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Old 04-23-2010, 01:07 PM   #1
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proper support for floor joists in my "man cave"


I have a 24' x24' garage. I want to build a "man cave" in the overhead space above. The former owners used it as a storage area by slapping down a few 4x8 plywood and a retractable ladder (for attics).

Problem is that the floor joists are 2x6 and span 24'. Never seen 24' 2x6....but sure as heck, there they are. And there is no support beams at all.

There does not appear to be any sagging since this area is really not used much except to hold xmas decorations etc.

I was thinking about sistering an additional 2x6 and then adding some support beams. As a DIY person, however, I would rate myself with somewhat basic skills. I can do carpentry but have never really dabbled in framing, so if I am way off the mark here I would REALLY appreciate any help or advice out there.

Problem is that the floor joists are 2x6 and span 24'. Never seen 24' 2x6....but sure as heck, there they are. And there is no support beams at all

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Old 04-23-2010, 04:10 PM   #2
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proper support for floor joists in my "man cave"


I am unclear whether you are planning to pull a permit, or want to build to code. If you do want to build to code, and/or pull a permit, you need to find out what the required floor loading is for your area, and for the use to intend. I don't think the code will have a specific load requirement for a "man cave"; the building inspector would decide what provision of the code applies. In my area, the space above a garage might be classed as living room space, which would probably be 30 pounds per square foot live load.

If you do need to support 30 psf, you would need 2x12 fir joists 16 inches on center to span 24 feet full. If you put in an intermediate joist support, the required size would be much lower. In any case, the simplest approach is to check with the building inspector on classification of the space, required loading, and required joist size.

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Old 04-23-2010, 04:30 PM   #3
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I don't know how they are not sagging 3/4 of an inch from their own weight on that span. To make it an inhabitable space, you will need 2 x 12's or an engineered product like TJI for that clear span.
Putting a support beam down the center of the garage, thus effectively cutting the span to 12' would allow smaller floor joists, but may not be very practical from an installation aspect.
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Old 04-23-2010, 06:28 PM   #4
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proper support for floor joists in my "man cave"


Depending upon the 2x6 wood species & grade I am only seeing around 10' 9" for a span @30psf L360
If you have 24' w/2x4 walls then you have ~23' 4" to span
Cut in 1/2 that equals 11' 8' for a span, so you would need (2) beams to use the existing joists
If you can put a beam in & sister in 2x8's that would work

Local lumber store may be able to size an LVL beam for the span
I had a heavy load for my addition & needed (3) 14" LVL's
Yours would be less load
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Old 04-23-2010, 07:39 PM   #5
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I am not a pro, I am a fellow DIYer.

That does seem like quite a lot of distance for a 2x6 already.

My garage ceiling spans about 21 feet, but they are trusses which yields an entirely different strength.

I don't have the definite answer for you, but I think some of the answers here point you in the right direction. In addition to the distance of the span, I think you will need to factor the spacing of the members. I would imagine 24" OC spacing would need stronger beams than 16" OC, for example. The design would be based on how much dead load and how much live load is needed.

You might be able to get this information from your local code enforcement authority, however, if they are anything like mine, they might be really confused by seemingly normal questions and never really give you a good answer.

You might get a good enough answer with a span table. I would inquire at your friendly neighborhood lumber yard (and then buy from them). I would avoid the big box home improvement stores at all costs. They may give you answers that are not only absolutely wrong, but utterly dangerous.

In my situation they told me that three 2x10 joists sistered together would be sufficient strength where I needed a beam to span 20 feet. In reality, I hired a structural engineer and his answer was that I need two LVL beams with 24" web sistered together. This is a _MAJOR_ difference and it is a beam that bears the majority of the weight of a project. Those LVL beams themselves weighed 500 pounds, not to mention their difference in load bearing ability, deflection, and whatever else.

To attach the sister joists, you can of course use nails. I used structural fasteners that are designed for this type of application and so far am very happy with them. Unlike most screws which tend to be brittle and are not rated to carry that kind of shear load, these have sufficient strength ratings, including higher shear strength and much higher withdrawal strength than a even a sinker nail.

Good luck.
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Old 04-23-2010, 09:45 PM   #6
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I have nothing to add of value but thought to comment that I'm surprised to hear so many 2x6's used as joists.

Why this ever seems like a good idea for anyone to do I have no idea.
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Old 04-24-2010, 09:33 AM   #7
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Originally Posted by Daniel Holzman View Post
If you do need to support 30 psf, you would need 2x12 fir joists 16 inches on center to span 24 feet full. If you put in an intermediate joist support, the required size would be much lower. In any case, the simplest approach is to check with the building inspector on classification of the space, required loading, and required joist size.
Forgive my naivty- but what is a fir joist?
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Old 04-24-2010, 09:35 AM   #8
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Forgive my naivty- but what is a fir joist?
Douglas Fir - it's a type of pine tree. Commonly used in house construction.
The species of wood used strongly affects the strength of the beam.
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Old 04-24-2010, 09:40 AM   #9
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Originally Posted by Frye View Post
I am not a pro, I am a fellow DIYer.

