Simple Solution
When wood lap or clapboard siding splits or cracks along its length, the result is unsightly, not to mention worrisome, because that crack will just continue to deepen.
Now, do you have to replace the siding? Well, 99 percent of the time, no, you do not. Here’s how to repair it.
First, pull out the nail or nails that are closest to the split. If removing the nails would damage the wood, you can cut them, from under the siding, with an oscillating multi-tool or hacksaw blade.
Next, use a chisel or stiff-blade putty knife to carefully pry open the split.
Then, apply a thick bead of waterproof wood glue into the split. Push up on the bottom edge of the siding to close the split and wipe off the excess glue.
Finally, you can tack-nail a 1-by-2 to the sidewall to hold the split closed until the glue dries.
You usually don’t even need to re-nail the split piece of wood, but if you choose to nail the split piece of wood, only nail about a 1/4-inch above the split.