Before You Buy a Door: A Professional Guide for Builders, Contractors & Designers

This guide explains the essential steps builders, contractors, and designers should take before ordering interior or exterior doors— including how to identify door handing, measure rough openings, size door slabs, determine jamb depth, and select the correct active leaf for double-door units. It also highlights key differences between interior and exterior measurements and offers practical tips to ensure accurate installs, fewer delays, and smoother project coordination across residential and commercial jobsites.

Tri-Supply has been supplying doors for Texans for more than 60 years. Whether you're planning a residential build, coordinating selections for a commercial project, or managing door packages across multiple job sites, our team is here to support you. Before you choose from our wide range of interior and exterior door options, there are a few essentials every construction professional should confirm. Getting accurate door measurements and proper handing upfront ensures smoother installs, fewer delays, and more efficient project planning.

Our experts are always ready to help over the phone or at any Tri-Supply location! We’ve also pulled together this professional door selection guide to help you prepare for ordering, staging, and installing new interior and exterior doors.

How Do I Determine the Correct Door Handing or Door Swing?

Correct door swing identification is one of the most important steps in construction project door planning. Right-hand, left-hand, right-hand active, left-hand active, inside opening, outside opening… it can be confusing! These details can affect hardware selection, ADA clearances, traffic flow, and even fire code compliance.

Fortunately, we can help simplify. There are two reliable ways to get the handing correct.

Method A: Stand Where the Door Opens Toward You

Imagine standing on the side of the door where you pull it toward you to open it. The side the knob is on determines the “hand” of the door.

• Knob on the right? Right-hand

• Knob on the left? Left-hand

Method B: Stand Inside the Frame

Stand inside the frame with your back against the hinge side. Swing your arm in the same direction the door opens. Whatever arm you use is the “hand” or “swing” of the door.

These methods will work whether the door opens inside or outside. Don’t let double or “french” doors confuse you. For those, handing is determined by the active door, meaning the door used most often.

How Do I Measure the Rough Opening for a Pre-Hung Door?

Getting an accurate rough opening measurements for doors is essential to keeping your project on schedule. A correct rough opening helps your team avoid onsite modifications, framing adjustments, and last-minute hardware changes.

Full Pre-Hung Unit

For a pre-hung unit that includes the door slab, frame, and/or trim, measure the height and width of your entire door opening, stud to stud, floor to header – called the rough opening. To measure accurately in an existing , you might have to remove some interior casing (moulding) to see the backside of the door frame.

Removing small sections of casing (interior moulding) may be necessary to see the full framing and get accurate door frame measurements.

Why This Matters for Pros

The accuracy of your rough opening measurements directly affect:

Door installation prep

Trim and casing fit

Jamb depth alignment

Lead time on custom-sized units

Punch-list and warranty performance

This step is critical whether you're selecting a single pre-hung unit or coordinating multi-unit door packages for an entire development.

How Do I Properly Measure a Door Slab for Replacement?

If you're replacing only the door slab, you’ll need to document the slab’s exact width, height, and thickness, as well as the size, placement, and spacing of all hinges. It’s also important to note the bore locations for latches and deadbolts and identify the bevel direction if the door has one.

Taking the time to capture these details ensures the new slab will fit correctly within the existing frame and align with current hardware. This is extra important in remodels, tenant improvements, or multi-phase residential upgrades where consistency matters.

How Do I Choose the Correct Jamb Size for My Wall Thickness?

Your jamb size is based on the stud width plus the finished wall material. Basically what you need here is the stud width plus the thickness of the finished wall. The thickness differs between interior and exterior walls, and even between units in different phases of the same job.

Interior Walls

Typically align with standard stud widths and drywall.

Exterior Walls

May include sheathing, insulation, brick, stone, or siding thickness.

Correct jamb sizing ensures a clean fit during trim installation and prevents gaps or overhangs.

What’s the Difference Between Measuring Interior vs. Exterior Doors?

Interior and exterior doors vary significantly in both construction and installation requirements. This affects how each one should be measured. Interior doors typically use standard jamb sizes, lighter-weight slabs, and simple bore patterns. They require very little in terms of weatherproofing.

On the other hand, exterior doors involve more complex considerations. Measurements must account for threshold height, precise jamb depth to accommodate multi-layer wall construction, and proper alignment of weatherstripping and the sill. Exterior units also require more accurate rough opening allowances because they include additional components such as the frame, sill, and brickmould. Taking careful measurements for exterior doors is especially important, as even small inaccuracies can lead to installation delays.

How Do I Identify the Active Door on Double or French Door Unit?

In double- or French-door systems, the active door is the one equipped with an operable latch. The inactive leaf typically uses flush bolts or surface bolts to hold it in place.

To identify the active leaf:

Look for the latch, not the hinges

Note which side handles daily traffic flow

Confirm ADA or design requirements for clearances

This matters for handing, hardware, and ensuring correct pre-hung unit fabrication.

Where Can Builders and Contractors Get Expert Door Support in Texas?

Tri-Supply is proud to support builders, contractors, and designers across Texas with professional door solutions for projects of every size. Whether you’re selecting a single custom entry door or coordinating an entire package of interior and exterior units for multiple job sites, our team delivers commercial and residential door options.

We provide accurate measurement guidance, jobsite-ready preparation support, and designer-focused product expertise backed by decades of local experience in Texas markets. Visit or contact your nearest Tri-Supply location to review measurements, explore options, or get pricing on the solutions that best fit your build.