# Caculating joists in a 16 foot span **URL:** \ **Category:** Factual Questions\ **Created:** [September 16, 2011, 3:27am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474 "2011-09-16T03:27:48Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [September 16, 2011, 3:27am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/1 "2011-09-16T03:27:48Z")
Can someone double check my math? It’s a bit rusty. Figuring lumber for a deck. 16 foot span with joists 16 inches on center. 3 in x 16 in = 48 in giving — 3 joists every 4 feet. --\>\>\>times 4 = 12 joists in the 16 foot span. Do I need to add in the starting joist (the one attached to the wall) too? making 13 joists? I’m thinking that I do. I’m buying my decking boards in 16 foot lengths so there will be no cuts. I tried googling a joist formula and no luck. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 16, 2011, 3:31am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/2 "2011-09-16T03:31:18Z")
Yes. One joist spans no distance. Two span 16 inches. Three span 32 inches. Etc. ---
**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\ **Post date:** [September 16, 2011, 3:33am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/3 "2011-09-16T03:33:06Z")
I get 13. ETA - including the starting joist. ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [September 16, 2011, 3:36am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/4 "2011-09-16T03:36:54Z")
Ok. thanks. I thought it was 13 but wanted to double check. 😉 Didn’t want to buy an extra joist I didn’t need. ---
**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\ **Post date:** [September 16, 2011, 3:38am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/5 "2011-09-16T03:38:27Z")
> [@aceplace57](#): > > Ok. thanks. I thought it was 13 but wanted to double check. 😉 Didn’t want to buy an extra joist I didn’t need. Just do me a favor and use joist hangers. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [September 16, 2011, 3:38am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/6 "2011-09-16T03:38:42Z")
Yes, you need thirteen joists (Personally, I’d refer to the outer two as rim boards.) You might want to think of there being twelve spaces between joists. Dumb question (and please, no offense intended), but how did you size the joists? ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [September 16, 2011, 3:41am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/7 "2011-09-16T03:41:23Z")
2x10 is pretty standard for deck floor joists. Some deck builders go 24 in on center, but I wanted mine stronger and went old school with 16 in on center. I’ll have a center pier for mid-point support. I’ll dig the holes and set my sono tubes this weekend. Maybe mix & pour concrete next week. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [September 16, 2011, 4:00am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/8 "2011-09-16T04:00:30Z")
What’s the span, and what type and grade of wood are you using? ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [September 16, 2011, 4:08am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/9 "2011-09-16T04:08:59Z")
16 x 16 deck. #2 grade ACQ pressure treated lumber. I was thinking of using 2x10’s. The rim plate lagged into the end joists of the house. I will definitely use joist hangers to meet code. ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [September 16, 2011, 5:05am UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/10 "2011-09-16T05:05:19Z")
I forgot, the actual decking boards will be 1x6 ACQ pressure treated lumber. I’ll use a 16 penny nail to space them apart. I’m going pretty standard with my decking lumber. We’ll typically have six adults and three kids on the deck during barbecues. That’s why I’m using a middle pier. So I’m only spanning 8 feet on each side of the middle pier. I rather over build a bit and exceed code. ---
**Author:** ![JBDivmstr](https://avatars.discourse-cdn.com/v4/letter/j/b4bc9f/32.png) [@JBDivmstr](https://boards.straightdope.com/u/JBDivmstr)\ **Post date:** [September 16, 2011, 2:33pm UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/11 "2011-09-16T14:33:13Z")
> [@aceplace57](#): > > 2x10 is pretty standard for deck floor joists. Some deck builders go 24 in on center, but I wanted mine stronger and went old school with 16 in on center. > > I’ll have a center pier for mid-point support. I’ll dig the holes and set my sono tubes this weekend. Maybe mix & pour concrete next week. I’m curious as to why you would pour a pier for a square deck that’s only 16’ x 16’. 😕 How high above the ground will the bottom of your framing be? If it’s not more than 18" - 20", and the framing will be connected to your house, then 16" x 16" x 4" concrete pads with a solid concrete block and cedar shims should be sufficient for that application. IMHO 🆒 ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [September 16, 2011, 7:03pm UTC](https://boards.straightdope.com/t/caculating-joists-in-a-16-foot-span/596474/12 "2011-09-16T19:03:44Z")
It’ll definitely be strong enough. I checked a few span tables, and 2x10s of #2 Southern Pine are good for 17-18 feet (Also, sorry for not minding my own business.)