clapboard butt spacing?

I am looking at LP 'Smartside' lap siding. The installation instructions say to leave a 3/16" gap at butt joints. I assume this is for expansion? You're supposed to caulk it, but it seems huge. Is this different than regular wood clapboards? I thought they were supposed to be snug at the joints.
Anyway, should the gap be proportioned to length? I have an 8' wall, where the Smartside comes in 16' lengths. I'd think that I could use a 3/32" gap?
Thanks, G
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Sawn wood products exhibit movement radially and tangentially to the grain - there is not a lot of movement along the grain. A sawn wood clapboard will expand and contract a small amount in height and thickness due to changes in humidity, and to a much, much lesser extent temperature. The length will not change.
LP's Franken-wood is made up of wood fiber and a lot of binder - plastic resins, and due to this there will be more movement in length, and that is why there is a need for an expansion gap.

Shorter lengths will allow narrower expansion gaps. This will also be dependent on the direction of exposure. South and West facing walls will heat up more from the sun, and will require larger gaps than North and East facing walls. Your local climate will dictate the gap requirements.
Call LP's tech support line if you want specifics.
R
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wrote:

I don't doubt what you say, but I have trouble picturing it. The boards will be nailed through the sheathing, into the studs. If the siding lengths change, ... what? ISTM, either the whole wall moves along with them, or the nails pull up. Or something.
G
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Siding nails are very thin and they can and do bend a bit. It's a system - everything in it moves. Nails, siding, sheathing, framing - everything.
R
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LOL... The Franken-wood siding and the cementious-siding also usually spec a small piece of felt paper which bridges behind the joint in addition to caulking... The felt paper is affixed above the clapboard you are joining and runs over the one directly below for a bit until you are solidly overlapping that full unbroken clapboard quite a bit and serves as a back up way of shedding water in case the caulking fails...
~~ Evan
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On Wed, 9 Nov 2011 05:30:06 -0800 (PST), Evan

I've read that; it's another puzzle, to me. If the caulk fails so the felt is needed, the felt would be exposed, to some degree. Exposed felt kind of deteriorates, IMO. I've been thinking of using a small piece of aluminum flashing, instead.
The whole thing seems odd, to me. I think of clapboards as being like shingles on a roof. But, on a roof, shingles are lapped so there's always a shingle layer below a joint. The Smartside calls for a 5" exposure on a 6" board; so, if the caulking fails over the years, the joint is open right to the underlayment.
I guess it must work, though.
G
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LOL!!.....
I first read it as "clapboard butt racing". My first thought was, "Is that on snow or wet grass?" ;)
nb
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