is board spacing necessary on a deck that won't get wet?

[nereo]

Looking to install cedar decking on an enclosed patio that does not get wet (completely protected from rain & snow).  I'd like to lay the boards right up next to one another so there are no gaps dirt to fill in.  Most of the guides dictate that I space the boards to allow for expansion when the boards get wet and allow for drainage, but I should have neither of these problems.

Question:  Can I lay the boards side-by-side with no gap for this installation?

Specifics (subject to adjustment as necessary)
5/4 x 6" premium red cedar.  Subfloor is 3/4" plywood on top of 2 x 10" joists.  There is insulation under the joists so this space really, really can't get rained on. I might remove the plywood and install cedar directly on top of joists... haven't decided yet.  Because there is insulation under the floor I really don't want gaps that can fill with dust/debris.
Cedar will be finished with Tung oil.
Entire space is 8' x 16'.  The space is well insulated but not climate controlled, so there are seasonal fluctuations in temperature.

[FIRE me]

It isn't just rain and temperature, it is also humidity. Keep in mind that interior wood floors have a gap between the wall and the floor (covered by the baseboard). The purpose of said gap is for seasonal expansion in high humidity.

The second big factor is moisture from under the floor. Many wood floor installs fail due to the installer neglecting to address moisture from a crawl space or slab. If you aren't dry below the floor, you are likely to have problems.

I am no expert, but by a Google search it looks like Cedar contracts and expands less than average. Lucky you. So provided you have a normal perimeter gap, you may get away with no gap between the boards.

Don't forget to acclimatize the wood before installing it.

Further reading:

http://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/

[paddedhat]

IMHO, you have a bigger issue here, that being, deck boards are a poor choice for this application. The material has a radius edge, and will provide hundreds of lineal feet of cracks to catch dirt. There will be gaps, at least on a seasonal basis, and there will always be dirt stuck in the radiuses, and gaps. Traditionally covered porch floors were done with tongue and groove flooring, with 3/4" thick Fir being a popular choice. Deck boards are really a bad idea for what is, essentially, an indoor application.

[nereo]

It isn't just rain and temperature, it is also humidity. Keep in mind that interior wood floors have a gap between the wall and the floor (covered by the baseboard). The purpose of said gap is for seasonal expansion in high humidity.

Thanks.  Did not mention in the OP but there will be a 1/2" gap along the walls, as you would have with an interior install.

The second big factor is moisture from under the floor. Many wood floor installs fail due to the installer neglecting to address moisture from a crawl space or slab. If you aren't dry below the floor, you are likely to have problems.

Moisture from below (I believe) shouldn't be an issue.  Beneath is a 6' high space that's also completely protected and has decent air circulation.  The underside of my patio has a vapor barrier and then insulation.   We'll certainly let the wood acclimatize before installing, but given that it will be early summer, it's hard to know what 'acclimated' is - one day it might be 85º, the next it might be 65º. 

[nereo]

IMHO, you have a bigger issue here, that being, deck boards are a poor choice for this application. The material has a radius edge, and will provide hundreds of lineal feet of cracks to catch dirt. There will be gaps, at least on a seasonal basis, and there will always be dirt stuck in the radiuses, and gaps. Traditionally covered porch floors were done with tongue and groove flooring, with 3/4" thick Fir being a popular choice. Deck boards are really a bad idea for what is, essentially, an indoor application.

Oh dear, this brings me back to where I started.  After originally planning on going tongue-in-grove someone that works selling flooring steered me away from this.  He said because it's a 3-season porch and it will be cold in the winter and hot in the summer, I should consider it an exterior install, even though it won't be exposed to rain/snow.  He urged me to "think of this like building a deck, not like installing interior flooring".

We love the idea of cedar decking - had a cedar deck on our last home. BUt my SO pointed out that if we used normal 3/16" spacing between boards they'd just fill up with dirt, since it couldn't 'fall-through' to below like it normally would.  We're ok with vacuuming the area occasionally, and I certainly could take of the radial edge of the boards.  That's a lot of freakin' sawing though (88 edges total).

[paddedhat]

Oh dear, this brings me back to where I started.  After originally planning on going tongue-in-grove someone that works selling flooring steered me away from this.  He said because it's a 3-season porch and it will be cold in the winter and hot in the summer, I should consider it an exterior install, even though it won't be exposed to rain/snow.  He urged me to "think of this like building a deck, not like installing interior flooring".

Classic case of a "pro" without a clue, handing out bad advice.  If you look at older style of pre-war home in N.A. you will often see covered front porches, on farm houses, four-squares, and others. These floors were always T&G and varied from hard yellow pines to edge grain fir. I replaced the fir on my parent's porch, using  mahogany T&G. It got a clear marine varnish finish, and looks stunning twelve years later.  . The original floor was ninety years old, and still functional. the weather had worn the very first inch of overhanging material, and it just looked tired.  If you have the boards already, I would definitely square the edges. Good luck.

