A windowsill assembly includes a beam member and an insert bracket. The beam member includes a first leg and a second leg, the first leg meeting the second leg at a joining region at proximal ends of the first leg and the second leg. The insert bracket includes a first portion and a second portion that are substantially perpendicular to each other to substantially match an angle between the first leg and the second leg of the beam member. The insert bracket couples the beam member to a support member.