41
Cracking
If a clean
fi
nish with no tile, etc. is desired, joints in the
non-combustible board and the transition to gypsum
board will require special attention if future cracking is
to be controlled.
Shrinkage and differential movement of the framing
and non-combustible wall board can transmit cracking
through to tiles, etc.
Be aware that temperatures on the non-combustible
wall surface above the appliance can exceed 200°F.
Below are some tips on how to best avoid any
cracking:
• Allow materials to dry thoroughly before
fi
nishing
the wall. Cement board has the ability to absorb up
to 30 percent of its weight in water and may shrink
as much as 1/8” over a 48” length when drying from
a saturated condition. Running the
fi
replace for an
extended period before
fi
nal
fi
nishing will help drive
out moisture.
• Always pre-drill screw holes through cement board
and use screws with self-milling head.
• Always use mesh tape over joints.
• Always stagger joints in wall board.
• Behind joints, double up studs or use studs “on the
fl
at” to add extra support to the joint. Adhesive on
the backside of wall board behind any joints can
help control differential movement.
• Use multiple, thinner coats of joint compound and
allow to dry thoroughly between coats.
• Ensure framing materials are dry.
• After
fi
nishing the wall, introduce heat gradually to
slowly dry any excess moisture rather than drying
too fast.
• Avoid notching cement board or tiles around corners
of window opening and instead provide a joint that
intersects the corner.
Installation Planning
QUALIFIED
INSTALLER