First Decide: Door Slab or Complete Prehung Unit?

A slab-only replacement reuses the existing frame and requires extremely accurate hinge, bore, bevel, and lock-preparation information. A prehung unit includes the slab mounted in a new frame and is generally the better choice when the jamb, sill, weatherseals, alignment, or water management is compromised.

Do not interchange slab size, frame size, unit size, and rough-opening size. They describe different parts of the opening. Writing “36 × 80 door” without labeling the measurement is not a complete order.

Tools to Bring to the Measure

• Quality tape measure and a rigid rule for jamb depth
• Long level, shorter level, and framing square
• Flashlight and inspection mirror
• Camera or phone for labeled photographs
• Moisture meter when deterioration is suspected
• Awl or probe used carefully on questionable wood
• Manufacturer order sheet or a consistent field-measure form

Record every dimension immediately and label what it represents. Never depend on memory, and never assume the left side matches the right.

1. Measure the Existing Door Slab

With the door open, measure slab width across the face and slab height from the top edge to the bottom edge. Measure thickness at an exposed edge. Take more than one width and height reading if the slab appears altered, damaged, or out of square.

For slab replacement, also document the hinge count, hinge size, corner shape, hinge locations from a consistent reference point, lock-bore diameter, backset, center-to-center spacing for handlesets, deadbolt preparation, door bevel, and any multi-point hardware. A nominally matching slab can still require extensive field work if these details differ.

2. Measure the Existing Frame or Unit

Remove only the trim necessary to expose reliable frame edges. Measure the outside width of the jamb from several locations and the outside height from the unit’s intended reference points. Document whether dimensions include a sill, subsill, brickmould, exterior casing, or another applied component.

The smallest clear condition often controls, but do not simply order to the smallest visible trim dimension. Identify what will remain, what will be removed, and what the manufacturer defines as unit size. Contractors can review Therma-Tru Fiber-Classic door styles and sizes while developing the replacement specification.

3. Verify the Rough Opening

The rough opening is the structural opening between framing members—not the visible opening between interior casing or exterior brickmould. When feasible, expose representative points behind the trim and measure width at the top, middle, and bottom. Measure height at the left, center, and right.

Check the opening for square by comparing diagonals and evaluate the framing for plumb, level, bow, and twist. Record the smallest dimension and the variation, then compare the complete information with the current door manufacturer’s required rough-opening clearance. Do not rely on a universal allowance; system requirements and field conditions vary.

4. Measure Jamb Depth From Finished Surface to Finished Surface

Jamb depth must coordinate with the wall thickness, sheathing, exterior finish, interior drywall or plaster, and any extension jamb strategy. Measure at several locations because older Quad Cities homes may contain plaster, added wall layers, replacement siding, or uneven masonry returns.

A jamb that is too narrow leaves awkward extension details. One that is too deep can project beyond the wall and complicate casing. Precise depth supports clean interior trim lines and an exterior that does not look improvised.

5. Record Door Handing and Swing Without Ambiguity

Handing terminology can vary by manufacturer, product category, and whether it is viewed from the exterior or hinge side. Avoid relying on “left” or “right” alone. Document all of the following:

• Which side contains the hinges when viewed from the exterior
• Whether the door swings inward or outward
• Which way the slab travels when opened
• Hardware location and active panel on double-door systems
• Clearance from stairs, walls, switches, furniture, and storm doors

Add photographs with arrows and confirm the definition used on the final manufacturer order form. The safest ordering process makes the intended movement unmistakable.

6. Inspect the Sill and Floor Elevations

Measure from the subfloor or finished floor as required by the planned installation, and document flooring thickness on both sides. Check the sill’s slope, support, projection, and relationship to porches, decks, steps, and landings. Confirm that the new sweep will clear rugs and finished flooring while still sealing against the threshold.

Soft wood, staining, corrosion, or a sill that moves under pressure signals a substrate or water-management problem. Do not size a new unit around deteriorated material that should be removed. Compatible Therma-Tru components and accessories can help complete the system, but sound support and correct installation remain essential.

7. Measure Brickmould, Exterior Trim, and Masonry Clearance

Measure the outside dimensions of existing brickmould or casing and the available opening between brick, stone, siding, or other fixed finishes. Record trim width, projection, setbacks, head flashing, and unusual returns. Determine whether new trim will cover old paint lines and whether siding or masonry repairs will be required.

The opening’s appearance deserves the same discipline as its fit. Centered sightlines, balanced trim widths, clean sealant joints, and deliberate transitions make a replacement look original to the home rather than inserted into it.

8. Treat Sidelites, Transoms, and Double Doors as Complete Assemblies

For an entry with sidelites or a transom, measure the overall frame, each component, mull locations, glass sightlines, rough opening, and fixed exterior constraints. Identify whether the sidelites are full-height, whether the transom is structurally separate, and how the system is supported.

Do not assume the existing proportions are standard. Review Therma-Tru sidelites and transoms when coordinating a new assembly, and verify every dimension against the selected configuration before ordering.

9. Document Hardware, Glass, Finish, and Performance Requirements

Dimensions alone do not create an order-ready door. Record bore preparation, backset, handleset spacing, multi-point locks, hinge and sill finish, glass design, privacy needs, grille pattern, paint or stain intent, exposure, and any required ratings. If the homeowner wants a smooth painted finish, compare the Therma-Tru Smooth-Star collection.

Verify current product limitations and code requirements for the specific project. Egress, safety glazing, fire ratings, accessibility, wind exposure, and local approval requirements can affect the final specification.

Common Exterior Door Measuring Mistakes

• Ordering from the slab dimension when a prehung unit is required
• Measuring casing instead of the structural rough opening
• Taking only one width and one height measurement
• Assuming a visibly square opening is structurally square
• Using “left-hand” without defining the viewing side and swing
• Ignoring jamb depth, flooring, brickmould, or masonry restrictions
• Failing to inspect rot hidden beneath trim or at the sill
• Ordering decorative glass before confirming sightlines and privacy
• Depending on nominal sizes instead of current manufacturer documents

Frequently Asked Questions