Replies

1. 
   DDay | May 14, 2006 03:33am | #1

   I'm in Metrowest boston.  If you outside the city, check with your building department, they should have info on the spans, and what they want in that town.  If your in Boston, from what I've heard, good luck dealing with them.

   The joist spans in my town for 2x10 @ 16 o.c. are a little more than 15'. 

   Also, in this area most lumber yards will do engineering if you buying the material through them, check with them and see if they'll run the numbers.

1. 
   TheDudeAbides | May 14, 2006 04:33am | #2

   I'm in the south shore. I'm just asking  generally speaking, but knowing that your town is 15' makes feel better.  I actually went to the inspector and asked him about any guidlines and got a blank stare... he didn't seem too interested in anwering any questions.

1. 
   DDay | May 14, 2006 05:42am | #3

   My town gave me a print out with a grid of 12 oc, 16 oc, 19.2 oc, 24 oc, etc and 2x6, 2x8, etc with the spans for each given the oc.  The building inspectors for my town are really easy going and always very helpful.  You might have caught yours on a bad day, call up and see if they have something similar.

2. 
   DDay | May 14, 2006 11:37pm | #5

   I need to correct myself from the earlier post.  The 15'5" span for 2x10's in my town is at 12" o.c., the 16" o.c. span is 13'4". 

   I found  the print out that the inspector gave me, it is table 3605.2.3.1C  It is titled Allowable spans for floor joists in decks and balconies.  See if they'll give you a photo copy of that.

    

1. 
   bobl | May 15, 2006 12:53am | #6

   http://www.mass.gov/bbrs/code.htm 

   bobl          Volo, non valeo

   Baloney detecter

1. 
   DDay | May 15, 2006 03:29am | #7

   thank you

2. 
   TheDudeAbides | May 15, 2006 04:46am | #9

   Thanks...I'll just do everything according to this, then I won't have to worry about it.  :)

    

2. 
   Matt | May 14, 2006 05:03pm | #4

   I'm assuming this is an open deck - no roof.

   >> Could I have a 4x12 beam span 12' between two posts with joists spanning 7', 16"o.c.?  Or should I just add another post in between and switch to a 4x10 beam. <<

   12' is a pretty long clear span for a conventional lumber beam.  Personally I'd go with the 10" beam with the 6' spans, but I'm not the one digging the holes. :-)  The 10" beam could easily span that.

   >> Could I span a 2x10 joist 13'6 16" o.c.? << yes

   >> Last one... is there any reason why I couldn't put two 4x4 posts on the same 10" diameter footing? (level change) <<

   10" footing to fit 7" if lumber is kinda tight.  More importantly footings are sized according to tributary load - how much actual deck the post(s) are supporting.  IMO, 2 posts together can be considered as 1.

   BTW - you can cut down on the span of your joists by using a drop girder with a slight cantilever on the joists - say 12"  Likewise, you can cantilever out your beam ends beyond the end posts, say 10", to shorten the girder clear spans.

   All of the above is dependant on what your local (state) codes require and specifically PSF ratings on decks.  Ours are only 40 PSF as we have no snow load to speak of.  You really need to get a span chart for your state/city.

1. 
   TheDudeAbides | May 15, 2006 04:40am | #8

   Forgive my ignorance, but ... what's a drop girder?

1. 
   Matt | May 15, 2006 01:56pm | #10

   >> what's a drop girder? <<

   When building floor systems, a girder or beam can be installed in either a flush or drop position.

   A drop girder is installed so that the joists sit on top of it.  Gravity and a few toenails enusre that the girder-joist connection stays in place.

   A flush girder is installed so that the joists butt into the girder - the top of the joists are flush with the top of the girder.  The flush girder-joist connection is reinforced with joist hangers, etc.

1. 
   TheDudeAbides | May 16, 2006 02:49am | #11

   ThAnKs

3. 
   PV Rich | May 16, 2006 05:33am | #12

   Check in with the Western Wood Products Association. They have a cardboard sliderule calculator for about $2. If you have a basic sense of what the physics of load and support is, you can easily figure out what is appropriate without guessing. A rule of thumb: With a beam (or any lumber) you square the width and cube the depth. A 4 x 12 ends up for strength 4x4=16 for width and 12x12=144 for depth. A 4x4 equals 4x4 + 4x4x4 = 80. A 2x8 = 2x2 + 8x8x8=516 much stronger for the same amount of lumber (4x4=16 and 2x8=16 see?). That said, big spans with big lumber produce bouncy floors OR mo' holes make bedda floors!
   Rich

1. 
   TheDudeAbides | May 16, 2006 10:37pm | #14

   "That said, big spans with big lumber produce bouncy floors OR mo' holes make bedda floors" 

   I guess my back and I grudgingly agree... :)

4. 
   storme | May 16, 2006 07:26am | #13

   Taunton sells a good book that covers this with a useful level of detail.

   http://www.taunton.com/store/pages/070449.asp