We consider skew-product maps on T2 of the form F(x, y) = (bx, x+ g(y) ) where g: T→ T is an orientation-preserving C2-diffeomorphism and b≥ 2 is an integer. We show that the fibred (upper and lower) Lyapunov exponent of almost every point (x, y) is as close to ∫ Tlog (g′(η) ) dη as we like, provided that b is large enough.