For prevention, use a wire piece and push it via the holes, so that they drain freely. If clogged severely, it's possible you'll consider re-drilling them to clear the blockage. Small cracks and holes (often made by thriving insects) will also be a motive for water seeping in and causing the wooden to rot. You may see them as brownish or yellowish stains on wooden, which regularly really feel spongy when touched. There are various kinds of colonial windows obtainable, every offering its own dist...