Will a siphon work from an in ground pool to a roof top back into the pool?

The pool surface that is the source must be higher than the pool surface that you are trying to siphon to. Water can only flow downhill on its own. Once you start a siphon, as soon as you raise the end of the hose above the source it will stop flowing. A pump as the other answer suggested is the only way.

No it will not work, unless your pool is on 2 different levels (hence 2 pools, one higher that the other). And even if this were the case, you would simply be emptying the higher pool into the lower one. So you would still need a pump to keep the level steady on the higher pool (pumping from low to high).

No it will not work, unless your pool is on 2 different levels (hence 2 pools, one higher that the other). And even if this were the case, you would simply be emptying the higher pool into the lower one. So you would still need a pump to keep the level steady on the higher pool (pumping from low to high).

If you have two pools one a foot to a meter higher surface than the other, you can siphon from the the high pool to the low pool provided your roof is less than about 33 foot = 10 meters higher than the high pool surface. Theoretically to 39 feet, but that is too much when the barometer reads low, and the flow rate would be very slow at over 33 feet, and the siphon difficult to get started. My experience is any light that gets in your hose causes pool algae to grow in the hose, so make sure your hose is opaque. The green hoses are not sufficiently opaque. Your pool filter pump can likely maintain the one foot (perhaps a meter) difference between the two pool surfaces, while filtering both pools.The high pool could be a tiny above ground pool, with any overflow water draining into the lower pool. Possibly you can connect the hose instead of the drain plug of the pool filter, but getting the siphoning action started will be difficult if the roof is more than a meter higher than the pool surfaces = pump assisted siphon with second pool not needed. Clearly a separate pump is simpler as it lets you stop the siphoning after sun down when the hose will likely cool your pool. I decided it was a waste of electricity to run the pool filter more than an hour after the last person got out of the pool, but most people forget to turn the filter on before they get in either pool. I only got about one degree warmer water with about 100 feet = 30 meters of hose in the sun, but a hot roof should be slightly better. Neil

You would have to use a pump.

If you have two pools one a foot to a meter higher surface than the other, you can siphon from the the high pool to the low pool provided your roof is less than about 33 foot = 10 meters higher than the high pool surface. Theoretically to 39 feet, but that is too much when the barometer reads low, and the flow rate would be very slow at over 33 feet, and the siphon difficult to get started. My experience is any light that gets in your hose causes pool algae to grow in the hose, so make sure your hose is opaque. The green hoses are not sufficiently opaque. Your pool filter pump can likely maintain the one foot (perhaps a meter) difference between the two pool surfaces, while filtering both pools.The high pool could be a tiny above ground pool, with any overflow water draining into the lower pool. Possibly you can connect the hose instead of the drain plug of the pool filter, but getting the siphoning action started will be difficult if the roof is more than a meter higher than the pool surfaces = pump assisted siphon with second pool not needed. Clearly a separate pump is simpler as it lets you stop the siphoning after sun down when the hose will likely cool your pool. I decided it was a waste of electricity to run the pool filter more than an hour after the last person got out of the pool, but most people forget to turn the filter on before they get in either pool. I only got about one degree warmer water with about 100 feet = 30 meters of hose in the sun, but a hot roof should be slightly better. Neil

You would have to use a pump.