The space between the outer and inner walls shall be filled with dry course sand (not crushed stone or gravel) or a weak mixture of cement mortar. All lumber used shall be well seasoned and free from loose knots, bark edges or decay. Report Title (OSH)Standards Topics Discussion 9/1/2021 RULE 1140 x RULE 1140 EXPLOSIVES 4) Brick Brick walls shall be 20 cm. (8 in.) .