All metal expands when heated. We have carefully designed the Pyro Cages to minimize the warpage that can result when the metal is hot, but it does happen to varying degrees. When burning organic material like limbs and leaves the effect is less than processed artificial material like plastic or wood that has glue or adhesive. While the panels can distort it does not impact the effectiveness of the unit - any gaps that can develop are still small enough to safety contain the burn. Generally, the effect is most noticeable during there first couple of burns and tends to even out with use.