A fire stop dimensioned to fit a selected size opening. The fire stop comprises end plates, a central shaft member retaining the end plates apart from one another, and a core between the end plates that comprises fire putty and mineral wool. The thickness and density of the mineral wool layer and/or the fire putty layer are chosen to achieve a particular fire rating within the constraints of the overall dimensional thickness of the slab. The fire stop has a low profile rendering it non-obstructing, can be inserted by a single installer from above a floor slab or into a wall, and further can be removed for inspection and/or replacement as needed. The fire putty layer swells under heat filling remaining gaps between the fire stop and the opening.