Question 101472
This is a Pythagorean theorem problem. You'd told the roof side is 7 m, and the height is 3.  So apply the theorem we know that 7^2 = 3^2 + x^2.  Simplifying, we  have 49 = 9 + x^2. So x^2 = 40, which means x = sqrt(40).

That would be a piece that goes half way across the roof, since the triangle I solved was from the center of the roof to the eave.

Therefore, each piece has to 2 * sqrt(40).