Abstract: A shut-off valve interactive between a sprinkler head and a riser pipe, and adapted to stop water flow in the event the sprinkler head is inadvertently broken off, includes a coupling sleeve suitably threaded to engage the sprinkler head and riser pipe, and having an annular frangible zone. A sealing member movably confined within the sleeve abuts the sprinkler head and is thereby held in a stand-by position which permits water flow. However, when the sprinkler head is broken off at the site of the frangible zone, water pressure forces the sealing member upwardly into abutment with an apertured washer, thereby stopping water flow.