Installing a dormer window? Choose the right scaffold.

On this job you take the wide Altrex RS52, with a working floor of 135 cm. The reason is not the lifting, because the crane sees to that, but everything around it: you take tiles off the roof and set them on the deck, you pass battens and boards up, and with two of you at the top you want to be able to get past each other. On a slim floor you are permanently in the other one's way, exactly when things have to happen quickly.

If a tight path or a gate is the only route round the back, the RS51 at 75 cm is the fallback. You give up working room, but you get where you have to be. Given a free choice, take the wide version: it saves fuss and climbing. Ask as well how many people are helping, since every extra pair of boots fills a narrow floor further. The models on offer are listed on the page where you rent a mobile scaffold for outdoor work, each with its build heights.

What you must be clear about is that a scaffold is not lifting gear. Nothing hangs off the build: no chain hoist, no rope with a dormer sash on the end, no pulley on the top rail. The unit goes up on the crane, and the loose gear you carry by hand or haul on a line down the outside. Confusing the two means renting the wrong kit for the heaviest act of the day. There is more background in the article on scaffolds when a dormer goes on.

The point you have to reach is the lower edge of the roof slope where the unit lands, and after levelling also the top of the unit itself. On a two storey terrace that lower edge sits around 6 or 7 metres and the highest point of the box around 9. On a house of three storeys everything shifts up a floor. To serve both points you rent inside a band of 8 to 12 metres and pick according to what your own tape tells you.

When you book you do not state a platform height but a working height, and the difference between the two is roughly 2 metres of reach. Standing on a floor at seven metres, your hands come to about nine, and nine is the figure that appears on the order. So count back from the highest thing you want to touch. If you are unsure which build belongs with that, compare the options on the page about working heights before you commit.

Choose too tight and you find out on the crane day, when there is no time left to arrange anything. What follows is that you stand on the top floor pointing with stretched arms while the unit hangs above you, and that you still need a taller build later for the finishing. An extra frame is quickly added if you take it into account at the booking, but not on the morning itself with a lorry in the street.

The crane decides the layout of your street, not the scaffold. A lorry with a loader crane has outriggers that go a long way out, room to swing the jib and a clear path over the house or the front garden. So pitch your build beside that path, at the side of the elevation or a few metres away from where the unit comes down. Talk it through with the supplier a week in advance, because he is the one who knows where his machine will stand and how far up it has to reach.

Under the wheels you want flat, hard ground: block paving, tarmac or solid slabs. Set the four stabilisers out and allow for the space they take, since on a crane day all sorts of people pass along your build. If part of it stands on the footway, keep a way through for pedestrians and ask the council which rules apply in your own street, and do that early where parking bays are being closed off for the crane.

Think as well about everything else arriving that day: the lorry with the unit, a trailer of tiles, perhaps a skip. Walk the pitch the day before with the supplier in mind and move aside anything at all that can roll away or topple over. If your drive falls away towards the road, read how to get the build level anyway in the article on uneven ground. A tower standing out of true is not something you will reposition on a busy morning.

The crane is there for half a day, but your rental period is far longer. Before the unit can go on, the tiles have to come off, the opening has to be cut and the structure has to be ready. After it comes levelling, fixing and making the whole thing weathertight. So reckon on three to five days with the build standing, depending on whether you do the lot or only the preparation and the finishing. Have the box installed complete and you hire for less, though rarely under three days.

Inside those days sit waiting moments outside your control. The supplier decides when the crane comes, and that date slips by a day easily enough with bad weather or an overrunning previous job. In a strong wind the lift will not go ahead at all, and a roof slope already open is not one you leave out in the rain for days. So set your period with margin on both sides: a day before the lift and at least two days after it for the finishing.

That margin costs little. The first 24 hours decide what you pay and everything following it counts with 95 per cent off, so a five day booking sits close to a three day one. That is precisely the reason to book wide on a job whose most important day is fixed by somebody else. You are paying for calm in your plan instead of for a second delivery. How to judge such a period on a bigger project is covered in the guidance on estimating rental days.

A floor 245 cm long gives you just enough length to serve a stretch of the roof edge without shifting every few minutes. From it you take the bottom rows of tiles off, pass them down and lay the roof slope bare where the opening will be. Where the box is wider than that floor you work in two pitches, and it pays to mark the second spot out the day before, so that nobody is manoeuvring a tower about on the crane day.

On the day itself the tower is mainly a passing post. One person up, one down, and between them go tiles, battens, screws and tools. Agree beforehand what travels in which round, because most of the time on such a day is lost climbing back and forth for something small. Everything you set on the floor goes against the guard rail rather than loose in the middle, so it does not get taken along when somebody squeezes past.

What people misjudge is how many operations follow the lift. Once the unit stands, the levelling, the screwing and the closing up begin, and all of it happens off the same floor. So keep the build in place until the box really is fixed and shut, even when the crane has long gone and the street lies empty. If you carry on yourself with fitting and sealing the dormer, you use exactly these positions for a few days more.

The parts come to you as a complete set, counted and whole: uprights, floors, braces, guard rails, toe boards and stabilisers. Book before 15:00 on a weekday and the van drives out to you the next working day. Choose a place for that stack that will not be in the way on the crane day, because the lorry with the unit needs the whole street side. A drive, a garage path or a strip of grass behind the gate works better than the footway.

The dates for bringing and fetching you pick while booking. Plan the delivery at least two days ahead of the crane, so you can build at a calm pace and get the first tiles off on the morning before. If the lift moves, pass your new dates on straight away; a finished tower waiting days for a unit only costs you room. Anyone driving themselves can take the set free of charge by appointment from the Zeeburgerpad in Amsterdam, provided the vehicle can handle the length of the uprights.

When everything is done we come and take the build away again. You dismantle it and lay the parts where the driver can reach them, ideally on the same stacking spot as at the drop off. Wait until the box is closed and nobody has to go up any more, because rebuilding for one forgotten screw is a waste. The fixed charge for bringing and fetching per province is in the overview of delivery costs.

The most important rule on this page: you hoist nothing from a mobile scaffold. The floor is made for people and hand tools and carries 200 kilos to the square metre, which comes down to load class 3 within EN 1004. A dormer weighs a multiple of that and belongs on the crane. Nor do you lift part of a box, a window frame or a pack of roof boards with a hoist off the guard rail. How that class relates to your material is set out in the guidance on loading.

On the crane day itself you do not stand on the tower while the load hangs above you or swings past. Wait until the unit rests on the roof and the hook is free before you go up. Beyond that the usual points hold: wheels on the brake before the first person steps on, never roll it with anyone or anything on board, and stop when it blows too hard. In wind the crane stops anyway, so there is nothing to do up there either.

Watch the people on the ground as well. When a dormer goes on there are neighbours looking, a driver walking about and somebody from the supplier calling round. Fence off the zone under your build, leave no tiles on the edge of the floor and work with tools on a cord. On a day as busy as that it is worth going through the rules that apply to work at height once more with everyone who is helping you.