What's the biggest gap you can grout?

The maximum allowable gap or joint width that you can successfully grout depends heavily on whether the product is sanded or unsanded grout. Unsanded grout contains extremely fine particles and is designed strictly for narrow joints measuring one-eighth of an inch or less; if used in wider gaps, it will severely shrink, crack, and slump out of place as it dries. Sanded grout, which includes graded silica sand aggregate, is formulated for wider joints ranging from one-eighth of an inch up to about one-half of an inch, providing the structural integrity needed to prevent major shrinkage cracking. For exceptionally wide gaps, stone joints, or heavy-duty masonry applications exceeding one-half inch, specialized high-aggregate grouts or mortar mixes must be utilized to maintain structural stability.