To do this you need two people.
Go to the turret in the construction building with a friend. Have your friend face the turret, then have him turn 180 degrees, left or right.
From here, he should walk forward and jump onto the railing in front of him. Have the friend walk about 2/3 the way up the railing to the point that his feet on the railing are equal to the stairs on his right.
He will see them when he gets there. From there, he can sprint jump torward the lower ledge that juts out of the wall in front of him. Have him kneel down in the corner where there is a gray pipe, and have him look down (I'm not sure him looking down is necessary, but it helps).
Now you walk up the same railing as he did, 2/3 the way up as he did. Here is the hard part. You have to sprint jump onto his head, and then up onto the ledge above him. This is very hard, and will take some time to get used to.
This next part will be hard the very first time, but once you understand it, it's very easy. You have to find a way to lie down while on the ledge, and then turn so you can crawl through the 2x4's ahead of you.
Just play around with it, you'll get it. From here, stand up,and walk very slowly forward untill you fall down about a centimeter. Turn to your right while on this ledge, and walk forward past 2 2x4's that are on your right.
At this point, just walk out onto the stairs, and up onto the roof.
This whole glitch is time consuming your first time, but the only really HARD part is the jump onto your partner's head.
[more cheats for Call of Duty 4 here]