptive chart your double 2x10 beam supported at 10'3" or less will handle joist spans of 8' so yes you can use a double 2x10 beam with 2x8 floor joists. And the beam could be a flush beam. 

This assumes that your roof load is being carried only by the posts/piers and not by the beam and joists. How are you figuring out your roof structure?


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## my5sons (Oct 29, 2008)

Thanks for the help jogr. Yes, the posts for the deck are supporting the roof structure as well. 

For the roof, I will be requesting that the builder be using the top plate of the house wall to tie into. I have already hired a builder to do the roof. It is a ranch style house and the deck and roof will tie into the front of the house and the side of the garage. The builder assures me that he can work with the post locations.

Here is a photo of my house (this was when a contractor did it, I have since torn it down and am starting over!):









Here is a webpage of what a Licensed Contractor did to my house before I fired him and tore the deck down to rebuild it myself. He passed the footing and was going to pass the framing inspection! 

http://webpages.charter.net/my5sons


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## my5sons (Oct 29, 2008)

jogr said:


> my5,
> 
> What you've drawn is not a cantilever. A cantilever would have no post and beam at the outside edge. Since you are putting a roof on this deck I think you should not use a cantilever.


The drawing is a 2' cantilever with 2x10 beam. The posts at the front of the porch are only holding the roof up, not the deck, no beam, just a rim joist. Will this work?


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## Aggie67 (Dec 20, 2008)

WOW! Wow.

That is what we call jughead work. I hope you didn't lay out too much money. That is some wild stuff.

The AWC's prescriptive guide for decks is a good source of info for the deck. I use it as a guide, but then I double check the IBC or local codes. For the roof, make sure your builder knows his stuff.

Also, now that you have a roof on it, I'd double check the wind loading. You're going to have to address the additional dead load, rain load, wind load and uplift since it's tied into both your new footings and the original structure. Last thing you want is the porch roof blowing off in a hurricane, and taking the roof of the house with it.


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## jogr (Jul 24, 2007)

my5sons said:


> The drawing is a 2' cantilever with 2x10 beam. The posts at the front of the porch are only holding the roof up, not the deck, no beam, just a rim joist. Will this work?


 
Just make that rim joist at the front a double 2x10 beam and you can eliminate all those other footings, posts and the beam that you drew for the "cantilever".


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## jogr (Jul 24, 2007)

It's going to be difficult to get any pitch on that roof if you have the deck as high as is shown in the picture. If you can drop the deck around 32" you could get a 4/12 pitch roof and still have headroom.


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## AtlanticWBConst. (May 12, 2006)

I wasn't orginally aware of the roof. My suggestion was based on deck only.

The roof's posts should be aligned with the deck support posts. That means that the posts should be directly above eachother, so that there is solid framing from the roof, to the footings.


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## my5sons (Oct 29, 2008)

jogr said:


> It's going to be difficult to get any pitch on that roof if you have the deck as high as is shown in the picture. If you can drop the deck around 32" you could get a 4/12 pitch roof and still have headroom.


Yeah, you know I thought the same thing when I was getting quotes, but none of the contractors ever mentioned the roof pitch. I can't drop the deck at all because I won't have anything to attach it to...or at the very least it will be difficult. 

The current house roof is a 4-12 pitch and if I use the top wall plate to attach to I will end up with approx. 8' of headroom on the porch. I think I see what you are referring to. If I use 2x10 on the top wall plate, I will not be able to do a 4/12 pitch. I wonder why the last contractor never mentioned this?


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## jogr (Jul 24, 2007)

A 4/12 roof pitch will drop 32" over 8' so you might have 8' headroom at current soffit but it will be down to 5 and 1/2 feet at the outside edge. A 3/12 pitch will drop 24" and that's getting pretty shallow for shingles.

Are you thinking you can't bolt a ledger to your foundation? Looks like brick veneer and its true that you generally shouldn't use brick veneer for bearing a deck but since the height would be minimal your local authorities might allow it. If not you could put in piers near the wall and make it freestanding.


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## my5sons (Oct 29, 2008)

jogr said:


> A 4/12 roof pitch will drop 32" over 8' so you might have 8' headroom at current soffit but it will be down to 5 and 1/2 feet at the outside edge. A 3/12 pitch will drop 24" and that's getting pretty shallow for shingles.
> 
> Are you thinking you can't bolt a ledger to your foundation? Looks like brick veneer and its true that you generally shouldn't use brick veneer for bearing a deck but since the height would be minimal your local authorities might allow it. If not you could put in piers near the wall and make it freestanding.


