# Pressure treated 2x12 span when flat



## bdog1234 (Feb 25, 2008)

Normally when building a deck you have wood joists every 16" or so and the span of the deck surface isn't much at all. I need to put wood decking on my fishing dock that has three massive steel I beams spaced 5' apart. Will pressure treated 2x12's laying flat be adequate? This type of stuff isn't my daily gig.


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## TimNJ (Sep 7, 2005)

:no:

Run your joists flush with the I beam then normal deck boards.


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## bdog1234 (Feb 25, 2008)

What's the max I can go? 

I work with dirt and welding. Wood is not my thing.


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## slowsol (Aug 27, 2005)

Before what? It bends so much that you fall into the water? Put a piece across two block or chunks of ibeam and see how bad it flexes at different spacing. Not very far would be my presumption. 


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## bdog1234 (Feb 25, 2008)

I don't know I guess I mean what is standard acceptable distance? My family will be using it and I don't want it collapsing. The steel structure I welded and am confident in it but I have no idea on the guidelines for wood. 

I have been a memeber here nearly ten years and will always try to help others if they have a question that falls under my area of expertise. I am no carpenter and anytime I need carpentry type work done I usually hire it out but I figured I could handle screwing some boards on top of the I-beams I just need some guidance on the spans. Sounds like I may need to add some intermediate beams.


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## bdog1234 (Feb 25, 2008)

TimNJ said:


> :no:
> 
> Run your joists flush with the I beam then normal deck boards.


If I add anything underneath I will do it with steel as that what I am comfortable with. The steel should last as long as I do and the wood will eventually rot so I want to keep it to a minimum.


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## Framer53 (Feb 23, 2008)

The way to do it is to install joists the 5' direction then install decking on top.


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## MOC (Mar 14, 2017)

If you really want to do wood on top and nothing else underneath...

Build timber mats by standing up (24)2×6s (gives a 36"mat) on edge and run allthread through each mat every 24" or so, attach from underneath with cleats.


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## bdog1234 (Feb 25, 2008)

MOC said:


> If you really want to do wood on top and nothing else underneath...
> 
> Build timber mats by standing up (24)2×6s (gives a 36"mat) on edge and run allthread through each mat every 24" or so, attach from underneath with cleats.


I don't want that. I want the minimum amount of wood. 

If I have to I will weld channels or purlins across the 5' and then screw the 2x12's to that. 

All I really need to know is what is the maximum distance I can have between supports under the 2x12's laying flat? Is it 16" or can I go 24", 30", 36" ? 

My I-beams are stout. They are cantilevered but will support a 3,000 lb load at the very end with less than an inch deflection over 30'. I don't envision more than 6 people ever being on the dock but the beams could probably take 30 people before it collapses. I just need decking on top of them to walk on.


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## wood_life (Aug 12, 2015)

Ok gotcha, I wouldn't go anymore than 16"OC for a 1.5" thick PT deck board laid flat.

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## dutchroots2 (Sep 20, 2016)

You could go 24". A lot of ordinary decks with 2x4s or 2x6s as the decking have joists 24" apart.


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## hdavis (Feb 14, 2012)

There are two parts to this. You won't find any prescriptive answers to your question - it's outside of normal builds, and an engineer would normally work out what needs to happen. You can go 24" prescriptive.

As a practical matter, if you're building to L/240 or L/360 (or whatever deflection you want to allow), Stick a couple 4X4s on the ground 5' apart, put a wet 2X12 across it, and start loading it up, and measure the deflection in the center. That will give you the answer, but it doesn't get you the stamp.

PT SYP is surprisingly strong - I have 2 2X10X8' boards I'll drive a car onto - not something I recommend.


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## bdog1234 (Feb 25, 2008)

I had an engineer design the main structure with the I beams. I figured the decking part was simple. 

Looking online at lumber it seems most decking is actually 5/4 nominal 1" actual so using 2x12's or 2x6's (1.5") should be considerably stronger. 

I think I will do the 4x4's with a board spanning them test. I wish I could just weld something for the deck but it would be slippery when wet and have sharp edges. My kids will be diving off of it.


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## Calidecks (Nov 19, 2011)

I've demoed many many wood 2x decks that had 32" spans that were there 30 years or more. 



_________


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## greg24k (May 19, 2007)

That should help you a little


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## heavy_d (Dec 4, 2012)

I've seen 2x6 deck boards on joists spaced 24 inches.. seemed okay but I wouldn't build it. 16" max for me.

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## hdavis (Feb 14, 2012)

You can buy 1 1/8" plywood rated for 48" interior floor spans - wood isn't as weak as it may seem. The issue with plank decks is having a defect in the board and potentially poor maintenance. In normal deck construction, the joist loading sets the joist spacing, so prescriptive doesn't really deal with maximum span for decking and there is plenty of safety margin. 

With plank decking and long spans, it's important to make sure the planks are in good condition.


