Waterproofing shiplap on dormer windows

[cl]

We have a house with lots of shiplap dormers and also a garage with a games room above that has two similar dormer windows. The ones on the house are quite waterproof but the ones on the garage aren't, when it rains the water drips on my work-bench which is very annoying.

I suspect that whoever built the garage cut a few corners when making the dormer windows.

Anyway, as far as I can tell the water is getting in at the corners where the ends of the shiplap are just exposed with no beading of any sort (they do have beading on the house). The gaps aren't huge but there's quite a lot of them and it would be a long job to squirt with a sealant gun. Does anyone have any better ideas (apart from totally rebuilding the dormers!)? For example are there any aerosol type things that I could squirt into the gaps? Outside access is reasonably easy, inside is more difficult.

[Tim Watts]

If the gap is deep, expanding foam maybe. However the sun rots the ordinary stuff - so if you can, use the foam to fill the gap behind and leave a crack to fill with sealant.

For the sealant, I recommend:

It sticks well (better than silicone) and is holding up nicely where I filled the gaps in my wooden window frames.

But for a better fix, could you glue some hardwood/cedar/oak/plastic profile over the edges?

Failing all of that, there is another version of PU foam made that is sun proof - it is called Waterfall Foam or variations on that. It's black PU foam designed for sealing between rocks in constructed water courses and waterfalls.

Not cheap, but you'll not need more than a can:

[cl]

That sounds good stuff, thanks for the recommendation. Being able to use it on wet surfaces is a big plus.

Yes, I've considered that.

Not so expensive really.