Hi,
I was wondering if someone can point me in the right direction? I’m trying to design a fairly large deck ( 30’ wide and 20’ deep). Due to it it’s depth, I figured that I could limit the joist spans by having 2 beams running parallel to the house. However I’m struggling to find beam span tables for this scenario. IRC table R507.5 explicitly states that it’s for “beams supporting joists from one side only”.
I think I can make the design work by limiting it to 18’ depth with single beam using 2x12s. But seems extremely expensive.
I would much rather work with 2×8 joists spanning 10’ and 2 beams with posts every 5’ 9” (the post spacing is calculated for footing pad). But I can’t find any specs for center beams.
Looking at the published beam span tables, it seems like a 2-2×10 beam would support joists spanning 18’. Does that imply that it also work if it was a center beam?
any and all guidance would be very appreciated. Thanks!
Replies
Can I ask why you need a 600 square foot deck? Are you having dance parties or something? My township uses Chapter 6 Prescriptive residential construction guide. I would not design anything until you have a copy of a deck guide in your hand. Call your borough or township and request a copy . You have some very wide beam spans. Some townships have restrictions on size.
Hi,
Thanks for responding. The deck size reflects my optimism for a post covid world where I can entertain large groups of people.
I've been following the DCA, but am trying to overcome its limited scope of single beam deck designs. Using the DCA, I can use 20' 2x12 joists with a 2-2x12 beam. But it carries a significant cost premium and my chances of finding a straight 2x12x20 are pretty slim.
So I'm hoping that by introducing a second beam, I can be less selective about my lumber and significantly decrease my costs
U can cantilever the joists (L/4) out past a drop beam. Meaning you would have a ~16' span with a 4' cantilever. It looks like u can accomplish this using 2x12 joists (DCA 6 span table attached 2018)(50psf total)(16" OC)(SYP)
or u could get to 19'3" total span with cantilever using 2x10 @ 12" O.C.
That would eliminate a lot of posts/ footers / beam required for dual parallel beams. which will reduce the cost. I think a single beam with 2x12 deck joists will be cheaper than a dual beam design using less expensive 2x8. The post and footers are the most work. Although with a dual beam you could potentially eliminate the ledger attachment and make the deck freestanding*
Consider using PT LSL for the framing. They are straight/ longer span (could reduce depth of joists) and can get a 20 year warranty, if u use joist tape. PT 2x12 will be really crooked at 20' . might add 3$ per square foot to use treated LVL.
If you use 2x8 PT LVL you could span 14' 10" from the ledger, plus a cantilever of 3' 8 1/2" get you out past 18' total span. Confirm (l/4) for pt lvl.
Hi,
Thanks for the response! Also thank you for telling me about treated LVL, this is the first time I'm learning about it.
Doing some rough calculations (using 16" joist spacing), a single beam design is about $1k more expensive than a dual beam design
Dual beam:
sistered joists 2x8x12: 46 x $35 = $1600
sistered beams 2x10x20: 6 x $70 = $420
footing concrete+hardware+posts : 11 x $120 = $1,320
total = $3,340
Single beam
lvl joists: 2x10x20: 23 x $140 = $3,200
sistered beams 2x10x20: 6 x $70 = $420
footing concrete+hardware+posts : 5 x $120 = $600
total = $4,220
I'm guessing labor costs for digging the footings would narrow the price difference. I need to think on this a bit more.
Hi,
-u r showing the beam material costs the same for both calculations.
-LVL will be a superior product and labor saver (they are nice to work with)
-Labor for digging footings, setting the columns, setting the beams will be about 30+ man hours? (0n the second beam only)-
Sounds like it will be a sweet deck!
Yea, I figured that using lvl for beam material is an independent factor that applies equally to both approaches. So I kept it constant.
Thanks again for all of the advice :)
Mike Guertin pointed me to fairfax's county building dept website, which contains this gem:
Typical Deck Details https://www.fairfaxcounty.gov/landdevelopment/sites/landdevelopment/files/assets/documents/pdf/publications/deck-details.pdf
Not sure how applicable fairfax county's code is to other localities, but it does provide a point of reference. The pdf has a table that correlates "beam influence" to post spacing. They define beam influence as the sum of joist half spans. So in my example beam influence = 10/2 + 10/2 = 10. Following table 3 in that pdf (and ignoring all other details), I see that in 2015, fairfax county would've been ok with me using a 2-2x10 beam with posts less than 6' apart.
Now I have to go decide how much I hate digging footings
Thanks for everyone's help