I bought a foaming nozzle from Drew Lafferty called the Bullet. He sells these to car wash operations. I'm using my All-Flo and 200' of 5/8" hose. My roof mix is pulled from one tank. I'm using water, SH, ammonyx lo and green apple for my mix. This is a nozzle only solution that does not use a metering system. Lafferty sells those systems and Drew is very helpful explaining all his products if you give him a call. The nozzle cost was somewhere between $50-60.
My experience with the foaming nozzle has produced mixed results. You must be able to walk the roof. It will only shoot about 5-7 feet. The foam produced is a wet foam and it takes some practice to lay it down properly. Too much and it will start to run and too little and when it dries there will still be spots. I also have found that getting the right amount of surfactant is key to producing good foam.
I mainly use it now near the edges to control runoff and use my regular nozzles to shoot the main portions of the roof. It's only rated at 1 gpm so it's a slow process using it on all sections of a roof.
This roof in the video is a mobile home so there wasn't alot of pitch but you will get the idea of how the nozzle works.