From verso of the card: "Sandbag walls should be 2 ft. 6 in. thick at the top and should overlap the window opening by at least 12 in. all round; the base should be wider to prevent the wall collapsing. Such a wall will keep out splinters from high explosive bombs and protect the glass of the window from being shattered by blast. The window must still be sealed against gas. The windows of refuge rooms should be small; it is also an advantage if they face soft ground rather than a metalled surface."