[nereo]

Oh dear, this brings me back to where I started.  After originally planning on going tongue-in-grove someone that works selling flooring steered me away from this.  He said because it's a 3-season porch and it will be cold in the winter and hot in the summer, I should consider it an exterior install, even though it won't be exposed to rain/snow.  He urged me to "think of this like building a deck, not like installing interior flooring".

Classic case of a "pro" without a clue, handing out bad advice.  If you look at older style of pre-war home in N.A. you will often see covered front porches, on farm houses, four-squares, and others. These floors were always T&G and varied from hard yellow pines to edge grain fir. I replaced the fir on my parent's porch, using  mahogany T&G. It got a clear marine varnish finish, and looks stunning twelve years later.  . The original floor was ninety years old, and still functional. the weather had worn the very first inch of overhanging material, and it just looked tired.  If you have the boards already, I would definitely square the edges. Good luck.

Trust me, I'm very much aware now that the advice of a "pro" is often not worth the dirt on my shoes.  The problem is knowing which advice is good and which advice is bad.  Our first inclination was to install tile, and god did that ever cause a firestorm, everything from underlayment to temperature swings to flex of the subfloor.  I couldn't find a consensus on anything.  Ultimately though we decided we'd rather have wood anyway.
I agree with what you are saying here because I've seen first-hand evidence of covered porches in Maine holding up just fine, even after a window's left open during a thunderstorm or snow blows through the cracks and muddy boots and bike are constantly stored there.  Porches seem to do just fine.... which is basically what i've got.
Alright, I know cedar's not a very hard wood, but we do love the look and it is nostalgic for us.  Any reason why tongue-in-grove cedar wouldn't work (provided I can find some) for this application?

[zolotiyeruki]

You say you have 6' underneath--why couldn't the dirt fall through the cracks and down 6' to ground level?

[paddedhat]

Alright, I know cedar's not a very hard wood, but we do love the look and it is nostalgic for us.  Any reason why tongue-in-grove cedar wouldn't work (provided I can find some) for this application?

No reason why T&G cedar wouldn't work just fine. I don't know who would of worked so hard to convince you that tile is a bad idea, but they are wrong. There are a bizzion feet of tile on top of 3/4" ply, and 2x10 joists. I've done it exactly like that for the last 25 years and have exactly zero failures. The key is in the prep. If the budget I tight, Hardibacker is a great product, if you want a bomb proof job, then go with Ditra membrane. It's up to you, if you had your heart set on tile, go for it. If cedar meets your needs, find some T&G. Good luck.

[nereo]

You say you have 6' underneath--why couldn't the dirt fall through the cracks and down 6' to ground level?

because there's 3/4" plywood as a subfloor.  Below that there's a moisture barrier, and then insulation.  There's several layers of 'stuff' (plywood, moisture barrier and insulation) keeping the dirt from falling through.

[nereo]

Alright, I know cedar's not a very hard wood, but we do love the look and it is nostalgic for us.  Any reason why tongue-in-grove cedar wouldn't work (provided I can find some) for this application?

No reason why T&G cedar wouldn't work just fine. I don't know who would of worked so hard to convince you that tile is a bad idea, but they are wrong. There are a bizzion feet of tile on top of 3/4" ply, and 2x10 joists. I've done it exactly like that for the last 25 years and have exactly zero failures. The key is in the prep. If the budget I tight, Hardibacker is a great product, if you want a bomb proof job, then go with Ditra membrane. It's up to you, if you had your heart set on tile, go for it. If cedar meets your needs, find some T&G. Good luck.

 
Thanks - appreciate the advice and reassurance.  I think we're going with the T&F cedar, both for looks and price. FWIW I posted the tile idea on the John Bridge forum - I guess there's a whole bunch of perfectionists there that want to make things as complicated (and bomb-proof) as possible.  Advice included doubling up the plywood to 1.5", then ditra, then long conversations about custom mixtures of mastic/thinset.  Following their advice it would have been $8-10 sqft.  Cheers.

[paddedhat]

I posted the tile idea on the John Bridge forum - I guess there's a whole bunch of perfectionists there that want to make things as complicated (and bomb-proof) as possible.  Advice included doubling up the plywood to 1.5", then ditra, then long conversations about custom mixtures of mastic/thinset.  Following their advice it would have been $8-10 sqft.  Cheers.

That's too funny. EVERY forum has at least a few of these guys. Post a question on how to patch up your beat old econobox car, and you end up with detailed instructions on how to turn it into a space shuttle, followed by ten pages of who is the REAL expert when it comes to DIY space shuttle building. Good luck with the floor.