Author Topic: 20x30 1 1/2 - full loft - decking & joists?  (Read 908 times)

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Offline NyTexan

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20x30 1 1/2 - full loft - decking & joists?
« on: March 22, 2019, 07:23:53 AM »
I could use a little help folks.  I plan to build a full loft with an open ceiling below using 4x12 DF beams spaced at 48" +/- per the plan specs.  My problem is it's near impossible to get 2x6 T&G decking here in central Texas at a reasonable price.  As an alternative, I am wondering if I can lay down 1x6 T&G decking instead (or sanded 1/4" birch plywood with finished side down) plus a layer of 3/4 OSB T&G subfloor on top of that. Thoughts?

Offline akwoodchuck

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #1 on: March 22, 2019, 07:44:52 AM »
3/4" won't span 48"....unless you add more framing perpendicular to the beams, in between the ceiling and floor layers....2x4s 12" o.c. would do nicely...
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Offline MountainDon

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #2 on: March 22, 2019, 08:02:21 AM »
What purpose will the loft itself fulfill??  Sleeping, storage,.....???
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline NyTexan

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #3 on: March 22, 2019, 08:24:04 AM »
I could use a little help folks.  I plan to build a full loft with an open ceiling below using 4x12 DF beams spaced at 48" +/- per the plan specs.  My problem is it's near impossible to get 2x6 T&G decking here in central Texas at a reasonable price.  As an alternative, I am wondering if I can lay down 1x6 T&G decking instead (or sanded 1/4" birch plywood with finished side down) plus a layer of 3/4 OSB T&G subfloor on top of that. Thoughts?

To clarify, here are some more details. The 20x20 1 1/2 story plan indicates that a full loft can be built by using 2-2x12 joists which span across the full 20 feet of the house. These joists are spaced 48" apart.  With 48' spacing, the plan also calls for 2x6 tongue and groove decking to be affixed perpendicular over the top of these joists. Instead of using 2-2x12's I will be using 4x12's DF beams since they look nicer as a part of the open ceiling below. The problem we've run into is finding/locating 2x6 tongue and groove decking here in Tx. Thus,  I am wondering if I can substitute the 2x6 decking with some other more readily available material. Since that decking needs to have a nice finish (as it will be exposed downstairs as part of the open ceiling) I am considering 1x6 tongue and groove instead of 2x6 tongue and groove (or 1/4" sanded birch 4x8 plywood panels). As for the upstairs, to give it more structural integrity we would put down 3/4' OSB 4x8 subflooring with a laminate floor finish on top of that. All of the loft space will be used for bunkbeds, chairs, etc. No plumbing, no bathroom. Thanks for your guidance all.

Offline NyTexan

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #4 on: March 22, 2019, 08:25:54 AM »
What purpose will the loft itself fulfill??  Sleeping, storage,.....???

It will only be used for sleeping. We will have 4 dormers as well so headroom should be OK for several built in bunk beds. We will lay down laminate floors over the OSB subfloor to finish it off.

Offline MountainDon

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #5 on: March 22, 2019, 09:50:07 AM »
Yes, 1-1/2" T&G is rated to span 48" maximum.

There are sheet goods that are made to span 48" but they will be 1-1/8" thick, maybe even 1-1/4". The 3/4" is normally rated as suitable for up to 24" maximum for floors. The sheets will be stamped. If there are two numbers, like 32/16, the first is allowable span for a roof and the second figure is the maximum span for floors. I am certain that using 3/4" will result in quite a spongy floor. The 1/4" plywood used for a downward facing cosmetic finish will contribute nothing to floor stiffness.

I believe you need to either find a supplier for the 1-1/2" T&G or change to 24" spacing, which would allow the beam depth to be reduced.
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline NyTexan

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #6 on: March 22, 2019, 02:44:00 PM »
Yes, 1-1/2" T&G is rated to span 48" maximum.

There are sheet goods that are made to span 48" but they will be 1-1/8" thick, maybe even 1-1/4". The 3/4" is normally rated as suitable for up to 24" maximum for floors. The sheets will be stamped. If there are two numbers, like 32/16, the first is allowable span for a roof and the second figure is the maximum span for floors. I am certain that using 3/4" will result in quite a spongy floor. The 1/4" plywood used for a downward facing cosmetic finish will contribute nothing to floor stiffness.

I believe you need to either find a supplier for the 1-1/2" T&G or change to 24" spacing, which would allow the beam depth to be reduced.

