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4x4 floor joists

18K views 35 replies 7 participants last post by  Nealtw 
#1 · (Edited)
I'm hoping to use 4x4 (nominal) wood beams as living room floor joists, but none of the online span tables/calculators seem to handle 4x lumber. Maybe an engineer type, carpenter, or internet sleuth can help me.

My clear span is 9'5", and I'd like the joist spacing to be no less than 16" o.c. Common lumber beams locally are Douglas fir and Spruce-Pine-Fir (SPF). I understand living room floors need to handle 40 psf live loads and 10 psf dead loads with L/360 deflection.

I'm willing to consider engineered wood beams and have seen 4x4 LVL, LSL, and PSL products. But those companies' span tables treat those beams as beams rather than floor joists. So, they aren't particularly helpful.

It's a tiny house on a trailer, so I want to keep the beam depth to no more than 3-1/2 inches (actual) to maximize headroom. These tiny houses are pretty much restricted to 13'6" in height (including trailer), so vertical space is at a premium.

The standard width of tiny houses on wheels is 8'6", and 4x4 SPF beams seem to cover their 7'10" clear span okay. However, from what I've been able to figure out so far, the extra 19" of clear span in our house rules out your typical 4x4 SPF or Douglas fir beams. Am I wrong? If not, then will any engineered 4x4 beams do the trick?

Another option I'm considering seems a little silly at first. Instead of laying a 4x6 SPF beam with the long edge vertical, I could lay the long edge horizontal. I realize that sacrifices lots of strength. Instead of being about 250% stronger than a 4x4 beam, it would be only about 150% stronger. But maybe that extra strength is all I need to get me the extra 19" of clear span while still preserving my 2 inches of additional headroom. What do you think?

Thanks in advance for any help you can offer.
 
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#4 ·
#10 ·
I'm hoping to use 4x4 (nominal) wood beams as living room floor joists, but none of the online span tables/calculators seem to handle 4x lumber.
That's because there's no "efficiency" in using, for example, a 4x4 instead of a 2x4. The depth is what is dominant in determining a span (and its loading; moment, shear). The 2x4 cross section would be adequate for a maximum span calculated, the extra 1" on each side of that 2" width would not contribute much more to that. Excluding timber and steel shapes (where you can thicken flanges), that is why structural wood cross sections are rectangular in shape, not square.

You will not find "official" 2x4 span tables.

My clear span is 9'5", and I'd like the joist spacing to be no less than 16" o.c.
What is keeping you from 12"o.c.?

I'm willing to consider engineered wood beams and have seen 4x4 LVL, LSL, and PSL products.
Where? Are you talking about for use as columns and posts?

It's a tiny house on a trailer, so I want to keep the beam depth to no more than 3-1/2 inches (actual) to maximize headroom.
You may need to go to tube steel. What about insulation under the floor?

Instead of laying a 4x6 SPF beam with the long edge vertical, I could lay the long edge horizontal. I realize that sacrifices lots of strength. Instead of being about 250% stronger than a 4x4 beam, it would be only about 150% stronger.
Your extrapolation does not apply exactly like that.
 
#13 ·
That's because there's no "efficiency" in using, for example, a 4x4 instead of a 2x4. The depth is what is dominant in determining a span (and its loading; moment, shear).
I read somewhere that a good rule of thumb for determining the strength of joists and beams is to multiply the beam's width by the square of its depth. So, assuming the same material is used, it is indeed much more efficient to use thin, deep beams. But a key priority for me is headroom, so I want to have shallower beams that are still strong enough. Using a 6(w)x4(d) instead of a 2(w)x6(d) might be a worthwhile trade-off for my purposes.

What is keeping you from 12"o.c.?
I'm planning to leave the living room's floor joists exposed, thus becoming the bedroom's ceiling beams and letting me poke my head between them while sitting up in bed or crawling across the bed to the staircase. Having 16" o.c. seems less claustrophobic than 12" o.c. in these situations.

Are you talking about [LVL, LSL, and PSL 4x4s] for use as columns and posts?
No. I'm hoping they also can be used as floor joists, even though that isn't their primary purpose.
What about insulation under the floor?
There will be insulation under the bedroom floor, at the trailer level. No insulation under the living room floor, which will be about 4 feet above the bedroom floor.

I hope all this is sort of making sense. If not, ask and I'll try again. Thanks for the input.
 
#15 · (Edited)
You can use this calculator it's free.

https://www.johnbridge.com/vbulletin/deflecto.pl

Using the calculator it says this.

"" Thank you for using the John Bridge Forums Deflect-O-Lator :)

For joists that are SYP or Douglas Fir, in good condition, 4 inches tall, 4 inches wide, 16 inches on center, and 9 feet long between supports, the deflection calculated is 0.307 inches.

This translates to a deflection of L / 352.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.""
 
#16 ·
Thank you. This is the first floor joist span table/calculator I've encountered that handles 4x sawn lumber beams. Since it appears to be designed for tile, I'm guessing it assumes a 40 psf live load and 10 psf dead load, since that is what you must deal with in kitchens and bathrooms.

