[ flat-wahyz ]
/ ˈflætˌwaɪz /
with the flat side, rather than the edge, foremost or in contact.
Also flat·ways [flat-weyz] /ˈflætˌweɪz/.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for flatwise
Placing it flatwise on supports close to its ends we find that its own weight causes a decided sag.Inventors at Work|George Iles