# Which is a stronger joist?



## ATHC

First up, I'm new to the board... great place and a wealth of info & help!

Concerning deck construction for large 6' hot tub, deck height under 3'-0".

1. Which is a stronger support joist: 4x6 or (2) 2x6

2. Advantages/Disadvantages

Thanks much.


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## maj

Depends on the span, which you don't give. Personally I would figure 1" of joist width per 1' of span. So if the span is 10', then usa 2x10 for joist. I would also space them 12" centers for the hot tub area.


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## Bob Kovacs

A 4x6 will be stronger, as it's actual dimensions are 3 1/2 x 5 1/2. two 2x6's would total 3 x 5 1/2, which is approximately 15% less cross-sectional area.

However, if you're talking about supporting a hot tub, you'd better get far more design info than this to work with- 6" deep anything won't span very far supporting a hot tub.........

Bob


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## Glasshousebltr

Some extra poles and footings under the tub would be a better route to go.

Bob


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## maj

Glasshousebltr said:


> Some extra poles and footings under the tub would be a better route to go.
> 
> Bob


I'll second that


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## Peladu

I would first verify with the HO if they ever plan on putting water in the tub. If not then you don't need additional support. Should they feel the need to add water, you may wish to "beef" it up.


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## Shamrock

I've done several using 2x10's on 12" centers over a 10' span and over time there is a noticable deflection...not terrible but definitely there. Currently make it a point of adding additional girder support in the middle these days for hot tubs.


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## ATHC

Bob Kovacs said:


> A 4x6 will be stronger, as it's actual dimensions are 3 1/2 x 5 1/2. two 2x6's would total 3 x 5 1/2, which is approximately 15% less cross-sectional area.
> 
> However, if you're talking about supporting a hot tub, you'd better get far more design info than this to work with- 6" deep anything won't span very far supporting a hot tub.........
> 
> Bob


I thought the 4x6 was stronger. This will be a new deck (extension). The HO currently has the hot tub (filled & in use daily) sitting on an existing 10' W x 20' L deck, span is 8' (12" overhang front/back) with 2x6 joists sitting on top a double 2x6 beam and no center support, 5 - 4x4 post spaced 5' on front and back. I was amazed it had not collasped when I first saw it. HO want's to add a 5' x 15' extension to move the hot tub out farther. No problem building the new to meet/exceed the requirements, I was just wondering if there was any real difference between two 2x6 or one 4x6 other then dimensionally. I've used both and know there is post beam/tie caps available for each.

This forum is really great!:thumbup: It's nice having a diverse group of professionals to discuss ideas/concerns. Much appreciated!


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## RobertCDF

You can use 2x6 for the joists if you like... but then I would put a beam every 2' under the joists... Other wise I would recomend double 2x10 at 12" O.C. and some large beam just depending upon what distance you want between the posts. But then again I just overbuild most everything.


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## old27

RobertCDF said:


> You can use 2x6 for the joists if you like... but then I would put a beam every 2' under the joists... Other wise I would recomend double 2x10 at 12" O.C. and some large beam just depending upon what distance you want between the posts. But then again I just overbuild most everything.



Robert-

I'm with you all the way on the beefier framing call........or he could use 2x4 for the joists...and then put a beam every 1' under the joists.

-msacras


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## RobertCDF

old27 said:


> Robert-
> 
> I'm with you all the way on the beefier framing call........or he could use 2x4 for the joists...and then put a beam every 1' under the joists.
> 
> -msacras


Lol it might be cheaper... you would just have to run the numbers.... or you could just put the tub on a slab and be done with it. Then build the deck around it. Personally I think this looks better.


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## bob the builder

RobertCDF said:


> Lol it might be cheaper... you would just have to run the numbers.... or you could just put the tub on a slab and be done with it. Then build the deck around it. Personally I think this looks better.



I like the privacy also...

Bob


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## nadonailer

Glasshousebltr said:


> Some extra poles and footings under the tub would be a better route to go.
> 
> Bob


Ditto - concrete tubes w/ 4x4's and post caps holding up 4x6's is what I'd do below the tub.............


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## Joasis

I guess I am missing something here, but why is 6X being considered? Just because they are already there? 6X as a joist and beam under the joist does not equal the strength of a 2X12. On the other hand, give a little thought to load distribution and exactly what weight is involved. My own hot tub weighs in at a hefty 1050# and holds 550 gallons of water @ 7# per gallon for 3850#, for a grand total of 4900lbs. It rests on a 7 ft square cabinet rim with a cross uqually dividing the tub base. Toss in a couple of adults, or more on a party night, and you have lots of weight to consider. You say you have less then 3 ft to work with, so the 2X10's or 12's may be a something to sell the HO on simply because it is better, safer, and if the deck sags even slightly under the tub, it will stress the tub and cabinet...more expense later. Just my 2 cents.


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## Riversong

ATHC said:


> I thought the 4x6 was stronger.


Like most things, it's not quite that simple. A 4x6 joist is theoretically stronger than a doubled 2x6 joist because it's 1/2" thicker, but the actual strength depends upon the knots and other inherent weaknesses of any particular piece of lumber (#2 lumber can have some nasty spike knots or large not-so-tight round knots or wane).

If the doubled 2x6s are well nailed (3 10d common every 16"), then they can be stronger than a single 4x6 because both the grain and the weaknesses can be supported by the sistered member.

It sounds like there's some good sense being shared here from years of experience, but I'm surprised that no one has mentioned using span tables or the formulas required for the relatively simple engineering of such a deck.

But, using standard lumber specs and the required 60 lbs/sf live load for an outside deck, plus adding a 5000 lb hot tub load spread over 4 or 5 joists (assuming 16" oc - that's an additional 1000 lb per joist), 2-2x6 PT yellow pine joists 16" oc will just about carry the design load, but be a little over-stressed in horizontal shear. With 4x6 joists 16" oc, the shear and deflection are OK (I'm assuming the new deck will also have an 8' span).

If you want to be sure there won't be problems, then going 12" oc won't be a bad idea for the area carrying the hot tub. And make sure your pier footings can handle the extra load, too (use bigfoot pier bases).

- Robert


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## ATHC

Riversong said:


> (I'm assuming the new deck will also have an 8' span).
> 
> If you want to be sure there won't be problems, then going 12" oc won't be a bad idea for the area carrying the hot tub. And make sure your pier footings can handle the extra load, too (use bigfoot pier bases).
> 
> - Robert


Thanks for all the input from everyone. Robert, your thinking was inline with what I was wondering. Both ways have their purpose, however there's numerous factors that can alter both outcomes. 

The new span will only be 6' and yes bigfoot pier bases are a must. The HO called yesterday to ask more questions, I'm still trying to steer them to a concrete slab with the deck built around the tub.

Thanks much!


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## Joasis

Maybe you could sell the HO on the idea of the tub going to concrete based on keeping the hot tub ridgid, and will help it in not cracking, ect as a selling point. Try all angles to get where you know you need to be.


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## Teetorbilt

joasis, where are you getting the light water? Mine weighs 8.336#s at 62*F.


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## kapena

Lighter water happens when you turn on the Jacuzzi jets. How light it gets depends on the "bubble factor".


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## bob the builder

kapena said:


> Lighter water happens when you turn on the Jacuzzi jets. How light it gets depends on the "bubble factor".


But you don't leave your jets on for load calcs.


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## Joasis

My bad, I was thinking about an installation awhile back that involved a fuel tank...fuel @ 7, water @ 8. I think I start typing and never edit so I can look and sound dumb...most all the time.


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