Cover the Grid

You must cover a 7x7 grid with L-shaped triminos and S-shaped tetrominos, without overlapping (flipping and rotating is permitted). What is the minimum number of pieces you can use in order to do this?

Remark: All pieces must placed within the board.


Consider all cells in the grid which lie in an odd row and odd column - there are 16 of them. Since each of the two pieces can cover at most 1 of these cells, we need at least 16 pieces. Giving an example with 16 pieces is easy.