Patching holes in plywood board?

[Syffuf]

After noticing some water damage around a windowsill on our top floor, I decided to investigate in the attic. I came to find that there are 2 holes (1 is the size of a softball and the other is about 4x that size) in the plywood board behind the siding. I can actually see the siding through the plywood. As it would likely be extremely expensive to have a contractor come in, remove the siding, and replace these boards, I was wondering if there is an alternative method for patching these holes from the inside? I've read about people sanding down the edges of the cut, then screwing in a plywood patch over the hole, then sealing the edges of the patch. If this is a terrible idea, please let me know as well.

I honestly have no idea what caused the holes. We've been in the house for about 1.5 years and are not the first owners. The holes look like they may have been cause by some sort of animal, or even a person with a hammer. There are no other signs of animal activity in the attic.

[pushkins]

There really is no reason to patch them for "structural reasons" however to close them off for rodent or air mitigation is a good reason.
A simple fix would be to cut a piece of plywood/OSB etc... larger than the hole, add a bead of caulk around the edges of the existing hole and attach the new piece from the inside over the hole with screws no longer than the thickness of both pieces of material.

As a note though you should not be able to see siding from the inside of the home directly, you should be looking at a house wrap (tyvek etc...) and fan board insulation (usually blue or green).

[Syffuf]

Thanks for the great info! Unfortunately, I can definitely see the siding through the holes. If there was some sort of house wrap, it may have but cut through as well. Is it possible to patch that, or will that require removing the siding and replacing that entire piece of house wrap?

[pushkins]

First try to see if there is any house wrap at all, try to feel around the hole from the inside and see if you can feel any there, if not then the point is mute as to install it would be cost prohibitive, with siding removal etc...
If it is there and it's just been chewed through (etc) then you might be able to push a new piece up under the existing and have it lap down over the bottom part of the existing obviously before you install the ply.

[HayZee518]

the only two things that could have made the holes are an insulation contractor blowing in cellulose insulation in which case they used a hole saw. and the second, maybe there was a wall thimble installed before the siding was put up.

[mrcaptainbob]

Water damage...

Syffuf, do you suspect the water damage might be from one or both of those holes?