2024-25_NTCA_Reference_Manual_dig_dwnld

Box Screed

Essentially a box screed is a jig used to apply mortar onto the back side of large-sized ceramic, marble and granite tiles that may vary in thickness or flatness, in order to achieve a uniform unit of thickness of the tile and mortar combined.

Place the 1/4 in. (6 mm) plywood on the surface of the tile and affix the suction cups. Remove the 1/4 in. (6 mm) plywood and place the tile face down into the box screed base. Trowel properly prepared mortar onto the back of the tile and screed off excess with a straight edge. Reach underneath the box screed and lift tile enough to grasp the suction cup handles. Place tile on vertical or horizontal substrate.

Example #1

Example #2

The illustrated box screed is specifically for a 12 in. x 12 in. (305 mm x 305 mm) tile. Materials and method of construction are the same as example #1. Overall dimensions – 16 1/4 in. x 42 in. (413 mm x 1067 mm). Place the tiles into the box screed from one end, pushing them through with no space between tiles. Trowel properly prepared mortar onto the back of the tiles and screed off excess with a straight edge. Remove and install tiles.

The illustrated box screed is made specifically for a 24 in. x 24 in. (610 mm x 610 mm) tile. The base is a Georgia Pacific 3/4 in. (19 mm) pine ply form 27 3/4 in. (705 mm) square, fastened to two 2 x 4s (48 x 98 mm) with #8 (4 mm) x 1 3/4 in. (45 mm) screws. The screed strips are high density polyethylene, drilled and countersunk and fastened to the base with #8 (4 mm) x 1 1/4 in. (30 mm) screws. The thickness of the screed strip is determined by the exact thickness of the tile used plus 1/8 in. (3 mm) of mortar. The 1/4 in. (6 mm) plywood piece is cut to the exact size of the tile. The base and the 1/4 (6 mm) plywood have corresponding holes cut to accommodate the suction cups shown.

GENERAL Best results are obtained when mortar is keyed into the substrate with the flat side of the trowel; then combed in one direction with the proper notched trowel. You now have a gauged amount of mortar on the tile and on the substrate. Placing the two gauged mor tar surfaces together, push tiles into the mortar and move them perpendicular across the ridges forward and back approximately 1/8 in. (3 mm) to 1/4 in. (6 mm) to flatten the ridges and fill the valleys. This technique can produce maximum coverage for large size tiles – with the corners and edges fully supported – without flat back-troweling or beat-in. Periodically remove and check a tile to assure proper coverage is being maintained. While the illustrated box screeds were made of quality materials for continuous use in NTCA training programs, jobsite box screeds can be made from plywood sanded one side for the base and any good hardwood for the screed strips. Construct the box screed to fit the particular size and thickness of the tile being installed, with the length determined by job requirements or individual preference. The tiles should fit snugly into the box screed to avoid mortar on the edges. Careful measurement of the thickness of the screed strips is essential for application of the proper amount of mortar. Screed strips can be screwed or nailed to the base. In either case, the screw or nail heads must be below the surface of the screed strips. Support the box screed on a solid work surface near the substrate being tiled for faster, easier and more efficient production.

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NTCA Reference Manual | 2024 / 2025

Chapter 8 | Thin-Bed Method Installations