# Butt or lap floor joists



## Nealtw (Jun 22, 2017)

[email protected] said:


> 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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## Nealtw (Jun 22, 2017)

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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## [email protected] (Nov 16, 2018)

Nealtw said:


> 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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## Nealtw (Jun 22, 2017)

[email protected] said:


> 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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## [email protected] (Nov 16, 2018)

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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## Nealtw (Jun 22, 2017)

[email protected] said:


> 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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## ZZZZZ (Oct 1, 2014)

[email protected] said:


> 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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## kwikfishron (Mar 11, 2010)

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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## carpdad (Oct 11, 2010)

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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## kwikfishron (Mar 11, 2010)

carpdad said:


> 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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## carpdad (Oct 11, 2010)

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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## Nealtw (Jun 22, 2017)

carpdad said:


> 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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## [email protected] (Nov 16, 2018)

kwikfishron said:


> 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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## Nealtw (Jun 22, 2017)

[email protected] said:


> 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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## [email protected] (Nov 16, 2018)

Nealtw said:


> 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?


Not in yet. Still planning. I may have over engineered it. 

Post and beam. Concrete footers every 6 feet allowing crawl space. I want the floor solid. No span longer than 6 foot. Would like 2' clearance under there. I have 18" under my house and it's a pain to work under there.


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## Nealtw (Jun 22, 2017)

[email protected] said:


> Not in yet. Still planning. I may have over engineered it.
> 
> Post and beam. Concrete footers every 6 feet allowing crawl space. I want the floor solid. No span longer than 6 foot. Would like 2' clearance under there. I have 18" under my house and it's a pain to work under there.


 How long are the beams and how are you attaching them to the foundation?


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## [email protected] (Nov 16, 2018)

Nealtw said:


> How long are the beams and how are you attaching them to the foundation?


30' beams attached to 4x4 posts using standoff post brackets set in concrete pillar.

Damn, I just thought of the beams being butted together over a 4x4 post. Wonder if that is okay.


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## Nealtw (Jun 22, 2017)

[email protected] said:


> 30' beams attached to 4x4 posts using standoff post brackets set in concrete pillar.


 4x4s are fence posts and are only uses in a house to replace multiple studs in a wall. So all posts are 6x6s.
I have built hundreds of houses all with 2x10 joists and overloading them is only considered at 16 ft or the plan for a stone covered island in a kitchen. 
With a tight crawlspace I would go up to a flush beam as was suggested with hangers at 12 ft. The beam would be engineered but likely 2 ply LVL.


Even if you don't have room for 2x10s at the foundation, the joists can be notched if you bolt a ledger under them. With solid blocking the beam can be hung at the foundation. The beams would be 10 ft long with only 2 footing and maintain the height of the crawl space. 
You would install the beam at 12 ft and 1/4" too low and start sheeting the floor with a half sheet.


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## Nealtw (Jun 22, 2017)

If you have a compressor/
https://www.securallfastenings.ie/product/stanley-bostitch-joist-hanger-nail-gun-mcn150/


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## [email protected] (Nov 16, 2018)

Nealtw said:


> 4x4s are fence posts and are only uses in a house to replace multiple studs in a wall. So all posts are 6x6s.
> I have built hundreds of houses all with 2x10 joists and overloading them is only considered at 16 ft or the plan for a stone covered island in a kitchen.
> With a tight crawlspace I would go up to a flush beam as was suggested with hangers at 12 ft. The beam would be engineered but likely 2 ply LVL.
> .


6x6. Corrected.

I like the idea of flush with beam. Am I in overkill with all the pillars? 4x6 beam should be good at 10' support?


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## Nealtw (Jun 22, 2017)

[email protected] said:


> 6x6. Corrected.
> 
> I like the idea of flush with beam. Am I in overkill with all the pillars? 4x6 beam should be good at 10' support?


 
I have never seen a 4x6 used as a beam, You might get away with 3 ply 2x10. I am not an engineer and have never looked at a span table, most of my suggestions are from experience. So anything less, I just don't know. When I don't know I go to an engineer.


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## maurlin9 (6 mo ago)

Question
Do lap joint for the flooring joint have to be nailed together or can they have gap, some up to 3 to 6 inches just to nailed to the bearer


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## HotRodx10 (Aug 24, 2017)

Oops! Original thread is 4 years old, and I'm not sure maurlin9 is asking.


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## Half-fast eddie (Sep 19, 2020)

He’s asking if lapped joists … do they have to touch where they lap the beam. Or could they be spaced a couple of inches apart. 

If you have solid blocking in the gaps to keep them from rolling over, gaps are ok.


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## Nealtw (Jun 22, 2017)

maurlin9 said:


> Question
> Do lap joint for the flooring joint have to be nailed together or can they have gap, some up to 3 to 6 inches just to nailed to the bearer


When one side changes to 12"OC for a kitchen the joists do not come together And it is fine.


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