Hi Joel, Good question. In these circumstances the water will flow out of the corners of the j channel. If there is a lot of water coming down, it will build up in the j channel until it flows over the face. The holes in the nail fin are higher than the face of the j channel so there should never be a problem with water overflowing the back. Also this j channel installation is underneath a roof so there will be very minimal water anyways. In these situations (especially when there’s no roof covering) it is imperative that the housewrap, ice & water, and flashing underneath the siding is done correctly to prevent any water damage. Vinyl siding helps keep water off from a house, but properly installed housewrap and flashing is what prevents it from contacting the underlayment and causing rot.