Exit the Mansion and turn to face it. Notice the low ledges on both sides of the entrance. The corner bug can be done with any of them.

For this example, we will use the right hand ledge. Go there, turn left and stand against the left hand wall. Side-step to the very left in order to reach the edge. You will notice that Lara's left foot is somehow in the air.

Turn left 45 degrees. You will see that Lara's right foot and hand are "inside" the wall.

Press the Jump button and do not let go of it. Lara will jump on the spot a couple of times (maybe three) and half of her body will be inside the wall.

This is an "illegal" spot for the game engine (Lara cannot be inside the wall), so it will try to get her out of there by tossing her upwards.

This is the result. After tossing her up, the game engine throws Lara outside the "illegal" spot. You will see that Lara is in the air and lands on the ledge above.

To reach the topmost ledge (therefore the roof), you need to perform the corner bug once more. So, stand against the next wall, side-step to the very left, turn left 45 degrees and jump on the spot.

And here you are! Lara will find herself on the topmost ledge!