Flooring for a storage loft

Old 11-30-04, 10:20 AM
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Flooring for a storage loft

I am building a storage loft in my garage above the garage door. The loft will be 9' high and have dimensions of 12'x20'. The design is very similar to a deck design except that a header will not be attached to the house.

The support posts will only be on the 12' side and there will be none under the 20' span. I was planning on using joists of 20' 2x12's at 12" on center since the span tables indicate that they can support 40psf live load and 10psf dead load for the 20', but it seems that I-Joists will be easier to use and be more stable. Does this seem reasonable that I use the I-Joists?

Another idea I had was to use two 20' LVLs as beams and then run 12' 2x12's as joists using joist hangers. Does this look like a better method?

Thanks in advance for the help.
Old 12-01-04, 10:20 AM
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It certainly is reasonable to use I-joists, such as the TJI/Pro joists. They are more expensive but also higher quality. I did not check stresses or deflections for the 2-2x12 spanning 20 feet but my guess is your pushing it with regard to deflection. If you use TJI joists at 12" centers spanning 20 feet you would have to use the TJI/Pro 150 with a depth of 11-7/8". If you are using this space as storage you may want to put a pencil to your dead loads and make sure 10 psf is enough. If you use a 40 psf LL and 20 psf DL I would go to a TJI/Pro 250 11-7/8" depth.

It may make sense to span your joists in the 12' direction. Generally, it makes sense to span the joists in the shortest direction. If you are going to use 2 ply LVLs as girders spanning 20 feet you'll have to use the 16" deep LVLs. If you go to a 3 ply beam (5-1/4" width) you could use the 14" deep LVLs. You'll just have to run the costs to see which makes the most sense. Be sure to provide adequate bearing length for the beams (for the 14" beam you need to provide 1.5" at the ends and 3.9" at intermediate supports (which I don't believe you have) and for the 16" beam 2.3" at ends and 5.7" intermediate).
Old 12-01-04, 11:59 AM
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thanks for the advice. Hopefully this will help me decide between the two methods.

Would the two LVLs support a similar load - 40psf live/20psf dead? My plan was to use 6x6" posts on each side to support the LVLs. I would just set the beams on the posts using caps. Would this be sufficient? Is this OK that there are no intermediate supports? This method seems easiest since the only hard work is getting the two 20' LVL girders up on each side and then just adding the joist hangers and joists. I have never used the LVLs and wasn't sure if they could hold the same load as the I-Joists in this configuration.

Also would it be useful to ledger the LVL at the front of the gargage - just like a deck on a house. Just lag bolt the header to the frame?

If I did go with the I-joists, what would I use to build the headers at the 12' sides? Can I use 6x6" posts and run double 2x12"s between them? Then use joist hangers to support the I-Joists? I heard a suggestion from someone that I should not mix engineered and dimensional lumber.

Thanks, Sam
Old 12-02-04, 08:52 AM
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depending on the type of storage

you might want to consider 125 psf live load! If you are goingto put big heavy boxes of books and stuff like that 40 psf LL will not be enough
Old 12-02-04, 09:57 AM
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Looks more like a second story

In the name of "doing it right the first time", it would be prudent to
design the support structure as though it is a second story.
An unsupported 20' span in wood is not enough.

Now steel fabrication is an entirely different proposition.
Old 12-02-04, 03:24 PM
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I have not seen any tables which show the 125psf - any idea where to look?


Do you really think this can not be accomplished without steel?

Thanks, Sam
Old 12-02-04, 04:05 PM
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First, you should not have to use a 125 psf LL. You would not want to define a storage load as live load because you are allowed an allowable stress increase for the short duration (typically live loads, such as snow, are defined as lasting up to three weeks). By most codes the live load for residential construction is 40 psf. You can estimate your dead load by calculating how much total weight you want to store divided by the area it is stored over. Your local code may also be able to give guidance.

Second, I designed a house earlier this year were the joists spanned 22 feet. You do not have to use steel. Plus the connections from steel to wood can be a pain and require a bit of fabrication.

The 2 LVLs spanning 20 feet will support a 40 psf LL and 20 psf DL. There should not be any problem with setting the beams on the columns. I would recommend using a proprietary column cap recommended by the beam manufacturer. Also be sure to brace the top of the beam adequately as recommended by the beam manufacturer (typically the subfloor sheathing is adequate bracing just be sure to nail it to the beams as well as the joists).

It may be useful to use a ledger beam since you would not have to construct footings for the columns. However, you would want to check that the existing wall is capable of handling the additional load from the loft.

If you go with the joists spanning 20 feet I would use 12 foot LVLs spanning between the timber columns. You could use 1-14" deep LVL or 2-11-1/4" LVLs. Acutally this looks like a pretty good option. If you do span the joists 20 feet be sure to use the TJI/Pro 250 or equal joists spaced at 12" centers. If you skimp on the joists your floor may have a springy feel because of excessive deflection or worse. Limit the joist defection to L/480 and you should be happy. Good Luck!
Old 12-03-04, 05:26 AM
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thank you for the advice! It really is a great thing that we can come to these forums and receive help on our projects.

Which of the two methods do you think will be the most stable and have the least spring?

1) 2 - 20'x5 3/4"x14" LVLs as girders and 12'x2"x14" joists
2) 2 14" LVL headers and TJI 250's at 12 OC

Thanks, Sam

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