If you have knots, splits or defects in the tongues or grooves you run the risk of allowing water to pass through the siding when driven by wind or gravity.
(you; have; knots, splits or defects in the tongues or grooves) (you; run; the risk of allowing water to pass through the siding)
you you
have run
knots, splits or defects in the tongues or grooves
the risk of allowing water to pass through the siding