How to figure deck boards

Hi all,
Because of water damage to the wall of my house I had to remove my existing deck. I was able to leave the outer frame but had to remove ledger board, joists and deck boards.
I have repaired water damage and I am ready to rebuild deck. The old deck had no joist hangers and honestly was not built very well. I am going to make it free standing with posts in concrete and 2x10 beams. More questions about that later.
I am trying to determine how much and what sizes of lumber I need. My big question is deck boards. The deck is just over 18 feet wide, and the longest 5/4x6 deck boards I can get are 16 feet. How do I figure out what lengths I should use? Obviously I would like the fewest number of seams and to not waste money buying the wrong lengths. I can't put them on at an angle without removing all the railings and it also sits partially against a wall. I have tried some of the online deck designers but they have me using 18 footers. Is there a software to give me the best layout for the deck boards?
Thanks, Jeff
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jdguay wrote: ...

Well, one way would be to look at what was used before...
To figure from scratch if that doesn't solve the problem, 18's would definitely be pricey even if you could find them so they're out of consideration plus the "just over" indicates they might not be long enough, anyway, perhaps?
Have to start w/ the precise overall length you need. Is it actually 18', or 18-3 or what?
If it were actually such that 18 would do, I'd use a mixture of 8's, 10's and 12's, staggering joints either randomly or in a specific pattern.
If it is actually a little over, then figure out the nearest multiple that gives the least cutoff waste when split over two or three.
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"Just over" is going to create a PITA. If exactly 18' you can use various combinations of 8, 10, 12, 16 to get there with almost no waste.
Personally, I'd rethink the size if you have not started yet and either scale back to 16', go up to 20', or at least stop at an even 18'. With even 18' you can go with 12 & 6, then 6 & 12, then 8 & 10, etc.
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OK, its exactly 18' 6". I can't change the width because the deck frame is still in place. I have some of the old deck boards but alot of them broke when I was taking them out.
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"jdguay" wrote

Interesting thread! I've not done this but would one seam be acceptable? Measure and see if you have a spot where the floor beam hits just right to use a 12ft and but up the end of something shorter to make up the difference? This would make a single seam all along which you may not want but seems easier? Perhaps if the floor beams are even, you could alternate the pattern so a seem on every other board and another set on the other side just like it?
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OK, that makes a difference. Along the inside of the two outer joists put a secondary joists just a few inches apart from the originals. Now run a 3" border along it and you still have an even 18' between to lay the boards and have nailers where they should be. Since wood expands mostly on the width, you'd only need a small space between to allow for expansion.
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jdguay wrote:

Then you're going to have some overage...
Basically, you may as well figure it's 20' 'cause there's no multiple of commonly available stock that's going to save you much.
If you're really concerned about wastage, the effort to shorten the span by the 6" would be the way to go.
I fiddled around a little and didn't come up w/ any combinations of LCDs that help...unless I overlooked one.
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dpb wrote:

The other alternative than making the overall smaller would be to run a border around the outer edge to bring it to 18-0 or slightly less. (Turn a design flaw into a "feature" ... :) ) If the decking is nominal 6", could use one of them as is and be just about right.
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