$P + \frac{1}{2}\rho {v^2} = {P_0} + 0$

$\left( {{P_0} - P} \right) = \frac{1}{2}\rho {v^2} = \Delta P$

Hence lift of the roof

$F = \Delta P \cdot A = \frac{1}{2}\rho A{v^2}$

$ = \frac{1}{2} \times 1.2 \times {\left( {40} \right)^2} \times 250 = 2.4 \times {10^5}N$

as pressure inside the roof is greater than outside the roof. so, force will act upward direction.