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## Calidecks (Nov 19, 2011)

heavy_d said:


> I've seen 2x6 deck boards on joists spaced 24 inches.. seemed okay but I wouldn't build it. 16" max for me.
> 
> Sent from my SGH-I337M using Tapatalk


I agree, 16" o.c. is best practices. 

_________________


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## bmcquin (Mar 10, 2008)

> I've demoed many many wood 2x decks that had 32" spans that were there 30 years or more.


When I first started building decks, 30 odd years ago, the guy I worked for had joist spacing of 36" with #1 grade treated pine 2x6's fastened with Duo-fast screw shank nails...... Granted, way back then the TYP was basically clear, we used any piece with knots for framing, and didn't move as much as the crap now days.


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## Houstondeck (Mar 28, 2017)

You can go 30'' centers with 2x material. We build over 100 decks a year with 2x material on 24" centers. 6" more won't hurt you for this application. Don't over think it, somebody guys on here think way harder they need too. Also, 2x12 isn't ideal because it will cup in the sun, 2x6 is better. 

Hope this helps


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## Irishslave (Jun 20, 2010)

Wait....you have 3 I-beams placed 5 feet apart? Why not use those beams as joist bearing girts and build a regular deck on top of them? Duh 

With 2x8's 16"o.c. you could cantilever at least 1 foot on either side perpendicular to the beam giving you even more surface


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## hdavis (Feb 14, 2012)

Probably he doesn't want to spend the extra money to overbuild.


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## ScipioAfricanus (Sep 13, 2008)

This thread is about 6 weeks old and you may have done the project already but I thought I would stick my nose into it also.

I would screw PT 2x6 to the underside of the 2x12 planks. I might even make a shallow rabbet into the bottom of the planks to fit the 2x6s into. I would suggest using a S/S or other water resistant screw of your choice about 3" or longer. The 2 x 6 only has to span about 95% of the plank length.

It would make a "T" shape in cross section. Very strong I tells ya.

Andy


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## Irishslave (Jun 20, 2010)

hdavis said:


> Probably he doesn't want to spend the extra money to overbuild.


Don't remember where I heard it but somebody once said 

"You get what you pay for"


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## hdavis (Feb 14, 2012)

Irishslave said:


> Don't remember where I heard it but somebody once said
> 
> "You get what you pay for"


Overbuild, and you don't get more than building just right. You do know T&G SYP planks will easily do 48" on a roof?

The biggest thing when you get into longer spans is making sure you don't have the grain running at an angle so it can split under load. In other words, you have to be choosy about your boards, or at least I am.


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## Irishslave (Jun 20, 2010)

hdavis said:


> Overbuild, and you don't get more than building just right. You do know T&G SYP planks will easily do 48" on a roof?
> 
> The biggest thing when you get into longer spans is making sure you don't have the grain running at an angle so it can split under load. In other words, you have to be choosy about your boards, or at least I am.


Yeah but theres no enormous live load on a roof plus those planks are protected by a water proof covering. I've walked on plenty of roofs I wouldn't want to make a point of hanging out on. Any deck or platform should be designed for a live load of 60psf IMO


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## Golden view (Feb 16, 2012)

Assuming you don't care about code or inspections:

If you want minimum overall height, use 4x for deck boards. I bet 3x would even work. Scaffold grade 2x10 can span 6 feet at reasonably high loads, so it's not out of the question that lower grade, wet, incised lumber wouldn't fail in your situation.

I've seen plenty of 2x6 T&G floors at 4' and even 5' span. If you tie the together somehow mid-span, it's pretty similar to a T&G floor.

Keep in mind the PT wood is really going to rust your steel.


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## hdavis (Feb 14, 2012)

I'm guessing he already figured out what he is going to do. His big deal is he wanted to use steel for everything except the deck boards. 

I trust he'll make a strong enough deck - he emphasized his family was going to be using it. I think if he was going to have inspections, the plans would have been approved already. If he was getting inspections, he'd either have to put a lot more steel in or get a stamp. or both. I think any steel construction around here has to be stamped, there is no prescriptive IIRC.

Fail load vs allowed deflection. Different deflections are allowed for different applications L/360, L/240, L/120, L/60. L/60 is for scaffold planks. Deflection at failure is probably in the L/20 range for a good PT SYP 2X plank. Irish is right, I wouldn't want me or my loved ones to walk on an L/60 surface, and probably not on an L/120 surface. It just isn't going to feel solid.

If you look at how I suggested testing to satisfy himself whether this would be OK for him or not, I said wet wood. This is the real life application, it has to be wet. The other detail is I said lay it across 2 4X4s. Checking that way actually builds in additional margin - if the plank was fixed to a rigid frame like it will be after actual installation, the deflection will be decreased over what he gets by not having it attached to anything. Why the extra margin? Some boards will be better than others. You don't want to go through installing them all to find out you have a little problem, and the fasteners and boards will degrade over time.

