When the blades are fully closed, they seal together and create a barrier from rain. There are grooves in the blades that will guide the rain water to the side beams of the pergola where a gutter sits just on the inside of the beam. Then from the beam gutters, the water will flow to the posts and down to the bottom of the post to drain. If you choose not to install your pergola with posts, you will need to purchase a downspout to connect to the corners of the pergola so that you can direct the rain water as desired.