18.3 More mass without touching the structure

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Fallout radiation is cut in half with each "half-value thickness" of material: about 6 in (15 cm) of water, 3.6 in (9 cm) of packed earth, or 2.4 in (6 cm) of concrete (full table in chapter 15). Wood and drywall barely count. Mass only protects in the direction where it is: a wall of water jugs protects from what comes from that side, not from what comes from above or the other sides.

More mass without touching the structure
What to use	How	Approximate effect	Watch out for
Water containers (5–7 gal jugs)	In a row or double row against the core walls that face the outside or rooms with windows.	A row about 12 in (30 cm) of water is two thicknesses: radiation from that side drops to ¼.	Weight: 8.3 lb per gallon (1 kg per liter). A 7-gal jug weighs about 58 lb (26 kg).
Full bookcases and boxes of books or paper	Against the wall, books packed tight, no gaps.	Packed paper slows radiation less than water per inch, but a full bookcase helps.	Anchor them to the wall (anti-tip) if children live there or there are large pets.
Dressers with full drawers, refrigerator, washing machine	Only if you can move them without hurting yourself.	Some mass; better than nothing.	Do not block doors or exits.
Full bathtub	If the shelter is the bathroom.	Mass on one side and a water reserve.	See section 18.7.
Mattresses	Over the people during a tornado.	They protect from debris and glass, almost nothing from radiation.	Keep them handy in the core.
Bags of earth or sand	Only as a last resort and if feasible.	Very good per inch (more than water).	Weight, dirt, moisture, pests, and your lease. See warning.