I am not sure I follow you. The peak of my current roof is approx. 13.5 feet above the top of my deck surface, and 5.5' above my top wall plate. If I do a gable roof spanning 32 feet, I can do a 4/12 pitch. 16 feet on the left dropping 5.5' and 16 feet on the right dropping 5.5 feet.

From what I understand a 4/12 pitch is the minimum for architectural shingles. I hope I'm ok. If I have to drop my porch overhead to 7.5 feet I will.


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## jogr (Jul 24, 2007)

my5sons said:


> I am not sure I follow you. The peak of my current roof is approx. 13.5 feet above the top of my deck surface, and 5.5' above my top wall plate. If I do a gable roof spanning 32 feet, I can do a 4/12 pitch. 16 feet on the left dropping 5.5' and 16 feet on the right dropping 5.5 feet.
> 
> From what I understand a 4/12 pitch is the minimum for architectural shingles. I hope I'm ok. If I have to drop my porch overhead to 7.5 feet I will.


I was assuming you would continue the current roof lines and not rebuild the roof over the existing house. So your roof would start at the height it currently is over the double plate and then decrease 1 foot for every 3 foot from the wall. 

I suppose you could put a gable end roof over it but it can't dive into the garage roof. That would leave you a flat valley between the current garage roof and the new porch roof which would leak. You could put a big gable over both the porch and garage but would that look ok? Is that the 32' you are describing? It would cost a lot more than dropping the deck 32". 

Just about everything I've read says 4/12 or steeper for shingles. I've seen occasional uses of 3/12 if absolutely necessary and if snow isn't a factor but 4/12 is probably a better minimum.


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## Wildie (Jul 23, 2008)

One thing that I would like to say is that using a ledger board is not a good idea! Two rows of beams, supported by piers, and keeping the deck removed from the building would be my choice! I just finished helping my neighbor put in new piers and beams. Rot had penetrated from behind the ledger into the rim joist and caused extensive damage!


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## my5sons (Oct 29, 2008)

jogr said:


> I was assuming you would continue the current roof lines and not rebuild the roof over the existing house. So your roof would start at the height it currently is over the double plate and then decrease 1 foot for every 3 foot from the wall.
> 
> I suppose you could put a gable end roof over it but it can't dive into the garage roof. That would leave you a flat valley between the current garage roof and the new porch roof which would leak. You could put a big gable over both the porch and garage but would that look ok? Is that the 32' you are describing? It would cost a lot more than dropping the deck 32".
> 
> Just about everything I've read says 4/12 or steeper for shingles. I've seen occasional uses of 3/12 if absolutely necessary and if snow isn't a factor but 4/12 is probably a better minimum.


Thanks jogr, you have educated me a lot and given me more thought about this project. I do not have the option of dropping the porch. For one, it just won't look right. The top of the windows would be hidden behind the top of the roof. I'm not interested in doing that.

Here is my other option. This is NOT my house, but one I found online that is built very similar. I could leave a gap in the opening of the entryway. The entry door is already set back 4 feet from the edge of the current roof.










I could then build a shade arbor over the top between the 2 gable ends of the porch and garage. Maybe there are other options too? (I am 100% against dropping the porch more than 5 inches becuase of the tops of the windows. )I know this maybe isn't ideal...I guess I don't have a choice other than to build a deck on the front of the house with no roof or skip the arbor and leave the house like the one pictured. I am not sparing expenses. I have used flashing and waterproofing membrane behind the ledger, flashing on top of the ledger and caulking all around. I also purchased $180 worth of BigFoot Footings, so I am interested in doing this the right way, but I guess after 4 contractors gave me bids and never mentioned the intersection of 2 gable roofs, I assumed I was ok. I will be sure to ask the guy about it.


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## my5sons (Oct 29, 2008)

By the way, I contacted GAF shingles and they said that 2/12 is the minimum slope when using their shingles. If I went with a shed style roof, I could probably make this work. I just have to overlap the underlayment a minimum of 19".


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## my5sons (Oct 29, 2008)

Here is another photo showing the idea of what I am talking about: (notice the opening between roofs ends.)

http://www.homebuyershandbook.org/ranch_style-homejpg.JPG


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## jogr (Jul 24, 2007)

The opening between the gables is a good idea. The contractors might have been planning to put a very shallow sloped "triangle" between the gables (I forget the roofing term at the moment) but it would have very little slope and could easily lead to water issues.


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## Wildie (Jul 23, 2008)

Personally, I think that extending the gable is the only option. Anything else would look like hell! Maybe a flat roof between the gables for the entrance, covered with a rubber membrane would be OK!


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