There were four categories of shelter strength. All protected from contamination, category 1 and 2 structures protected from direct hit by aerial bomb, while category 3 and 4 structures protected only from falling rubble and indirect result of an attack (shrapnels, blast accompanying an explosion).
Category 1 shelters were dedicated for central authorities and command posts of Territorial Air Defence stations. They were the most resistant and the required minimum thickness of the reinforced concrete ceiling amounted to 275 cm. The most durable ones were supposed to provide safety during a direct hit of an aerial bomb up to one tonne of weight, however the majority protected against a bomb weighing 250 kg.
Category 2 shelters used ceilings of minimum thickness amounting to 210 cm. Designed in office buildings of central authorities and state offices, political organisations, and health centres, they served also as Territorial Air Defence command posts at key workplaces, districts or Citizens’ Militia stations, as well as to secure particularly precious museum exhibits.
Other institutions, workplaces and residential buildings were equipped with category 3 and 4 shelters where ceilings were minimum 30 and 15 cm thick, accordingly.
Following the implementation of regulations connected with the nuclear weapon threat at the end of 1950s, shelters were classified according to their strength in view of the power of explosion and the distance from the epicentre.
Another way to protect civilians against the results of air raids was to transform some cellars and basements into makeshift air raid shelters. The adjustment consisted primarily in securing external openings (windows, doors), supporting ceilings, sealing the structure with a product preventing penetration of poisonous substances, preparation of emergency exit and providing equipment enabling people to stay there (water reserve, lighting, toilets, benches).
It was also planned to build air-raid trenches, that is long pits, 180-220 cm deep and approx. 100 cm wide, with timbered walls and a proper covering, dedicated to hide people during an air raid. They protected only against shrapnels and the blast accompanying an explosion. The equipment consisted of benches, dry toilets (hermetically sealed containers) and a water reserve. Persons staying inside were supposed to be equipped with gas masks.
Trenches and protected spaces were to be constructed in previously selected places, following readiness announced by the Territorial Air Defence.