I needed to put a Velux on the side of a roof valley on the house, so as to be able to do easy maintenance myself. I bought a Velux 140*78 with a flashing kit, and the total cost was around £280UK, I think. It took all of a perfect May day to do, between me and son. We worked from the inside. The early 20th century roof is made of tiles nailed to horizontal boards on top of vertical joists, built to last! I drilled holes through a board with one of those spade type wood drills, and joined them up with a jigsaw. The boards were a good inch thick, more than I expected. I used a power saw - the reciprocating type - to cut through a joist, in order to pull in the cut piece of board and make a pilot hole. Then we lifted as many tiles as we could reach, and put them inside the loft. Once we could get outside and stand in the valley, we took off the rest of the tiles, and pulled the nails, then cut the hole etc as per instructions. The Velux instruction books are comprehensive, but have no words, so you need to have rehearsed putting it all together somewhere downstairs first. We separated the frame from the sash, and lifted the frame - which wouldn't go up through the trap - with a rope up the outside. The sash with glass is heavy, and though I thought of taking the glass out, in the end lifted it complete up the inside with a chain hoist originally bought for moving a Rover v8 engine. There are two instruction books, one for the window and one for the flashing. Unfortunately they sort of interleave, and you have to move from one to the other and back again as you work, which caused the only real error of the day as we missed out a piece of metalwork, and had to unscrew some bits to put it on. In the book, there's a nice picture of a chap putting the sash back in on his own. Not a chance! It's heavy, and althought the pop out studs which lock the parts together are very clever, the little arms they are on are a pig to get back in the grooves, as they wobble. Also you need to do this twice whilst setting up, once to make sure everything is square, then again at the end. It's not fun. I'd never done tiling before, and I'd bought some fat headed nails from Wicks, which meant we had to stop and go back there to buy small headed galvanised 40mm nails, which are the right ones. Hanging the tiles back wasn't difficult, just tedious. I managed to nail tiles down the sides of the window to partially overlap the flashing, but just slid the ones closest to the window under the others, where they stay by friction - so far anyway. At the end we felt we'd done quite a professional job, and the next day it tipped down, so I had the satisfaction of looking out of my new window with no leaks at all!