That does seem like quite a lot of distance for a 2x6 already.

My garage ceiling spans about 21 feet, but they are trusses which yields an entirely different strength.

I don't have the definite answer for you, but I think some of the answers here point you in the right direction. In addition to the distance of the span, I think you will need to factor the spacing of the members. I would imagine 24" OC spacing would need stronger beams than 16" OC, for example. The design would be based on how much dead load and how much live load is needed.

You might be able to get this information from your local code enforcement authority, however, if they are anything like mine, they might be really confused by seemingly normal questions and never really give you a good answer.

You might get a good enough answer with a span table. I would inquire at your friendly neighborhood lumber yard (and then buy from them). I would avoid the big box home improvement stores at all costs. They may give you answers that are not only absolutely wrong, but utterly dangerous.

In my situation they told me that three 2x10 joists sistered together would be sufficient strength where I needed a beam to span 20 feet. In reality, I hired a structural engineer and his answer was that I need two LVL beams with 24" web sistered together. This is a _MAJOR_ difference and it is a beam that bears the majority of the weight of a project. Those LVL beams themselves weighed 500 pounds, not to mention their difference in load bearing ability, deflection, and whatever else.

To attach the sister joists, you can of course use nails. I used structural fasteners that are designed for this type of application and so far am very happy with them. Unlike most screws which tend to be brittle and are not rated to carry that kind of shear load, these have sufficient strength ratings, including higher shear strength and much higher withdrawal strength than a even a sinker nail.

Good luck.

What is a truss? and where would I get these structural fasytners? Are they like a screw? And are you saying instead of re-beaming the whole garage that I run a single support beam down the middle across the (current) joists? BTW- yes the current joists are 16" OC.
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Old 04-24-2010, 09:46 AM   #10
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Douglas Fir - it's a type of pine tree. Commonly used in house construction.
The species of wood used strongly affects the strength of the beam.
Ohhhhh- so I would definitley want to specificy that kind of wood at the lumber yard. VERY interesting! I thought the only option (in framing) was generic pine. Sounds like there are many "grades" of pine.

I've built furniture and knew that there many options (i.e. Oak, cedar, Poplar etc) in that realm. Never realized framing also has nuances as well.

Having said that, what is the kind of pine that you see at a DIY...
and what other grades of pine exist and is there some kind of scale for that?
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Old 04-24-2010, 09:51 AM   #11
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proper support for floor joists in my "man cave"


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Originally Posted by mike d View Post
Ohhhhh- so I would definitley want to specificy that kind of wood at the lumber yard. VERY interesting! I thought the only option (in framing) was generic pine. Sounds like there are many "grades" of pine.

I've built furniture and knew that there many options (i.e. Oak, cedar, Poplar etc) in that realm. Never realized framing also has nuances as well.

Having said that, what is the kind of pine that you see at a DIY...
and what other grades of pine exist and is there some kind of scale for that?
http://en.wikipedia.org/wiki/Lumber

It's wikipedia but quite accurate and informative - covers softwood and hardwood.

DF is most common and, thus, least expensive - cost goes up from there. Believe it or not, cost can jump skyhigh depending on what's available and how much is in stock of hardwoods.
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Old 04-25-2010, 01:24 PM   #12
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Mike,

My advice isn't always popular, but you can't go wrong with it. Find a local engineer or insured DIY consultant to set the job up with properly sized joists/beams/columns/footings/fasteners. What you're after isn't difficult, and shouldn't take long to crank out, but a lot of times it's a big gamble getting structural questions answered on an internet board. Folks buy $300 entertainment centers, and they come with instruction manuals. Why invest a couple thousand on a DIY project, and risk doing it without the same level of detail?

If you're into learning how to go about the process the right way, the best place to start is at awc.org. They've got some really great information.
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Old 04-25-2010, 01:28 PM   #13
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2 threads on same issue merged
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Old 04-25-2010, 09:56 PM   #14
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I have nothing to add of value but thought to comment that I'm surprised to hear so many 2x6's used as joists.

Why this ever seems like a good idea for anyone to do I have no idea.
Cheapest possible thing. Most tract/spec house builders are looking to save every single dollar they can on costs, so they will max out the span of everything they can, especially a garage ceiling; designed to support nothing but the sheetrock.

I agree it is poor practice, but that is just the way it is in lower end housing, where the profit margin is minimal to begin with, and several hundred dollars in the cost of the house is often signifigant to whether the potential buyer can afford it or not.
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Old 04-25-2010, 10:06 PM   #15
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proper support for floor joists in my "man cave"


2x6 ceiling joists? Are you sure these aren't manufactured trusses?

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