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Old 07-09-2011, 10:48 AM   #1
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deck beam length vs post spacing


I'm in the planning stages for a deck and at this point, I'm just trying to learn as much as possible, consider different options, etc. When built, it'll just be a deck, but at some point, I'd like to make it into a sunroom, so I want the structure to be able to support that in the future.

One thing I don't understand is beam length and post spacing. Ideally, I'd like to have 16' long beams, supported only at the ends. From what I've read, it looks like this is possible, but would require some fearsome beams. The thing I don't get is that there doesn't seem to be any allowance for adding more beams. So, my question is, if I add more beams, would it be possible to make each beam a little less fearsome?

Hopefully, the attached picture gives some idea as to what I'm thinking about. The red part represents the house, which is on 2 sides of the proposed deck. The beams are gray and the posts are black, with the joists in blue.

Thanks.
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Old 07-09-2011, 11:06 AM   #2
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deck beam length vs post spacing


You could probably have an engineer design something to span 16' and take the load, however, you are going to have issues with deflection. When you walk on the deck, it's going to feel bouncy. If it were me, I would span 8' with a second set of posts/beam. It will feel much more sturdy - especially if you are looking to convert it to finish space in the future. A couple of extra post holes won't kill you for a small deck like this. Lumber will probably be cheaper too with the shorter span.

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Old 07-09-2011, 11:44 AM   #3
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deck beam length vs post spacing


Agreed. No reason it cant be framed like a regular deck. Although you plan to add weight in the future, I think you'll be okay as long as the weight is evenly distributed to proper foundations.

The engineering guys will be along soon.
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Old 07-09-2011, 01:00 PM   #4
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deck beam length vs post spacing


What really jumps out at me when I first saw the drawing is the concept of attaching it to the house and the associated problems with ledger attachments and penetrating to house surface with regard to known future mold, moisture and durabilitiy problems.

Make sure the basics of flashing to shed water and not funnel that into an area behind the primary moisture barrier and si seal all penetrations.

The structural criteria is really just not the "canned" span design tables for load carrying capacity. Very often, the deflection and response to moving loads can cause other problems regarding usability and comfort. This takes a lot of experience and analyzing from afar gives a good feel initially, but makes you realize what is like to live with an over-analyzed design.

If you contemplate converting to a different use in the future, make sure you have a reliable, relatively rigid deck since the criteria becomes different when you convert to a living space and not just a deck.

Fortunately, you can have good lateral strength for such a small deck from a lateral stability standpoint.

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Old 07-09-2011, 01:01 PM   #5
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deck beam length vs post spacing


I don't know what code you are designing to, but if your town requires IRC 2006 (mine does), then you should check Table 3, which gives you the allowable beam span for various sized beams, based on the joist span. In your case, the joist span is 4 feet, which is pretty short. For a 6 foot joist span, you can use 3 southern pine 2x12's as the beam and span 17'5" based on the table.

I don't particularly recommend this long a beam span, previous posters correctly noted that the deck is likely to feel bouncy. Better perhaps to install a few more posts as discussed, shorten the beam span, and you can extend the joist span to 8 feet or so, a more common span for joists.
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Old 07-09-2011, 02:19 PM   #6
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deck beam length vs post spacing


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Originally Posted by Daniel Holzman View Post
I don't know what code you are designing to, but if your town requires IRC 2006 (mine does), then you should check Table 3, which gives you the allowable beam span for various sized beams, based on the joist span. In your case, the joist span is 4 feet, which is pretty short. For a 6 foot joist span, you can use 3 southern pine 2x12's as the beam and span 17'5" based on the table.

I don't particularly recommend this long a beam span, previous posters correctly noted that the deck is likely to feel bouncy. Better perhaps to install a few more posts as discussed, shorten the beam span, and you can extend the joist span to 8 feet or so, a more common span for joists.
Thanks---I'd seen that code document, but didn't quite know how to interpret that table.

