The phrase "a flat sheet of" is correct and usable in written English.
It can be used when describing a specific type of material or object that is flat and in sheet form, such as paper, metal, or plastic.
Example: "She carefully placed a flat sheet of paper on the table before starting her drawing."
Alternatives: "a level piece of" or "a smooth panel of".
Say you have a flat sheet of paper with a particular crease pattern.
Wallboard is essentially a flat sheet of plaster sandwiched between sheets of special paper.
The leaflets consist essentially of folds of endocardium (the membrane lining the heart) reinforced with a flat sheet of dense connective tissue.
Reel microfilm and microfiche (a flat sheet of film containing multiple microimages reduced from the original) were popular methods of document storage and reproduction for several decades.
Between these two extremes are other departures that have pushed the print wildly beyond its former definition as a flat sheet of paper suitable for framing.
Similarly, a flat sheet of thick tooling leather, from tandyleatherfactory.com, could be used as a desk blotter or as a top for a bedside table.
A typical array, some 28 by 24 inches overall, can have an echoing area equivalent to that of a flat sheet with an area of some 1,600 square feet (150 square metres).
\scriptstyle H \ = \ 2 \sqrt{\frac {\gamma} {g \rho}}\scriptstyle x - x_0 \ = \ \frac {1} {2} H \cosh^{-1}\left(\frac {H}{h}\right) - H \sqrt{1 - \frac{h^2} {H^2}} Pouring mercury onto a horizontal flat sheet of glass results in a puddle that has a perceptible thickness.
A rectangular-shaped honeycomb module that has triangular channels was manufactured by packing alternately a flat sheet and a corrugated sheet of the coated wire mesh to a certain thickness.
The obtained composite of PP containing PBT nanofibers was compressed into a flat sheet and piled with a sheet of high-density polyethylene (HDPE).
Tissue segments were then opened along the mesenteric border, cut into a flat sheet, and rinsed free of luminal contents with phosphate buffered saline (PBS; Gibco Invitrogen, Carlsbad, CA., USA) prior to being vertically mounted in the Ussing chamber.
