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Old 12-21-2008, 08:34 PM   #1
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joist span/beam spans


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.


Last edited by my5sons; 12-21-2008 at 08:39 PM.
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Old 12-21-2008, 10:37 PM   #2
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joist span/beam spans


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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Old 12-21-2008, 11:11 PM   #3
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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.
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!

Last edited by my5sons; 12-21-2008 at 11:18 PM.
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Old 12-21-2008, 11:51 PM   #4
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joist span/beam spans


Is this what you mean?

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Old 12-22-2008, 08:38 AM   #5
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joist span/beam spans


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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Old 12-22-2008, 10:52 AM   #6
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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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Old 12-22-2008, 11:04 AM   #7
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joist span/beam spans


Quote:
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Remember that all has to be laid out on a structural detail, to submit to the building dept for approval.
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Originally Posted by 12penny View Post
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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Old 12-22-2008, 12:04 PM   #8
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joist span/beam spans


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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Old 12-22-2008, 12:14 PM   #9
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joist span/beam spans


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

Last edited by my5sons; 12-22-2008 at 12:20 PM.
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Old 12-22-2008, 12:29 PM   #10
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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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Old 12-22-2008, 12:51 PM   #11
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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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Old 12-22-2008, 01:16 PM   #12
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Originally Posted by my5sons View Post
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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Old 12-22-2008, 01:28 PM   #13
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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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Old 12-22-2008, 02:09 PM   #14
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joist span/beam spans


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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Old 12-22-2008, 02:57 PM   #15
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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?


Last edited by my5sons; 12-22-2008 at 03:01 PM.
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