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
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
Call LP's tech support line if you want specifics.
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.
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...
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.
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