# Warped Mull Post? A Shim Repair Before Replacement > When the mull between a sidelight and an entry door warps, the door may not latch. How UTS ServicePros recommended a shim repair in lieu of replacing the unit. Source: https://utsservicepros.com/blog/warped-mull-post-sidelight-door-shim-repair --- Technical Guide 6 min read # Door Will Not Latch Beside Its Sidelight? Shimming a Warped Mull Instead of Replacing the Unit A manufacturer asked UTS ServicePros what to do about an entry door that would not seat into its lock mortise and latch because the mull post between the door and its sidelight had warped. Instead of replacing the whole unit, Warranty Service Manager Stephen Somers recommended a repair that works and is the proper process: remove the interior quarter round on the sidelight edge touching the mull, then shim between the sidelight panel and the mull post to move the mull back at the center, where the latch plate meets the handle set. The result is a door that latches, with reveals corrected top to bottom in the 1/8-inch range. One caution comes with it: if the mull keeps warping, the shims leave little room to adjust further. Stephen Somers Warranty Service Manager, UTS ServicePros ssomers@utsservicepros.com972-633-6806 Published October 1, 2026 Updated October 1, 2026 Recommended repair in lieu of replacement: shim between the sidelight panel and the mull post to correct a warped mull. UTS ServicePros diagram; the full-size version is below. Share this article: ## The Call: A Door That Will Not Latch Beside Its Sidelight A manufacturer reached out to UTS ServicePros about an entry door with a sidelight. The mull post between the sidelight and the door had warped, and the door panel was not seating into the lock mortise and latching. The choice was to replace the whole unit or to repair it by bringing the mull back into line. UTS ServicePros advises manufacturers on calls like this regularly, and here it recommended a repair in lieu of replacement, laid out step by step in the diagram below. ## What the Mull Post Does A sidelight is a companion product installed on one or both sides of a door. In the window and door industry’s own glossary, published by the Window & Door Manufacturers Association, a sidelite is “an operable or non-operable product that is designed to be a companion product installed on one or both sides of an operable door or a fixed door.” The mull post, or mullion, is the member where the two units meet. Pella’s guide to the parts of a door puts it simply: “A mullion, sometimes referred to as a mull, is where two units are joined together.” On a door like this one, the mull post sits between the sidelight and the latch edge of the door, and it carries the latch plate. Manufacturers anchor the mull with the latch in mind. JELD-WEN’s installation instructions for its custom wood and fiberglass door systems anchor the mull post to the sidelite with screws 16 inches on center “and under the lock strike location,” and have the installer inspect the unit for “square, level, plumb, twist and even door reveal throughout the fastening process.” ## Why a Bow in the Mull Stops the Door From Latching In this case the mull post had bowed toward the sidelight at its center. That opens the gap between the mull and the door edge at exactly the height of the latch plate and the door handle set mortise, while the top and bottom stay closer to where they should be. Manufacturers set the gap around a door to a narrow, even range. ProVia’s installation manual for single entry doors with sidelites calls for “a 1/8″ - 3/16″ margin at all corners of the door slab,” including the margin between the door slab and the mull post. Therma-Tru’s installation instructions have the installer make adjustments “so the margins between the door and frame are even top to bottom,” and its instructions for doors with venting sidelites say to adjust the lock side mull post so the weatherstrip contacts the door surface “equally at the top, middle, and bottom.” ## The Repair The repair, in lieu of a replacement of the unit, is to remove the quarter round on the interior, along the edge of the sidelight that is directly touching the warped mull. Once that quarter round is removed, shims go between the sidelight panel and the mull post. They apply enough pressure to the mull to move it over at the center, where the latch plate meets the door handle set mortise. That provides enough clearance for the door panel to seat into the lock mortise and latch, and corrects the reveals from top to bottom in the 1/8-inch range. The repair works, and it is the proper process for a mull that has warped this way. The repair, step by step, viewed from inside. Not to scale: the bow in the mull and the reveals are exaggerated for clarity. UTS ServicePros diagram. ## How It Lines Up With Manufacturer Guidance No manufacturer document we found describes this exact field repair. The principle behind it, shimming at the center where the margin is wide, does appear in their own troubleshooting. ProVia’s troubleshooting table for units with a sidelite on each side lists a “strike-side margin is too wide only near the strike (center)” and gives the remedy: loosen or remove the sidelite screws to shim as needed in that location to achieve proper margin, and “shim tighter in the center of the strike sidelite.” Therma-Tru’s instructions make the same point about where shims belong on the latch side: “Close the door and carefully shim between the jamb and the opening behind the adjustable strike plate area.” ## The Caution This repair comes with one concern: if the mull continues to warp. If the shims go in at this location and the mull warps further, the shims will not allow much room to adjust any further and could cause the panel to run at the latch. ## Why Recommend a Repair Before a Replacement Replacing the unit is the other option, and it is a bigger job than it sounds. Every job is different, but any interior trim in the way has to be removed before a new unit