Installing a roof window? Choose the right scaffold.

To install a roof window you hire a mobile scaffold in its wide form, the Altrex RS TOWER 52 with a deck width of 135 cm. That deck holds more than you alone: the frame you set down for a moment, the circular saw, a box of screws and the insulation going inside. On a narrow platform all of that lies in your walking route, and you notice it the moment you turn with a board. Which builds are free is shown among the towers standing ready for hire.

The deck is 245 cm long, and for a roof window that works out well. A standard window is narrower than that length, so you reach the whole opening from one position without moving while you set it. If the tower stays put, your line of sight on the opening stays the same and you keep an overview of how the lead meets the tiles on both sides. That saves measuring errors you would otherwise spot only once the window is already in.

You take the slim 75 cm version only when the route to the back of the house allows nothing wider. You give up working room with it, exactly at the moment you and a second person are tilting a frame over an open slope. If the loft side has to be boarded from inside and no ordinary build fits up the stairs, that is a separate matter; the room scaffold for indoor work is meant for it, with a build that passes an interior door.

For a roof window you measure neither to the ridge nor to the gutter, but to the sill of the future opening. That is the point you have to reach standing up. If the window sits low in the slope you stay close to the roof edge; if it comes higher, your build goes up with it. On a terraced house of two storeys with a pitched roof you usually land somewhere in the 8 to 12 metre band. Ask your supplier for the clear size of the window beforehand.

Then translate that height into the build you order. On top of the platform height counts about 2 metres of reach, the distance over which you can still work comfortably from the deck without stretching. For a window whose sill sits at nine metres, your top deck ends up at around seven. How many sections go with nine metres of working height is set out in the overview for that height, so you know in advance what is in the van.

Choose slightly too high rather than too low. With your deck below the sill you work with your arms above your head while lining up a frame, and it nearly always ends up sitting skew in the slope. Half a metre too high and you simply step down a deck panel. Check the height you set out from the loft as well, because inside and outside measurements diverge more than you expect on a pitched roof.

The tower goes directly under the spot where the roof window will come, not somewhere alongside it. From the platform you want to look square on to the opening, otherwise you cannot see whether the frame lies true in the slope while you set it. So look from inside first to see where the window falls between the rafters, measure the distance to the wall and mark the build out on exactly that line outdoors. Standing half a metre off costs you the most time while levelling.

Under the wheels belongs flat, load bearing ground. Paving, block setts or a drive will do; fresh soil, newly laid turf and soft garden ground will not. The four stabilisers need room all round, and with a roof window you are often in a back garden where a shed, a pergola or a mature hedge makes that awkward. Prune or move whatever is in the way before the parts arrive. For a garden that runs downhill, there is advice on working on a sloping surface.

Keep space free below for what comes out of the roof. Making a new opening sends tiles, pieces of batten and sawdust downwards, and you want all of that caught in one place rather than spread over the terrace and the borders. Put a wheelbarrow or a tub beside the build and keep the zone directly under the platform empty while anyone is up there with a saw running. Warn the household not to walk through there during those days.

Installing a roof window usually takes three to four days, including the morning the build goes up. On the first day you make the opening: tiles off, battens cut through, the trimming framed and the roof made watertight again. On the second you set the frame, fit the lead and the flashing and lay the tiles back around it. What follows is the inside work. If you are only swapping an existing roof window, you finish sooner, because the opening is already there.

That inside work takes more time than people think. The reveals have to be insulated, boarded and finished, and that happens in steps with waiting time between them. Meanwhile you do not want to lose the tower, because there is nearly always a moment when something outside needs adjusting: a tile that will not close, or a strip of lead that refuses to sit down neatly. So reckon with a hire period running past the inside finish rather than to the day the window goes in.

Keeping it standing longer has been made deliberately easy. After the opening 24 hours, which count in full, 95 per cent of the rate comes off every following hire day. Holding on to the tower for a week instead of three days therefore costs far less than you expect, while it buys you the calm to finish inside and go over the outside once more. Over a longer period the sums are worked through in the comparison between hiring by the day and by the week.

From the 245 cm deck you work the whole opening without moving the tower. Start by lifting out the tiles over an area wider on all sides than the window itself, so you can get at the battens and the counter battens everywhere. Do not lay those tiles loose on the platform but in stacks against the wall side, where they are out of your walking route and cannot topple against the guard rail as you turn with a board or a frame.

Setting the window is a two person job. One stands on the scaffold and holds the frame, the other works from the loft and takes it through the opening. That division only works if you have room on the platform to turn with a window in your hands. Take the glazing out of the frame before it goes up; separately each part is considerably lighter, easier to steer and far less vulnerable in a sudden gust of wind.

What gets underestimated is how often you go up and down during those days. Forgotten screws, the wrong size of lead, the sealant gun lying below: each time you spend ten minutes on the ladder. Lay everything out on the deck before you start, in the order you will need it, and hang a screw pouch on the rail. Only move the tower once the opening is closed and the lead around it is in place.

You get the tower delivered as a set of loose, counted parts. Booking is open until 15:00 on a working day; whatever is in by then goes into the van for the next working day, and the date is yours to pick. Choose the day before your first sawing day, so building and opening the roof do not fall in the same session. Make sure somebody is at home to point out the unloading spot. The charges for bringing and collecting are given per province including VAT.

Putting the tower together is your own job. The set comes with a manual and an instruction video, and with two people you are up at height within a couple of hours. Decide in advance where the parts may lie: a drive or a stretch of pavement, and not the spot where the frame will come out of the van. Lay the sections down in order, so you are not hunting through the pile for the right brace halfway through assembly.

We collect again on the day written into your booking. You dismantle the build and lay the parts together complete, somewhere reachable from the road. Go through the count as you strip it, so that no brace or deck panel is left in the loft or among the shrubs. Sweep sawdust and sealant residue off the deck panels before you hand them back, because on this job there is always some of it lying about.

Every platform in the fleet belongs to load class 3 of EN 1004: 200 kg per square metre. When you install a roof window you are up there with two people, a frame, a pack of insulation and a saw, and that limit is less generous than it looks on paper. Do not put all the heavy items on the wall side but spread them across the deck, and take down at the end of the afternoon whatever you no longer need that day.

Brake the wheels as soon as anyone steps on and never move the build with tools or a person aboard, not even across a few slabs. Stop in strong wind, certainly while the roof lies open: a loose sheet of film or a board then works like a sail. Once the build has been shifted, satisfy yourself that all four stabilisers bear again before you resume sawing, and that the rails on the roof side are still tight.

The particular risk on this job is the opening itself, and you make it with your own hands. Once the tiles are off there is a hole in the slope that you lean over from the tower and that somebody stands under inside. Do not work above one another, cover the hole as soon as you stop and leave nothing loose on the slope, not even a sawn off piece of batten. The remaining agreements are gathered in the guidance on working at height.