The reason for the long span is that I'd like to park cars beneath the deck, and if I can get away without any posts in the middle, that would leave just enough space for our 2 cars. It's not a huge deal, but if it can be done without too much additional trouble or expense, I might do so.
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Old 07-09-2011, 02:53 PM   #7
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deck beam length vs post spacing


...and the plot thickens.
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Old 07-10-2011, 10:32 AM   #8
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deck beam length vs post spacing


have you considered steel I beams?

also, with cars parking underneigh you should consider designing the deck such that it can lose any one of the posts supporting the beam without collapse so when you give your 16yr old daughter the keys to the car the people on the deck will be safe. .. just say'n
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Old 07-10-2011, 10:55 AM   #9
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deck beam length vs post spacing


will your carrier insure a post foundation under a living space?
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Old 07-10-2011, 01:17 PM   #10
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deck beam length vs post spacing


Since you are just in the planning stage, no harm discussing options. In my town, a deck is designed to IRC 2006, which uses a total of 50 psf load for design purposes. This is greater than the load on an equivalent floor for a house, which could be 20, 30 or 40 psf, depending on the use of the room. So in most cases, the structural requirements for deck framing will exceed that for an equivalent occupied room. There is a catch, however.

The deck design typically does not incorporate deflection criteria directly, just strength criteria, because you are unlikely to cover your deck with tile, which is very sensitive to vibration and deflection. An occupied room is always designed both for strength and deflection, and deflection often governs, especially if you plan to use large format porcelain tile or natural stone tile. If you are, you may need to design the deck for deflection in addition to strength, in which case you need to either have a good understanding of deflection theory, or read the code very carefully. If you never convert the deck to occupied space, this is not an issue.

If you do plan to consider conversion to occupied space, you need to show this on the plans you give to the building inspector, else you are running the risk of getting your conversion permit denied if the inspector feels that the framing, though acceptable for a deck, is not acceptable for occupied space.

As to the park the car under the deck issue, the previous poster who suggested you think about impact on the posts is spot on, that would be a very unfortunate, potentially fatal event, and totally possible. Build some means of prevention of that event into the design. And if you plan to park cars under occupied space in the future, discuss this with the building inspector, there are probably all sorts of rules to prevent carbon monoxide poisoning. These may be found in the section of code governing over/under garages.
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Old 07-10-2011, 02:35 PM   #11
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deck beam length vs post spacing


Quote:
Originally Posted by Daniel Holzman View Post
Since you are just in the planning stage, no harm discussing options. In my town, a deck is designed to IRC 2006, which uses a total of 50 psf load for design purposes. This is greater than the load on an equivalent floor for a house, which could be 20, 30 or 40 psf, depending on the use of the room. So in most cases, the structural requirements for deck framing will exceed that for an equivalent occupied room. There is a catch, however.

The deck design typically does not incorporate deflection criteria directly, just strength criteria, because you are unlikely to cover your deck with tile, which is very sensitive to vibration and deflection. An occupied room is always designed both for strength and deflection, and deflection often governs, especially if you plan to use large format porcelain tile or natural stone tile. If you are, you may need to design the deck for deflection in addition to strength, in which case you need to either have a good understanding of deflection theory, or read the code very carefully. If you never convert the deck to occupied space, this is not an issue.

If you do plan to consider conversion to occupied space, you need to show this on the plans you give to the building inspector, else you are running the risk of getting your conversion permit denied if the inspector feels that the framing, though acceptable for a deck, is not acceptable for occupied space.

As to the park the car under the deck issue, the previous poster who suggested you think about impact on the posts is spot on, that would be a very unfortunate, potentially fatal event, and totally possible. Build some means of prevention of that event into the design. And if you plan to park cars under occupied space in the future, discuss this with the building inspector, there are probably all sorts of rules to prevent carbon monoxide poisoning. These may be found in the section of code governing over/under garages.
Thanks for the info on deflection vs strength. I've done a couple of tile jobs, so I'm familiar with the deflection issue. I hadn't thought about it in this context.

As for the posts being damaged by a car... That points to another advantage to having the full 16 foot span without posts in the middle. There is actually a lot of existing concrete on the side where the posts would go, so there's (almost) no way that a car could touch the posts---a tank maybe, but probably not a car. The house side might be a possible concern, I suppose. So, with lots of beams, I might be able to argue that a couple of them would have to be damaged (which would require an enormous gash in the side of the house) before catastrophe would ensue.
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Old 07-10-2011, 04:06 PM   #12
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deck beam length vs post spacing


Quote:
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When built, it'll just be a deck, but at some point, I'd like to make it into a sunroom, so I want the structure to be able to support that in the future.
You are planning on making this a sunroom in the future. You now have to design it as a room, not a deck. All plans now are to design it as an addition because that's what it will be.

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