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Old 12-08-2018, 01:55 PM   #1
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Butt or lap floor joists


Can you butt floor joists together over a girder? Or must they be lapped?
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Old 12-08-2018, 02:05 PM   #2
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Re: Butt or lap floor joists


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Can you butt floor joists together over a girder? Or must they be lapped?
Yes you can do either if you have enough bearing on the surface.
What type of girder do you have. How big is the top surface and is it flat or is it like a 2 ply roof truss?
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Old 12-08-2018, 02:11 PM   #3
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Re: Butt or lap floor joists


One problem with butting them is when the floor flexes there is a slight lift at the wall beam or truss, when they are lapped that lift is controlled more by the lapped joist and the lift will be less noticeable.
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Old 12-08-2018, 04:57 PM   #4
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Re: Butt or lap floor joists


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Yes you can do either if you have enough bearing on the surface.
What type of girder do you have. How big is the top surface and is it flat or is it like a 2 ply roof truss?
Walls plus 3 rows of 4x6 beams at 6, 12 and 18'. Each beam supported every 6' by pier.
Joists will be 16oc. Hoping to get 24' width by butting 2x8x12's over the center beam. Joist longest span will be 6'.

Is it 1-1/2 bearing required on beam? I would sister 2x8 piece on each side of butt joint?
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Old 12-08-2018, 05:10 PM   #5
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Re: Butt or lap floor joists


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Walls plus 3 rows of 4x6 beams at 6, 12 and 18'. Each beam supported every 6' by pier.
Joists will be 16oc. Hoping to get 24' width by butting 2x8x12's over the center beam. Joist longest span will be 6'.

Is it 1-1/2 bearing required on beam? I would sister 2x8 piece on each side of butt joint?
If they are a full 4" you can but them, I would joist the top with a 2x4 along side at or near the top. So they stay lined up when you are sheeting them.
I prefer the lap but that would be a lot more lumber and a lot more thinking for the lay out and which way to jog so you don't waist plywood.


Remember that the first mark beside the joist is at 15 1/4" then 16" after that. That puts the first set of nails at the 16" mark.
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Old 12-08-2018, 05:21 PM   #6
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Re: Butt or lap floor joists


Nice. Not really a lot of talk on this that I could find. All examples show lapping. Should be plenty sturdy. Thx
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Old 12-08-2018, 05:29 PM   #7
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Re: Butt or lap floor joists


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Nice. Not really a lot of talk on this that I could find. All examples show lapping. Should be plenty sturdy. Thx
You wont need any bridging, I think, at least with 2x10 they are needed for 7 ft of span.
Remember to check your layout on the free end of the plywood as you go, you don't want them leaning and getting away from layout. I usually just mark the layout on the first row of plywood and then the first row after a beam or wall.
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Old 12-08-2018, 07:14 PM   #8
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Re: Butt or lap floor joists


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Can you butt floor joists together over a girder? Or must they be lapped?
If this is part of a new home construction or a permitted addition, check with your local building inspector for code requirements.
.
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Old 12-08-2018, 07:47 PM   #9
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Re: Butt or lap floor joists


I prefer butting the joist so the layout stays the same. You only need an 1-1/2" bearing.

Think about your sheathing and how changing the layout an 1-1/2" by lapping the joist will affect where your sheets break.
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Old 12-08-2018, 08:21 PM   #10
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Re: Butt or lap floor joists


Nealtw says full 4" mid beam. This is usual bearing for floor joists on the rim side, 2". If you don't have 4" beam, I'd lap them esp because you want min 2 nails each face. Less bearing surface, you may split the joist ends too many times. Floor joists keep the opposite walls from spreading out as well as being solid anchoring system for deck.
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Old 12-08-2018, 08:41 PM   #11
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Re: Butt or lap floor joists


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Nealtw says full 4" mid beam. This is usual bearing for floor joists on the rim side, 2". If you don't have 4" beam, I'd lap them esp because you want min 2 nails each face. Less bearing surface, you may split the joist ends too many times. Floor joists keep the opposite walls from spreading out as well as being solid anchoring system for deck.
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Old 12-08-2018, 09:52 PM   #12
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Re: Butt or lap floor joists


1.5" is for the engineers. I don't think it's enough for the nails that are needed without the toe nails spliting the ends - but I just followed the traditional way so I can't say for sure 1.5 will not work. Sometimes I stop sinking the nail head to avoid splitting the endgrain. I was thinking about going back to my post and adding: butting the joists and strapping them together will work, but the joists must have nails into the base plate.
Bridging or blocks can be added later. As long as the layout marks are ok on the decking, the joists should be plumb. Adjust the joists to the marks before the field is nailed off. Anything off is fairly obvious so pay attention to plumbs and squares.

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Old 12-08-2018, 10:17 PM   #13
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Re: Butt or lap floor joists


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1.5" is for the engineers. I don't think it's enough for the nails that are needed without the toe nails spliting the ends - but I just followed the traditional way so I can't say for sure 1.5 will not work. Sometimes I stop sinking the nail head to avoid splitting the endgrain. I was thinking about going back to my post and adding: butting the joists and strapping them together will work, but the joists must have nails into the base plate.
Bridging or blocks can be added later. As long as the layout marks are ok on the decking, the joists should be plumb. Adjust the joists to the marks before the field is nailed off. Anything off is fairly obvious so pay attention to plumbs and squares.
I think most inspectors want to thin the but joint is in the middle so you have the 1 3/4". I asked about the 4" to know if we had to do the speech about being care with where that but joint is. We would never do a but join on a laminated beam, a little un even shrinkage and you are in trouble.
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Old 12-09-2018, 12:44 PM   #14
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Re: Butt or lap floor joists


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Originally Posted by kwikfishron View Post
I prefer butting the joist so the layout stays the same. You only need an 1-1/2" bearing.

Think about your sheathing and how changing the layout an 1-1/2" by lapping the joist will affect where your sheets break.
I like this also. I could account for the lapping when adding my sheathing - but it just seems easier to butt. I thought about hangers, but am just not sure of them. A lot more nails, and work. Everyone seems comfortable with them though.
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Old 12-09-2018, 12:49 PM   #15
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Re: Butt or lap floor joists


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I like this also. I could account for the lapping when adding my sheathing - but it just seems easier to butt. I thought about hangers, but am just not sure of them. A lot more nails, and work. Everyone seems comfortable with them though.

I should have asked what you have below like basement or, you have a lot of posts and beams that could have had a better design for a basement. are they in already?
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