"Understanding Your Home" by Building Inspector Mark Visser
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Slab door. Flush door.
Holow & solid-core

Common widths are 24, 30, 32, and 36 inches. The standard height is 80", which can be trimmed if necessary. Make sure you trim from the bottom of the door, not the top!
Rough Stud Opening (RSO) for slab doors: Two inches (50 mm) wider than the door itself and two inches (50 mm) higher than the actual door. Most interior doors are 6'8" high (80") so the height for the RSO would be 82"

Slab door. Flush door.
Holow & solid-core

Slab Doors (Flush Doors)

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Slab door. Flush door. Hollow-core and solid-core door

Slab doors, also known as flush doors, are used for sliding closet doors, pocket doors and for hinged doors leading to rooms throughout the home. Hollow-core interior doors are 1 3/8" (33 mm) thick while solid-core doors are mainly used for exterior doors as required by building and fire codes. Solid-core outside doors are 1 3/4" (44 mm) thick. Interior hollow-core doors usually have a wood grain texture, are factory primed and ready for painting. The doors do not have pre-drilled holes for door handles or mortised for hinges.
Hollow-core mahogany or masonite slab doors have been around for ages but today's door of choice is the popular hollow-core, two to six panel molded composite slab door. They are made of an outer wood frame and a molded Masonite ( hardboard) skin on front and back. The name hollow-core doesn't mean that the door is 100% hollow. The inside of older slab doors consists of glued in folded cardboard or thin wood slats which gives more strength to the door skins and provide an even door thickness. Newer hollow-core flush doors are filled with a high density fiberboard
A solid block of wood is installed within a hollow-core door to accommodate the installation of the door's hardware.
Solid-core doors were either solid wood or filled entirely with a low-density particle board. Solid-core flush doors (especially foam-core ones) are commonly used as exterior doors because they provide more insulation and strength.

Molded two panel hollow-core masonite
door with wood grain texture finish

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