Diagonal Layout in a Sloped Room With Control Joints
A 24 ft x 36 ft banquet room in a historic building is getting 24 x 24 porcelain tile with 3/16 in. grout joints in a diagonal layout. There are four 6 ft-wide doorways, one centered on each wall, and a 12 ft bar along the north wall. The general contractor wants full tiles centered at the main entry on the south wall and no cut tile narrower than 6 in. anywhere. The slab slopes 1/8 in. per foot to a floor drain 4 ft from the center. Control joints run at 12 ft intervals in both directions. The installer proposes to snap a diagonal line from the main entry, start with full tiles there, cut at all walls, and grout the control joints flush with the field.
1. Develop a layout strategy that satisfies the centered-entry requirement and the 6 in. minimum cut rule. What must be checked before setting tile?
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Start by snapping centerlines through the main entry and perpendicular to it. A diagonal layout is controlled by the diagonal module, which for a 24 in. tile is about 33.9 in. across the corners. If full tiles are centered at the entry, the cuts at the side walls will fall where the half-room dimension lands within that module. For the 24 ft width, half the room is 144 in.; 144 divided by 33.9 is about 4.25, leaving a cut near 8.5 in. For the 36 ft length, half is 216 in.; 216 divided by 33.9 is about 6.37, leaving a cut near 12.5 in. Both are above 6 in. in the simple case, but the bar, doorways, and drain can change the result. Dry-lay or snap a full grid and check every wall and the bar toe. If any cut falls below 6 in., shift the layout by half a module or add a border rather than accepting a sliver. Confirm the drain slope and tile flatness before committing.
2. Evaluate the proposal to grout the control joints flush. What is the correct treatment, and what other movement joints are needed?
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Grouting control joints flush is wrong. The slab control joints are designed movement locations, and tile must not be rigidly bonded across them. The joints should continue through the tile as movement joints filled with an ASTM C920 sealant compatible with the tile and grout, often with a backer rod. In addition, the perimeter must have a flexible joint at all walls and fixed objects, including the bar. The 36 ft dimension exceeds the usual 20 to 25 ft spacing for interior movement joints in a tile field, so at least one intermediate movement joint should be laid out in that direction. The 24 ft dimension is at the upper end and should be reviewed. The joint layout must be shown on the setting plan before tile is cut.
3. What installation concerns does the floor drain slope create for 24 x 24 tile, and how should the tile and grout be selected?
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A 1/8 in. per foot slope over 4 ft creates about 1/2 in. of fall to the drain. Large, rigid tile cannot easily follow that slope without lippage, hollow spots, or rocking. The installer should use a medium-bed mortar where needed, back-butter the tile, and check flatness and coverage frequently. Near the drain, it may be better to use a smaller tile or a cut pattern that follows the slope rather than forcing a 24 x 24 tile across the depression. For 3/16 in. joints, a sanded grout is normally appropriate; confirm it is compatible with the porcelain surface and the expected traffic. Use sealant, not grout, in the control and perimeter joints. Mock up a few tiles across the slope and at the bar to verify the layout before setting the field.