Determining joist spans

[John_M]

Just curious....

How do you calculate the span length of a joist when you double it up.  Say a 2x10 vs a 4x10 (a 2x10 doubled up).

Also, do you need to consider the weight of the flooring.  My example would be 2x6 flooring vs 3/4 plywood.  Does this affect what you would use.

Obviuosly the distance between joists makes a difference too.  You can move them farther apart when you are using 2x6 T&G flooring right?

I want to build 18  feet wide and use T&G flooring.  Not sure what to do about joists.  I have seen several pictures of cabins (Mickey's maybe) that had the look I wanted but I'm unsure what to use as joists.  I would like an open floor plan on the first floor.

[Daddymem]

Sounds like a FirstDay Cottage system.  They use 2x4 sandwiched between 2x8s (or 2x10s), 2x6 T&G decking, and 2'-11" between beams.  Some good pictures on their site: http://www.firstdaycottage.com/pictures/building/Building01.jpg
Of course you would have to check your codes (if any), but they have these up in 28(?) states.

[JRR]

For some of the easiest reading on the subject, I recommend a look through "Architectural Graphic Standards" by Ramsey & Sleeper.  This highly illustrated text covered "everything" concerning building structures of a few floors, and was the student bible on many campuses.  You'll probably have to go to the library to find a copy.  There was an older and newer format ... I prefer the older, such as edition #5.

[jraabe]

This house http://www.countryplans.com/20w_loft.html uses both a standard joist system (24" o/c) where plumbing and wiring are important, and 2x6 decking and an exposed double joist system (48" o/c spacing) where you want the open beam look. This particular building is 20' wide and this is about the Max with standard lumber. However, you can also go 18" (and have an even stiffer loft floor).

The plans show how to combine the two systems and keep the finished floor the same level. (This is not in Arch Graphic Standards but that is a great book.) Here is the lower cost alternative: See "Graphic Guide.." here http://www.countryplans.com/books.html.

[joepittsburgh]

John,

I know this is an old post...I've looked for the answer with no luck so far.  What I am looking for is the beam spacing for open joists and 2x6 decking.  I'm looking at the plans and the closed system has 16 inch spacing.  I would like to use 4x12 White Pine but am not sure how to calculate the beam spacing.

Sorry --- I just found it on another page --- 48 inch.  Does anyone know where I can get a tool like the span calculator to see if 4x12 Eastern pine would work 48 inch on center...Thank you

[rocking23nf]

if you mean you are spacing your supports 4 feet apart with a 2x6 decking laying flat?  your gonna go right through that. Maybe im confused by your layout.

[MountainDon]

I was going to ask what the plan was for flooring on top? Then I got distracted and left the page....  

I would think the decking/flooring would have to be much thicker than finished 2x if you were running joists 4 feet apart. That might work for a ceiling with no attic load at all.    I think 4 feet can be used if the beams are acting as rafter ties with a cathedral ceiling; everything open.

[dug]

I used 2 by 6 tongue & groove in my loft with the beams spaced at 32 in. and it seems very solid.

[Don_P]

There is a safe load table for timbers. It is a bit different than what you might be used to, it requires some homework/ know how.
http://www.awc.org/standards/wsdd.html

Allowable spans for floor decking can be found in R503.1. Inch and a half T&G can go up to 60" spans depending on bending strength and stiffness (read the footnotes on that table). Laminated decking can go farther.