It confirms that a 3-1/2 x 3-1/2 (actual) Douglas fir beam won't handle a 9.5' clear span. Nor would a 3-1/2 x 4-1/2 beam. Unfortunately, it doesn't allow me to test a 3-1/2 x 5-1/2 beam.

At this point, I'm leaning towards some sort of engineered 3-1/2 x 3-1/2 beam, as long as it can be used as a floor joist and isn't too expensive.
 
#18 ·
I'm not familiar with the permitting requirements beyond trailer registration and its roadworthiness, but I'm sure there will be something somewhere. It seems without an understanding of structural design and the use of "rules of thumb," it might be a good idea to hire an architect or engineer to help with the basic structure. A few hundred dollars of time, if no stamps are required, may be a good investment.
 
#20 · (Edited)
Someone at a tiny house forum came up with an outside-the-box (as I defined it) alternative. I was fixated on the idea of running the floor joists the width of our 10' trailer (along the length of our bed). Another option would be to run the joists across the width of the bed. Even a king-sized bed would leave me with a clear span that's only about 6.5'. According to a floor span calculator I was pointed to, Douglas fir 3.5x3.5 (actual) beams should be able to handle that with up to 22" o.c. spacing.

I can foresee two possible issues with this suggestion, neither of which is a deal breaker.

First, my plan is to have a wall of storage drawers and shelving on both sides of the bed. So, putting a ledger board along the top of these storage walls will effectively remove 3.5" of shelf space at the top of the walls. (Is a 3.5" ledger board even practical? I've never seen one that narrow.)

Second, when I sit up in bed, poking my head between the exposed beams, and face forward, then I'll be staring at a beam running directly across my field of view instead of gazing down the length of the beams.

I'll do more investigating of engineered wood and 9'5" clear spans, but it's nice to know I have another option available to me.
 
#24 ·
I can't help but wondering if you're reading to much into this.

Does the trailer frame already have beams.?
And will the floor rest on these beams.?
The bedroom floor will rest on the trailer beams. The living room will be about four feet above the bedroom floor, so the living room will need floor joists. Unlike nearly 99% of other tiny houses, our bedroom will be below the living area instead of above it (in a loft).
 
#31 ·
Sorry, I'm a little late to this discussion, since it took place late November.
That Deflect-O-Lator floor joist calculator is a joke. It only checks deflection, not stress. Your floor joists will be critical in stress.
First of all, your span is 9'-5", so the closest chosen span should have been 9.5 ft., not 9 ft.
Second of all, your wood section is 3.5" x 3.5", not 4" x 4".
Here are the (correct?) results:
>>>>>>>>
Thank you for using the John Bridge Forums Deflect-O-Lator :)

For joists that are SYP or Douglas Fir, in good condition, 3.5 inches tall, 3.5 inches wide, 16 inches on center, and 9.5 feet long between supports, the deflection calculated is 0.627 inches.

This translates to a deflection of L / 182.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.
>>>>>>

L/182 IS TOO MUCH DEFLECTION! You want L/360 deflection or less.
That means L/370 or L/380 would be sufficient, but L/182 is twice as much deflection as allowed.

Regardless, the Deflect-O-Lator does not check stress, so it is USELESS.

I did a proper engineering analysis, and a 3.5 x 3.5 SPF #2 (4x4) is INADEQUATE by 72%. The controlling factor is stress. Depth required is 4.19" .

If you properly glue and screw 3/4" plywood subfloor to the top, then it is only inadequate by 6%. Much closer to actually being workable.

A much better FJ could be made by ripping a 1-3/4 x 9-1/4 LVL into two 4" strips and nailing them together to form a (2) 1-3/4 x 4" LVL. This would be ADEQUATE by 18%. That is what I recommend.
 
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#32 ·
A much better FJ could be made by ripping a 1-3/4 x 9-1/4 LVL into two 4" strips and nailing them together to form a (2) 1-3/4 x 4" LVL. This would be ADEQUATE by 18%. That is what I recommend.
Thank you for your analysis. I'm curious about one thing. Why nail two 1-3/4" LVLs instead of using a single 3-1/2" LVL? Intuitively, I'd think the continuous bonding of a single beam would be stronger than a pair of nailed beams, but I certainly could be wrong.
 
#34 ·
Using a 3.5" wide LVL beam is fine, and saves the work of nailing. Two 1-3/4" LVL beams will end up just as strong as a 3.5" wide beam, as long as there is sufficient nailing to stabilize the two 1-3/4" beams to each other.
 
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#35 ·
For 7' span,
3.5" x 3.5" Doug Fir #2 at 24" o.c. is Adequate by 8%.
3.5" x 3.5" Doug Fir #2 at 19.2" o.c. is Adequate by 35%.

3.5" x 4" tall LVL at 24" o.c. is Adequate by 91%.
3.5" x 3.5" LVL at 24" o.c. is Adequate by 28%.
Amazing how much more margin an additional 1/2" in height buys you.
 
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