Decks, Patios, Porches and Docks - beam construction

Is it possible (or recommended) to butt join two pieces of PT wood together to make a beam?

I went to Lowe's to use their deck planner just to see what might be involved in building my deck. It will be about 12' x 21', at the most 24" above ground, against the back of my house but freestanding. The way it came out of their system, it uses two beams (made from 2x8's on both sides of the 4x4 posts, with 4x4 spacers between them) running parallel to the house. There are 4 posts for each beam. The deck will overhang 2 or 3 feet against the house and it's opposite side. The joists will sit on top of the beams. I hope this makes sense and somebody can picture it.

So, to my question. Looking at the materials sheet, the 2x8's don't come long enough to span the 20' or so length. My guess is they would have to be "created" by butting pieces against each other. Is this ok? If the joint is positioned between the posts correctly, I would think it would be alright. Effectively, you're just cantilevering each piece over the respective posts.

Maybe the joints should be staggered? One joint at one long end of the deck and the other at the other end?

Incidentally, the decking material will most likely be composite, possibly angled to the deck sides, so the joist spacing will most likely be 12" centers.

2X8s are available in longer lengths but you might have to order them. Visit a lumberyard.

Crocostimpy-

I have found that the Lowe's and HD deck planning services are a bit lacking in the common sense department, although they are useful to get your mind wrapped around cost/materials.

With a 12'x21' deck size - you definitely want your built-up beams to run the 21' and use standard 12' 2x8's as joists.

Here's my suggestion on how to build up your 21' beam:

1. Yes - you need 4 posts. You can cantilever your beam up to 2 feet over the outside post ends if you so desire. It makes things easier when sizing - you can let your beams run a bit long and then cut them to match your outer rim board.

2. Therefore - I'd place posts 2', 7.66', 13.33', 19' and here's why: For your beam - you have to butt your joints ON the posts, and its better to stagger your butts than put them all on one post. Now - run a 14' 2x8 from one side, over the 2' post to the middle of the 13.3' post, then finish it with 8' worth of 2x8. For the second part of the beam - run a 14' 2x8 from the end over the 19' post to the 7.6' post and finish it off with a 8' worth of 2x8. Now - your butt joints are staggered on the 7.6' and 13.3' posts. And you have 6 inches of wild to play with on both ends that you can cut off to perfection at the end of framing.

3. To space your beam for the 4 posts - you can do it the way Lowes said - but I think its great if you can 'biscuit' your butt joints. My preference would not be to bolt your beam to the sides of the 4x4 post. Rather..I would use 1/2" treated plywood spacers to build the thickness of your beam to 3.5 inches - matching the tops of your 4x4 posts. Then use a Simpson post to beam connector and just place that beam right on top of the posts. Take advantage of the 1/2 treated and use it as a long biscuit running through your butt joints. You'll need to nail the heck of this built up beam with 16D galvanized nails - but only at the locations of the spacers. This design lets water fall through too which should keep the beam dry and rot free. I cut my spacers like home plate with the point facing skyward so they don't catch water. I made mine 6 inches thick and placed them every 2' oc.

If you prefer to use the bolting method - then I would think it better to get a 4x6 to give you some more 'territory'. Remember - when you bolt to the sides - its the strength of the bolts that holds things up. I'd notch a 4x6 on two sides and bolt through it before I'd bolt to the sides. That way the wood and the concrete get the bulk of the weight.

4. Your two beams can be wherever you want..but again - I'm a big fan of the cantilever design helping to hide any small mistakes in measurement you make with the beams. Two feet max though. I'm not sure that 2x8 joists can span 12 feet unsupported by code...so you may have to cantilever to shorten that length.

Also - I vote for attaching to the house..but that's another discussion!

I didn't do diagonal -but I'm a big fan of it. Stengthens your deck too.

Forget special ordering longer 2X8's -- it's not the extra money or the wait. Splicing them works, but put the butt joint of one 2X8 over one post, and the butt joint of the other one over a different post. A 12' or 16' 2X8 starting from the left end and ending over the farthest post possible, and the other 2X8 starts at the right end and ends over a different post. Get a 3 2X8 long enough to fill in the leftover end.

Well the beams won't sit on top of the posts, they will be bolted to the sides. If I do that, I like the idea of using 4x6's for those posts so there's enough meat there to bolt them to. I also like the idea of getting the longer boards and just using one piece. I haven't decided which way to do it yet. I suppose I should look into the price vs. hassle angle.

Thanks for the replies.

The beams sitting on top of the posts is stronger. But, if you are going to side mount them and thru-bolt them, the same thing applies -- stagger the joints.