Thanks MountainDon.  I found 1-1/8 subfloor sheets that are rated for 48" span.  If I keep the 4x12X20 joists "as is" at 48" OC - I suppose I can first nail and glue down the cosmetic 1/4" plywood and next nail and glue down the 1-1/8 subfloor sheets on top of the cosmetic sheet. Sound about right? On a related note, using balloon framing where how would I affix the subfloor sheets when I get to the two 20' end walls? Do we need to have a 4x12 up against/parallel to the end walls?

Thanks again.

Offline Don_P

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #7 on: March 22, 2019, 06:12:27 PM »
I would not exceed 42" oc spacing with #2 DF. The 1/4" will sag unless you nail it up into the subfloor. A 2x12 DF at the walls would support the subfloor at the ends if attached to the studs, that can be done with blocking between the studs and nails from the backside if the fasteners need to be hidden. It can also be 4x there. Don't forget fireblocking between studs at floor level to stop flame spread up the balloon framing between floors.

Offline NyTexan

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #8 on: March 24, 2019, 11:50:52 AM »
A related follow up question. The 4x12x20 beams are pretty pricey. As an alternative, I'm wondering if I go with double 2x12's how they look? Do they get glued together? Do you finish/box them out with trim? Anyone have pic's they can share?

Thanks,


Offline NyTexan

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #9 on: March 27, 2019, 09:32:57 AM »
Bump to see if anyone has some insight on this ...

Offline Don_P

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #10 on: March 27, 2019, 05:53:00 PM »
I would frame it with #1 or #2Dense or better SYP on 16" centers and apply 1x6 T&G to the underside. It will be more readily available materials, a better, to code, floor and easier to run utilities etc.
Span calc here;
https://awc.org/codes-standards/calculators-software/spancalc

Offline CabinNick

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #11 on: March 27, 2019, 06:13:30 PM »
I am doing pretty much exactly what you are trying to do - 20' span, spaced 48" OC, with 2x6 tounge and groove decking.  I could not meet the span/load with 4x12 at that spacing - had to bump it up to at least 5x12.  I ended up milling my own 6x12 DF beams.  That decision was based largely on advice from this forum.  You might read some of that thread at "NE Oregon 20x30+ 1.5 story".  Finally starting our build in 2 months if all this snow ever melts!

Offline NyTexan

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #12 on: March 30, 2019, 07:38:50 AM »
Thanks for the advice everyone. For the loft floor joists, I think I’m going to use double 2x12x20 SYP #2 spaced apart at 32” OC (they will be supported by “let-in” ledgers on opposing walls and will be nailed to every other wall stud (which are 16” OC).  For the loft floor, I’ll use 1x6 tongue and groove sanded pine glued and nailed on top of the 2x12 joists (this is really just for cosmetics to ensure it looks good as a first floor ceiling). I’ll add another 1 1/8 OSB subfloor on top of the 1x6 cosmetic flooring to reduce loft floor bounce.  Lastly, I’ll cover the seams between the built up 2x12 joists with 1” trim with 3/4” reveal on both sides. This should make it look more like solid 4x12 beams running across the first floor ceiling. One thing I’m questioning is whether or not I should actually use 3 married together 2x12x20 pieces instead of just 2. It would undoubtedly provide more support but I’m wondering if it’s overkill. Also, how do I marry those 2x12’s together knowing that it will be exposed ceiling? I’m not sure if I want a bunch of nail heads showing. Perhaps I drill a small countersink hole for wood screws and back fill with  wood dowel plugs over the screw heads. Thoughts?

Offline Don_P

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Re: 20x30 1 1/2 - full loft - decking & joists?
« Reply #13 on: March 30, 2019, 03:18:27 PM »
2 members would get you to a 30psf "sleeping rooms" floor, 3 members would get you to a 40psf floor in #2 SYP, and would allow 1/3 of the material to be basically hidden. You are going to be pretty hard pressed to find nice, flat, wane free, appearance grade stock. I'd go roll through a pile at the building supply looking to see if it is available and to your taste.

I don't think you will be able to get a smooth flat bottom edge for attaching the strip to. I'd be tempted to nail one joist to each side of the stud and drop the middle joist down about an inch below the bottom edge of the others. It would not be on the ledger so needs to be well connected to the other joists near each end, they are "hangering" the middle ply. That would provide a detail that breaks the plane. If that needs gussying up you could apply a quarter round or cove in that step. I'd use the fastenmaster type screws and clamps, both C and deep jaw bar clamps to pull together and align crowns while screwing. You'll need to use scrap to keep from marring the face and 4 hands. Plugs would be fine, if you have access to a drill press and bandsaw you can get countersinks and matching plug cutters and make face grain plugs out of the same framing material, I'm guessing a 3/8 or 1/2" set would be about right.

 

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