An insulated overhead door that rubs the header on a sunny afternoon and seals fine by evening isn't broken. It's bowing with temperature. Here is what causes it, why Lower Mainland buildings see it both summer and winter, and what actually reduces it.
We get two versions of the same call. In summer: “the top of the door scrapes the header after lunch.” In winter: “there’s daylight at the top of the door in the mornings.” Both are usually the same thing: overhead door thermal bowing. The Door & Access Systems Manufacturers Association (DASMA) covers it in Technical Data Sheet #185, Thermal Bowing of Garage Doors with Bonded Core Sections.
What overhead door thermal bowing is
Most insulated sectional doors are sandwich panels: a steel face on each side bonded to a foam core. DASMA describes thermal bowing as an inherent characteristic of doors with insulated bonded-core sections. It is not considered a product defect.
The mechanism is simple. When one face is much warmer than the other, the warm skin expands relative to the cool one and the door bows toward the warmer side. DASMA says all insulated panels will show some bowing when there is a temperature difference from one side to the other. The insulation keeps the two faces at different temperatures, which is why it happens.
Outward in summer, inward in winter
Most articles only cover the hot-sun case. TDS #185 points out that bowing isn’t only outward: in a cold winter, a heated building can make a door bow inward.
| Situation | Warmer face | Bow direction | Symptom DASMA lists |
|---|---|---|---|
| Sun on the outside face, cooled or shaded interior | Exterior | Outward | Top section rubs the top of the opening (header) |
| Cold day outside, heated shop or warehouse | Interior | Inward | Gap between the top section and the top of the opening |
In the Lower Mainland the winter case is common in heated auto shops, fleet bays and food facilities. The gap at the top lets heat out and rain in, and people often blame the weather seal first. Our weather seal service page covers real seal failure. Check for bowing before replacing seals that are fine.
What makes one door bow more than another
TDS #185 lists the factors. Here they are with what each means when you’re specifying or troubleshooting a commercial door:
- Colour. DASMA’s example: a brown insulated door facing the sun on a hot day, with an air-conditioned building behind it, bows more than the same door in white. Dark brand colours on a sunny elevation are a known risk.
- Door size. The larger the door, the more potential bow. Without reinforcement, DASMA says the bow on a 20 ft wide door could exceed several inches. Commercial doors are often wider than that.
- Orientation. A south-facing door in direct sun is more likely to bow than a north-facing door out of direct sun.
- Thermal breaks. Doors that are “thermally broken” slow heat transfer and so can bow more. Better insulation can mean more bow, which surprises buyers. Our insulated commercial doors page explains the trade-offs, and what R-value numbers really mean.
- Facing material. DASMA notes aluminum and plastics expand more with heat than steel.
Thermal bow vs sag vs damage
Not every bowed door is thermal. A quick way to separate them:
- Thermal bow comes and goes with temperature and sun. The door is flatter in the morning or at night, or once inside and outside temperatures even out.
- Sag on a wide door stays there all the time. It usually means a missing, loose or undersized strut.
- Impact or wind damage stays put and usually shows creases, cracked stiles or a hinge pulled out. That’s a panel or section replacement question, not a thermal one.
How thermal bowing is reduced
DASMA is clear that when these conditions are present, thermal bowing can’t be eliminated. It can be minimized to the point where it has no appreciable effect on how the door works. The sheet names these approaches:
- Reinforcement. Typically horizontal struts across the back of the door. On wide commercial doors this is the main fix, and it also helps with wind load. See our note on wind load ratings.
- Track adjustment. Depending on the door system, the track can sometimes be set to give an outward-bowing top section more room at the header.
- Header and jamb seal adjustment. An adjustable seal can be set to close an inward-bow gap without jamming the door in summer.
- Manufacturer guidance. DASMA says to contact the door manufacturer for its recommendations, since what works depends on the door system.
TDS #185 also says that, because of the dangers involved, repairs and adjustments must be done by a trained door systems technician with proper tools and instructions. Track and seal changes on a sprung commercial door aren’t a maintenance-staff job. Our roller and track repair page covers that work.
If you’re buying a new insulated door
You can’t remove overhead door thermal bowing, but you can plan for it:
- On sunny elevations, weigh dark finishes against a lighter colour, or plan for extra struts.
- Ask what reinforcement comes standard at your door width, and what the manufacturer recommends for high-sun or heated-interior applications.
- If the space behind is heated or cooled, mention it at quote time. That temperature difference is what drives the bow.
- Make sure header and jamb seals are adjustable.
DASMA’s framing that bowing is not a defect matters if you’re thinking of a warranty claim. Get the door assessed first, so the conversation with the supplier is about the right problem.
Questions property managers ask us
Is thermal bowing a defect in my insulated overhead door?
No. DASMA TDS #185 describes thermal bowing as an inherent characteristic of doors with insulated bonded-core sections and says it is not considered a product defect. It can be minimized but not eliminated.
Why does my door rub the header on sunny afternoons?
When the outside face is much warmer than the inside face, the warm skin expands and the door bows outward. DASMA lists the top section rubbing the header as a typical field issue of outward thermal bow.
Why is there a gap at the top of the door in winter?
In a heated building on a cold day, the inside face is warmer, so the door can bow inward. DASMA lists a gap between the top section and the top of the opening as the typical symptom.
Does door colour really affect bowing?
Yes. DASMA’s example is a brown insulated door facing the sun with an air-conditioned space behind it, which bows more than an equivalent white door in the same application.
How is thermal bowing fixed on a commercial door?
Typically with horizontal struts across the back of the door, plus track or header and jamb seal adjustment where the door system allows. DASMA says to follow the manufacturer’s recommendations and to have a trained technician do the work.
Book a look at your doors
Call (604) 206-5727 or use the contact page. We answer around the clock for commercial door failures. New commercial clients: 10% off labour on your first service call or repair. Mention NEW10 when booking (one per business, not combinable with maintenance-contract pricing, work booked by Dec 31, 2026).
