check valve thumping

I just installed a 2' verticle check valve between my pool solar pump and the solar pannels on the roof. I need the check valve because I'm installing an additional pump for a water fall into the pool. When I shut the solar pump off the water draining back from the roof starts the 2" pipe to the roof and the check valve thumping which last for about 2 minutes and can be heard clearly in the house. Any suggestions how to get rid of this thumping? How about a releif valve? or possibly a horozontal spring check valve??

You probably need a vacuum vent [relief] valve on the roof at the highest point. Cant say for sure with the information given. If you do the waterfall with pool water you may as well forget the solar heating as it will give up the accumulated heat very quickly... Unless you are in palm springs and need to cool the water in summer.

Are you in an area where these panels can freeze?

bob...