A baking sheet should not be placed on the oven floor because it is closest to the lower heating element and receives direct heat; the bottom of the food can burn while the top remains raw. A sheet or foil on the floor also blocks rising air, disrupts heat distribution, and may damage the oven floor coating. Always place the sheet on a rack.
The oven floor is just above the lower heating element in electric models, or on the surface of the compartment that conceals the element in some models. It is the hottest surface in the oven and transfers heat by conduction, meaning through direct contact. The bottom of a baking sheet placed on the floor is exposed to much more heat than the oven air. The result is predictable: the bottom of a savory pastry or cake browns in a few minutes while the top is still uncooked. Moving the same sheet to a rack resolves this problem on its own.
The second problem is airflow. Heat in an oven comes not only from the heating element but also from circulating hot air, which rises from the bottom and circulates across the top. A baking sheet covering the floor or foil laid there blocks this circulation; the lower area of the oven overheats while the upper area stays cooler than expected. The same recipe does not produce the same result in the same oven every time. In a convection oven, any obstruction in front of the fan has the same effect. That is why the bottom of the oven should never be completely covered.
The third problem is the oven itself. Many manufacturers explicitly prohibit covering the floor with foil or placing a baking sheet there; reflected heat strains the lower heating element, the floor enamel can crack over time, and foil may stick to the floor from the heat. Removing stuck foil can also pull off the coating. If the manual gives a clear warning about this, follow it. To catch drippings, place foil on an empty baking sheet on the rack below the food, not on the floor.
There are other ways to get bottom heat when it is needed. For pide, pizza, and savory pastries with soggy bottoms, move the baking sheet to a lower rack; if desired, use a baking sheet or pizza stone preheated with the oven. This method provides controllable bottom heat, not excessive heat from the oven floor. Use an oven mitt when removing the hot baking sheet. If the bottom of a dish browns too much, place a second empty baking sheet underneath the first; the layer of air between them softens the heat and delays browning on the bottom. The air between the two sheets acts as a simple heat shield.
A few exceptions are described in the manufacturer’s manual. Some ovens recommend a special position near the floor for pizza or bread; this recommendation is specific to that model and is not a general rule. In gas ovens, the compartment containing the flame is already enclosed, and the floor is usually not used. The rule is: food always goes on a rack in the oven, the baking sheet fits securely on the rack, and the oven floor stays empty, clean, and uncovered. Wipe away crumbs that fall onto the oven floor after each bake.