A sprinkler includes an outer case and a riser mounted for reciprocation within the case. The riser supports a nozzle, an impeller, and a drive linkage connecting the nozzle and the impeller. A diaphragm valve is mounted in outer case beneath the riser and includes a valve member vertically reciprocable within a valve support housing to engage and disengage a valve seat. The valve seat can reciprocate relative to the valve support housing to reduce mechanical loading.