Cut precise holes in cement board for toilet flanges and water lines by measuring twice, marking clearly, and cutting with the right tools to avoid damage and gaps.

Start by taking exact measurements from fixed reference points like the finished wall and center of the flange or water stub. Transfer those measurements to the cement board and mark both the hole outline and a relief line about one quarter inch larger for fitting and tile allowances. Use a straight edge and a pencil to score long lines, then switch to an oscillating multi tool fitted with a diamond or carbide blade for controlled plunge cuts around curves and corners. Support the board with two by fours placed under the cut area to prevent flex and to keep the blade from nicking underlying substrate or adjacent boards. For small round holes for water lines consider a diamond hole saw on a rotary drill, drilling from the top side slowly to avoid chipping, and for very thin edge trimming a utility knife with multiple passes can clean the perimeter after the main cut. Always wear a dust mask and eye protection, seal cut edges with thinset or bituminous tape if required by tile manufacturer, and dry fit the flange or fixture before final tile to confirm alignment.

Common mistakes include overcutting the hole which leads to visible gaps and weak tile edges, so measure carefully and cut conservatively leaving a small allowance for adjustment that you can trim away. Another frequent issue is cutting without supporting the board which causes breakouts on the underside; use temporary two by fours under the cut area and cut with slow steady pressure to prevent splintering. People also forget to protect adjacent flooring or underlayment from dust and debris which can complicate plumbing; lay down a drop cloth and vacuum frequently while you work and clean the hole edges before installing the flange or supply escutcheon.

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