Why does my garage door bow or warp in the sun?

A garage door that bows outward in afternoon sun and returns to flat by morning is not a defective door. It is a physics problem specific to bonded-core insulated doors. DASMA TDS-185 addresses thermal bowing of bonded core sections as a documented industry phenomenon. Colorado's high solar intensity, low humidity, and frequent temperature swings make this issue more common here than in many other parts of the country. Understanding why it happens, when it is normal, and when it signals a real problem saves you an unnecessary warranty call and helps you decide whether a replacement makes sense.

What causes thermal bowing

A bonded-core garage door has two steel skins with polyurethane foam bonded to both of them. The foam and steel are stuck together. When one skin heats up, it cannot move on its own. Direct sunlight can heat the outer steel skin to 120 F or higher. The inside of the door stays near the air temperature, maybe 80 F on a hot summer day. The outer skin wants to expand more than the inner skin. Because both skins are bonded through the foam, the section bows outward. The bow shows up in the middle of the panel. That is the point farthest from the rigid top and bottom rails.

The temperature difference between outer and inner skin drives how much the door bows. In Colorado, this difference is larger than in most states. Colorado gets more solar energy per square foot at altitude than states at sea level. The air is also dry. Dry air reduces the haze that normally filters some solar intensity. A black or dark brown door on a south-facing garage in Colorado can reach outer skin temperatures above 140 F on a clear June afternoon.

Which doors are most susceptible

Bonded polyurethane core doors (Intellicore and similar) have the highest bowing risk because the foam is chemically attached to both steel skins. The bond is strong in shear, which gives the door its structural rigidity and dent resistance, but that same bond forces the two skins to deform together when heat loads are uneven.

Non-bonded polystyrene doors use foam panels that sit loosely between the steel skins or inside a steel frame. The foam can shift slightly relative to the skins. This means the heat expansion is not fully transferred from one skin to the other. Polystyrene doors still bow somewhat in severe heat, but less than bonded polyurethane.

Single-layer steel doors have no foam at all. With only one steel layer, the door expands uniformly. These doors do not bow the way insulated doors do, but they also provide no thermal or sound benefit.

Door type	Bowing risk in direct sun
Single-layer steel (no insulation)	Low (no differential expansion)
Two-layer polystyrene insert	Moderate
Three-layer bonded polyurethane (Intellicore-type)	Higher
Dark color, south/west facing	Higher than light color, same type

Door orientation matters too. South-facing and west-facing doors get the most direct afternoon sun in Colorado. A south-facing dark door in Denver can receive 60 to 70 percent more solar energy per square foot per day in summer than a north-facing door. More solar energy means a higher skin temperature difference and more bowing.

Is the bowing a defect or normal behavior

Thermal bowing is normal if the door flattens back out when the temperature equalizes. Check the door in the morning before the sun hits it. If it looks straight in the morning and bows in the afternoon, that is the normal thermal cycle. This is not a defect.

Permanent bowing that does not flatten overnight is different. Permanent bowing means the foam has started to separate from one or both steel skins. This is called delamination. When the bond between foam and steel fails, the outer skin can expand and contract freely. Over many thermal cycles, the outer skin shifts position and stays bowed even when cool. This is a structural failure. It may be covered under the door's warranty.

To check: look at the door early in the morning after a cool night. If it is still bowed, press gently on the center of the bowed section. A delaminated panel feels hollow compared to an intact panel. It may also sound different when you tap it, or show bubbling along the edges where foam meets the frame. Report this to your door manufacturer or installer. Most major manufacturers cover delamination under warranty as a separate issue from normal cosmetic bowing.

How to reduce thermal bowing without replacing the door

You cannot eliminate thermal bowing on a bonded-core door in direct intense sun, but you can reduce it:

Choose a lighter door color. Light colors reflect more solar energy and reduce peak outer skin temperature by 20 to 40 F compared to black or dark brown. This directly reduces the skin temperature differential and the amount of bow.

Add an awning or overhang. A roof overhang, pergola, or retractable awning over the garage opening can shade the door during peak afternoon sun hours. Even partial shade helps by reducing direct solar load.

Improve ventilation inside the garage. A hotter garage interior means the inner skin is also warmer. That reduces the difference between inner and outer skin temperatures. A garage that traps heat makes bowing worse. A ventilation grille in the eave or a ridge vent in the garage roof lets hot air escape. This keeps the inside cooler and lowers the skin temperature difference.

Consider polystyrene insulation. If you are replacing the door and bowing is a concern, a three-layer polystyrene door provides most of the thermal and noise benefit of polyurethane with lower bowing tendency. The R-value will be lower (R-10 to R-14 vs R-18 to R-20), but the door is less susceptible to the cosmetic issue. For most Colorado Front Range homeowners, the difference in practical insulation performance between R-14 and R-20 is smaller than the difference between an insulated door and an uninsulated one.

Park inside to reduce interior heat. On the hottest days, leaving one car parked inside the garage raises the interior temperature significantly. A shaded garage interior stays cooler, which reduces the temperature gap between inner and outer steel skins and lessens bowing. This is not always practical but is worth noting if you have the option.

When to call for service

Call your installer or manufacturer if: the bowing is permanent and does not flatten by morning, the sections show visible gaps between the panel face and the frame edge, the door no longer closes flush with the floor or side seals, or the door has begun to bind in the tracks during opening and closing. Permanent deformation can pull the door out of alignment with the track, causing binding and uneven wear on rollers and hinges.

G Brothers serves the Denver metro and Front Range and can inspect a bowing door, advise on warranty coverage, and provide free estimates on replacement if the door has delaminated. If you are not sure whether your door is bowing normally or showing signs of delamination, a service visit is the fastest way to find out. Same-day service is available.