A drywall manufacturer reveals the questions most frequently on the minds of its products' users.

Drywallers in a pinch can call National Gypsum Co.’s toll-free technical-services hotline for help about specifications, code regulations, product usage, installation and more. Following are the questions its three experts hear the most and the answers they provide.

Q. My home was in a flood and the wallboard in my basement was exposed to floodwater. Do I have to replace the wallboard in my basement?

A. Yes, wallboard that has been soaked by floodwater can be a permanent health hazard. When the wallboard finally dries, there will still be contaminates dried inside the core and to the wallboard paper facings.

Q. My fire-rated wood-framed design was tested with nails. Can I fasten the wallboard with screws as an alternative to nails?

A. Yes, screws can be used as an alternative to nails in fire-rated designs provided they are substituted, one for one, and the head diameter, length and spacing are equal to or exceed the requirements for the specified nails.

Q. What is the recommended temperature for taping and finishing wallboard?

A. For taping and finishing wallboard, the room temperature should be maintained at no less than 50 degrees F for 48 hours prior to application and continuously thereafter until completely dry.

Q. When constructing a fire-rated wallboard partition, must the exact stud size specified in the tested design be utilized?

A. No, specified wood or steel stud sizes are minimum unless otherwise stated in the design. Wood-stud walls may contain studs treated with fire retardant, as well as untreated wood studs. The hourly ratings for steel-stud partitions are maintained when the steel studs are of a heavier gauge and/or larger dimensions than specified in a design.

Q. Can I apply moisture-resistant gypsum wallboard (green board) to a ceiling?

A. Yes, moisture-resistant gypsum wallboard may be applied to interior ceilings provided framing does not exceed 12 inches o.c. for 1/2-inch-thick board and 16 inches o.c. for 5/8-inch-thick board.

Q. May glass fiber insulation be added to a fire-rated partition design when it is not specified as a component?

A. Yes, glass fiber insulation of a thickness not exceeding the depth of the stud is permitted to be added to the stud cavity.

Q. Is gypsum wallboard a suitable base for the direct application of gypsum plaster?

A. No, gypsum plaster will not properly bond to wallboard face or back paper. Gypsum lath or gypsum base for veneer plaster (blue board) must be utilized when a board product is specified. Gypsum plaster base products have a special absorptive face paper designed to permit a strong plaster bond.

Q. What is the fire rating for 5/8-inch Type X wallboard?

A. Five-eighths-inch Type X wallboard by itself has no fire rating. Fire ratings are assigned to a complete construction assembly (design) that may include 5/8-inch Type X wallboard as a component.

Q. When should a level 5 gypsum board finish be specified?

A. This level of finish is highly recommended where gloss, semi-gloss or enamel paints are specified or where severe lighting conditions occur. This highest quality finish is the most effective method to provide a uniform surface and minimize the possibility of joint photographing and fasteners showing through the final decoration.

Q. What is the proper fastener length when applying wallboard to wood framing?

A. Nail penetration into the framing member should be 7/8 inch for smooth shank nails and only 3/4 inch for annular ringed nails, which provide more withdrawal resistance. For fire-rated assemblies, greater penetration may be required. Type W wallboard screws should penetrate wood framing a minimum of 5/8 inch.

Q. Is Ready-Mix joint compound permitted for use finishing joints on exterior soffits?

A. No, a setting (hardening) type joint compound is recommended. Setting joint compounds are not affected by humidity once set and dried, unlike a drying-type Ready-Mix joint compound, which will become soft with prolonged exposure to high humidity.

Q. Do base layers in fire-rated multi-layer wallboard designs require joints be taped?

A. No, base layers in multi-layer systems are not required to have joints or fastener heads taped or covered with joint compound.