Sometimes deck prescriptive codes just don't work for a specific family. I know of one instance where a ~400+lb woman broke through. Code doesn't require you to design for that load, but if it's your wife, Mom, whatever, you probably should.


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## hdavis (Feb 14, 2012)

Irishslave said:


> Yeah but theres no enormous live load on a roof


Up here there is.


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## srexteriors26 (Apr 13, 2017)

I walk my fat a$$ on a flat 2x12 all the time on staging but everyone down below closes there eyes and crosses there fingers. You could add a vertical 2x to the bottom of each piece of decking bUT to me it sounds like your not done building your dock.


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## A&E Exteriors (Aug 14, 2009)

I once loaded a 95 Taurus onto a 16' trailer using 2 - 10' 2x12's.

That said I'd go 19.2" oc with your purlins


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## tonyb (Apr 12, 2017)

For what your use is 24" on center will work just fine. You could also get true 2" thick instead of 1.5". At any real lumber yard. That could be 16" center. Make sure you crown the board b4 i stall.

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## hdavis (Feb 14, 2012)

A&E Exteriors said:


> I once loaded a 95 Taurus onto a 16' trailer using 2 - 10' 2x12's.


That's pushing it, but they do hold up pretty well. Once you get the front wheels past the half-way mark, you're almost home free.:thumbsup:


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## A&E Exteriors (Aug 14, 2009)

hdavis said:


> That's pushing it, but they do hold up pretty well. Once you get the front wheels past the half-way mark, you're almost home free.


It was super sketchy....unloading it was even more so


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## hdavis (Feb 14, 2012)

A&E Exteriors said:


> It was super sketchy....unloading it was even more so


You'd think it would be the same on and off, but it isn't. Once you're on, you're pretty much committed.:laughing:


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## shanegavitt (Mar 2, 2017)

the 2x12's cant span 5' in that direction they will deflect like crazy, reduce your span to 12" or 16" for deck boards.


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## Dirtywhiteboy (Oct 15, 2010)

shanegavitt said:


> the 2x12's cant span 5' in that direction they will deflect like crazy, reduce your span to 12" or 16" for deck boards.


No they won't,,,, so it's 12" to 16" for a 2x12 but 2' for ¾" ply:blink:


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## shanegavitt (Mar 2, 2017)

you really use 2' o.c. for 3/4" plywood? I use 16" o.c. for 23/32" advantec subfloor with 3/4" wood flooring on top of it. im not saying 2' o.c. isnt adequate just curious.

theres only 3/4" difference in depth between 3/4 and 2x12's thats not a big difference considering plywood is designed without grain to distribute load


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## Dirtywhiteboy (Oct 15, 2010)

shanegavitt said:


> you really use 2' o.c. for 3/4" plywood? I use 16" o.c. for 23/32" advantec subfloor with 3/4" wood flooring on top of it. im not saying 2' o.c. isnt adequate just curious.
> 
> theres only 3/4" difference in depth between 3/4 and 2x12's thats not a big difference considering plywood is designed without grain to distribute load


Put 2 saw horses 5' apart and screw a 2x12 to each side and tell me it deflects like crazy and I'll tell you go on a diet,,, just saying,,,,
Yes we put 4x8 rafters 4' feet apart and use 2x6 T&G and it's fine.


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## shanegavitt (Mar 2, 2017)

you are right i have used 2x10s for scaffolding and i think its fine for humans.


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## Morning Wood (Jan 12, 2008)

I think 2x12's would probably be fine at a 5' span. You could screw a 2x6 on the flat from above and suck it up against the 2x12's. It would run perpendicular to the 2x12's. That might help a little with cupping and deflection. 
Now, if it were me, I wouldn't use pt for a finished surface. Id add joists between the I beams and use 5/4 composite or wood.


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## Dirtywhiteboy (Oct 15, 2010)

shanegavitt said:


> you are right i have used 2x10s for scaffolding and i think its fine for humans.


Yea but what about dogs or even cats:blink:rabbits:blink:


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## hdavis (Feb 14, 2012)

23/32" Advantech is actually interesting - I'm a big fan of it. It's span rated at 48" for roofs, and 24" for floors, plus there are actual tables available from Huber that give much more information on the loads the different thicknesses will handle with various support configurations. You'd be surprised what you can do with the stuff, especially if you aren't putting tile over the top.


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## Jonbuild (Nov 18, 2013)

This project I dominoed the deck boards together with exterior grade dominos, not sure if it will hold up, but it was really solid when I finished, prior to me replacing the old deck he had 4" log screw up to the bottom of the floor boards as a joist, but not bearing on anything, the dominos keep the boards aligned


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## JFM constr (Jul 26, 2014)

would it not of been easy and beefy looking to drop 4x treated material down with some lag screws and be done .wonder what op ended up doing


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