goes in, and a full door unit is a heavier, longer job than a shim repair. UTS ServicePros is the manufacturer’s arm for due diligence. When a repair restores proper operation and alignment without replacing the unit, the manufacturer should see that option, with its limits stated plainly, before deciding. Start a manufacturer consultation to set up door inspections and service through UTS ServicePros. ## Technical references - [The Window Glossary](https://www.wdma.com/the-window-glossary) — Window & Door Manufacturers Association (WDMA). Definition of a sidelite as a companion product installed beside a door. - [Parts of a Door](https://www.pella.com/ideas/doors/entry-doors/parts-of-a-door/) — Pella Corporation. A mullion, or mull, is where two units are joined together. - [Installation Instructions for Custom Wood and Fiberglass Door Systems with Hardwood Frames (JII-90072)](https://cmd-jeld-wen.s3.us-east-2.amazonaws.com/assets/documents/4142175481.pdf) — JELD-WEN. The mull post is anchored to the sidelite, including under the lock strike location; inspect for even door reveal. - [Installation Instruction Manual: Single Entry Door with Sidelite(s), FrameSaver (Version 3.2, September 22, 2026)](https://entrylink.provia.com/doc.aspx?id=910) — ProVia. A 1/8- to 3/16-inch margin at all corners, including slab to mull post; troubleshooting for a strike-side margin too wide only at the center. - [Installation Instructions for Pre-hung Door Systems](https://www.thermatru.com/globalassets/customer-support/how-to-center/documents/installationinstruction.pdf) — Therma-Tru Doors. Shim behind the adjustable strike plate area; margins even top to bottom. - [Installation Instructions for Single Door with 2 Venting Sidelites Systems](https://www.thermatru.com/globalassets/customer-support/how-to-center/documents/ventedsideliteinstallation.pdf) — Therma-Tru Doors. Adjust the lock side mull post so the weatherstrip contacts the door equally at the top, middle and bottom. Table of Contents ## Frequently Asked Questions Common questions about warped mull posts on doors with sidelights. What is a mull post? The mull post, or mullion, is the member where two units are joined together. On an entry door with a sidelight, it sits between the sidelight and the latch edge of the door and carries the latch plate. Why does a warped mull stop a door from latching? When the mull bows toward the sidelight at its center, the gap between the mull and the door edge opens at the height of the latch plate and the handle set mortise, so the door panel does not seat into the lock mortise and latch. Can a warped mull be corrected without replacing the unit? Yes. Remove the interior quarter round on the sidelight edge touching the mull, then shim between the sidelight panel and the mull post to move the mull over at the center. The door latches again and the reveals are corrected from top to bottom in the 1/8-inch range. Watch for one risk: if the mull continues to warp, the shims leave little room to adjust further. What is the risk with a shim repair? If the mull continues to warp, the shims will not allow much room to adjust any further and could cause the panel to run at the latch. What reveal should an entry door have? Manufacturers set a narrow, even range. ProVia’s installation manual calls for a 1/8- to 3/16-inch margin at all corners of the door slab, and Therma-Tru’s instructions call for margins that are even top to bottom. ## Related Articles Technical Guide ### [Door Warranty Problems: Check the Frame, Sill and Hinges Before Replacing the Door](https://utsservicepros.com/blog/door-warranty-disasters-frame-warping-seal-failure-hardware-breakdowns) A door that will not seal, latch or keep water out puts the manufacturer’s name in front of an unhappy homeowner every day. Many of those calls do not trace to a defective door: a slab that looks warped usually hangs in jambs that are out of plane, and door makers’ own troubleshooting guides trace many leak, sag and latch problems to how the frame was set and sealed. Getting the diagnosis right on the first visit is what protects the brand. Technical Guide ### [Foggy or Cloudy Window Glass? Seal Failure, Low-E Coating Failure and How the Glass Gets Replaced](https://utsservicepros.com/blog/foggy-window-glass-seal-failure-low-e-coating-replacement) Fogging, cloudiness or white speckling between the panes of a window means the insulating glass seal has failed. Moisture gets into the sealed space between the panes and, over time, damages the Low-E coating on the inner glass surfaces. It cannot be cleaned or wiped away. The repair is to replace the insulating glass unit in the existing sash or frame, not the whole window. UTS ServicePros does that field glass replacement across the lower 48 states and western Canada two ways: dispatched by the manufacturer when the glass is under warranty, or quoted directly to the property owner when the labor, or the whole repair, is not covered. Technical Guide ### [Does Your Entry Door Look Warped? Usually the Door Is Straight and the Jambs Are Not](https://utsservicepros.com/blog/entry-door-looks-warped-jambs-out-of-plane) The most common entry door complaint UTS ServicePros inspects is a “warped slab”: the door is closed and latched, but the top corner on the latch side sticks out past the frame and does not seal against the weatherstrip. Most of the time the door is straight. The jambs were not set on the same plane in the rough opening, with either the hinge jamb leaning in at the top or the latch jamb leaning out at the top. The only repair is to adjust the installation, and the manufacturer usually does not cover it: the original installer is required to go back and set the jambs true and plumb. ## Deliver the Manufacturer's Promise Nationwide UTS ServicePros provides the nationwide network, documentation discipline, and liability protection that transforms warranty service from a cost center into a competitive advantage. 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