UL Design U901

4-Hour Fire Rated U901 assembly

UL Design U901 — Four-hour concrete masonry wall (rated 4 hours both bearing and non-bearing) built of UL Classified concrete blocks of various designs carrying Classification B-4 (4 hr), laid in a full nominal 3/8 in. mortar bed (1 part Portland cement to 2-1/4 to 3-1/2 parts sand, hydrated lime up to 50 percent of cement volume) with vertical joints staggered. Portland cement stucco or gypsum plaster, if used, adds 1/2 hour to the classification. Lower-classified D-2 or C-3 blocks may achieve the 4-hour rating when all core spaces are filled with loose masonry fill. UL permits various unit designs, so nominal thickness varies by manufacturer. (Details per the cited source; confirm item-level options against the full published listing before specifying.)