The difference in one table

What is a door slab?

A slab is the swinging door panel without a new frame assembly. It may be ordered blank or with selected hinge and lock machining, depending on the supplier and product. The existing jambs, sill, stops, weatherstrip, hinges, casing, and flashing remain unless separately repaired or replaced.

A slab can work well when a damaged or dated door is being replaced inside a high-quality frame that is still square, plumb, dry, and correctly sized. It is not a universal shortcut. Field-fitting an exterior slab requires precise measurement and experienced carpentry.

What is a prehung exterior door?

A prehung door arrives with the slab mounted in a new frame. Depending on the ordered system, it may include hinges, sill, weatherstrip, corner seals, brickmould, jamb options, and other components. Therma-Tru emphasizes the performance of a complete door system using coordinated parts.

Contractors can review available Therma-Tru door components and accessories to understand why the frame, sill, weatherstrip, hinges, and seals matter alongside the slab itself.

Prehung does not mean automatically correct. The unit must still match the rough opening, wall thickness, swing, exposure, sill condition, flooring height, exterior trim, and applicable installation requirements.

When a slab replacement makes sense

• The existing frame is square, plumb, level, and securely attached.
• The jambs, sill, threshold, and sub-sill show no rot or water damage.
• The weatherstrip and stops can provide a continuous seal.
• The new slab is available in the required size, thickness, bevel, and handing.
• Hinge locations, hinge size, backset, bore, and latch preparation can be matched accurately.
• The existing rough opening and frame are not causing the current door problem.
• The contractor is prepared to fit, machine, finish, and adjust the slab professionally.

A slab is often most defensible when replacing a locally damaged panel in a relatively recent, documented door system. It becomes less attractive when the frame’s history and dimensions are unknown.

When a prehung door is the better investment

• The frame is rotted, split, twisted, or out of square.
• Daylight, drafts, or water appear around multiple sides of the opening.
• The sill, sweep, weatherstrip, or corner seals are worn or incompatible.
• The homeowner wants a different door width, height, swing, jamb depth, or configuration.
• Sidelites, a transom, multi-point hardware, or a coordinated entry system are being added.
• The original installation or rough-opening flashing needs correction.
• The contractor wants factory-controlled slab-to-frame alignment as the starting point.

For a larger entrance, explore Therma-Tru sidelites and transoms as part of the complete design rather than trying to treat each element as an unrelated field modification.

Slab-only measurements that must match

Nominal width and height are not enough. Two doors described as 36 by 80 inches can differ in actual dimensions, thickness, edge bevel, hinge preparation, and lock machining. Record the existing slab and frame before ordering.

1. Measure actual slab width, height, and thickness at several points.
2. Identify inswing or outswing and viewed-from-which-side handing terminology.
3. Measure every hinge size and its location from one consistent reference edge.
4. Record hinge corner radius or square-corner preparation.
5. Measure bore diameter, backset, centerline height, and deadbolt spacing.
6. Confirm latch-edge bevel and the door’s relationship to the stop and weatherstrip.
7. Check top, side, bottom, and sill clearances rather than copying only the old slab.

Do not assume the old door was fitted correctly. Measuring a defective condition perfectly only reproduces it.

Prehung measurements and specifications

A prehung order begins with the complete opening, not the old slab. The contractor should determine rough-opening width and height, wall thickness, jamb depth, exterior cladding, interior finish, sub-sill condition, finished-floor elevation, and available clearances.

• Unit size and required rough opening
• Door swing and hinge side
• Jamb material, width, finish, and extension requirements
• Sill type, finish, depth, and accessibility considerations
• Brickmould or exterior casing configuration
• Lock bore, deadbolt, multi-point hardware, and backset
• Door style, glass, grids, privacy, and Low-E options
• Exposure, overhang, storm-door conditions, and finishing requirements

Why the existing frame condition controls the decision

A frame can look acceptable beneath paint or aluminum capping while the lower jambs and sub-sill are deteriorated. Probe suspicious areas, inspect staining and fasteners, check sill support, and look for previous sealant repairs. If the frame moves, the slab will move with it.

A new fiberglass slab does not straighten a twisted jamb, rebuild a soft sub-sill, or reconnect flashing to the wall’s drainage plane. The contractor must separate the visible door problem from the opening problem.

Weather performance depends on the complete assembly

Homeowners often describe a slab as “energy efficient,” but the installed opening controls performance. Glass package, slab construction, weatherstrip compression, sweep contact, sill adjustment, corner seals, frame insulation, flashing, and lock engagement all contribute.

A prehung system provides coordinated components, but workmanship remains decisive. The unit must be installed square, plumb, level, securely fastened, properly insulated, flashed, sealed, and adjusted according to current instructions. A premium assembly installed into a poor rough opening can still leak air or water.

Aesthetics: slab replacement can expose old proportions

A beautiful new slab inside narrow, damaged, or dated trim can make the surrounding frame look worse. Evaluate the complete entrance from the street: door panel, glass, jambs, brickmould, sidelites, transom, storm door, hardware, lighting, house numbers, steps, and siding transitions.

For a wood-grain appearance, compare Therma-Tru Fiber-Classic fiberglass doors. For a smooth painted finish, review Therma-Tru Smooth-Star doors. The collection decision should support the home’s architecture, not merely fit the opening.

Cost comparison: include all labor and missing components

A slab usually has a lower initial material price, but the contractor may need to machine hinges, bore hardware, bevel edges, trim height, replace weatherstrip, repair the sill, refinish the frame, and adjust the stops. Those tasks require time and carry fit risk.

A prehung unit costs more and requires removal of trim and the existing frame, but it may reduce the amount of custom fitting needed and allows hidden opening conditions to be addressed. Compare the finished scope, warranty responsibilities, callback risk, disposal, trim work, paint or stain, and customer disruption.

Common ordering mistakes

• Ordering from nominal slab size without measuring actual dimensions.
• Using ambiguous left-hand or right-hand language without a swing diagram.
• Copying hinge locations from different reference points.
• Ignoring slab bevel, thickness, or hinge corner preparation.
• Ordering a prehung unit without confirming jamb depth and finished floor height.
• Assuming brickmould fits every siding, brick, stone, or trim condition.
• Failing to include sidelites, mullions, transoms, or hardware in overall dimensions.
• Promising performance before the frame and rough opening are inspected.

Frequently asked questions