Learned a new tool : FFD 3x3x3 As you can see in the photos, I have modelled the roof plank by blank by copy and pasting. After grouping and positioning the roof, I realized that the roof was too short. And if i were to ungroup and change the length, the shape will somehow change.
FFD 3x3x3 allows you to change length or rescale things at only the particular control points that you use. So, solved my problem.