I am putting fiber cement claps on a small building. I've seen two ways to
do the corner boards. Either put the corners on the building first, then
butt the siding -or- put up the siding with the corner boards on top of the
Do you have a preference? The installation instructions for the hardiboard
show it both ways. I was thinking of putting the corner boards on top of
the siding would be a lot easier because I wouldn't have to be so precise
with the cuts on the claps.
There are a bunch of ways to do it. I like to nail vertical furring
strips to the corners, run the siding, then nail the corner boards to
the furring so they cover the ends of the clapboard. You should still
seal all cut ends of fiber cement product with paint, even if they are
Depends on the width and thickness of the corner boards you want to
use. Generally about an inch to an inch and a half less than the
corner board width. Allow for the corner boards overlapping at the
I also like to use an expanded polystyrene corner board, such as Azek.
Cornerboard on top of siding
* Easier measuring of siding.
* Ten times as much caulk.
* Harder to get the cornerboard straight.
Siding butted to cornerboard
* Lots less caulking.
* You have to be careful measuring.
* Put the cornerboards on first.
* Be sure to put factory ends together in the field (where two pieces
of siding butt up to each other). The joint will be prettier.
* Get those butt joints nice and tight.
* Leave just a little space for expansion, no more than 1/8", between
the siding and the cornerboard.
* Don't align the top edges of adjacent pieces. Hold a straightedge
under the bottom edge of the siding to align the new piece to the one
already on the wall. The width varies just a little, and the bottom is
what will be visible.
* Caulk all the vertical joints. Don't caulk the bottom edges--water
needs a way out.
* Rent or buy the scissors to cut the HardiBoard. The dust from sawing
it is awful.
I'll try to put this into words...
We did 8" lap smooth fiber cement on our new garage. On the
recommendation of the building matls supplier, we installed 1" thick
corner boards (I think they were deck boards) then installed the
siding, relatively tight to the corners, but not perfect, then
installed more fiber cement siding vertically on the corners, covering
up the 1" boards and lapping over the siding about 3/4" and covering
up the not-so-perfect cuts. Overall it looks pretty good, and was
easy to do.
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