Replacing a roof window? Choose the right scaffold.

For a swap you want a wide build: the Altrex RS TOWER 52, 135 cm across. Unlike a painting job, there is glass on your deck here. The old unit coming out and the new one going in both stand up there at some point, and you want to set them against the rail without having to squeeze past them every time. What else stands available is shown on the page with the full mobile scaffold range.

The deck length of 245 cm decides whether one position serves the opening. A common roof window is narrower than that, so you stay in the same spot for the whole job and never have to roll the tower with a half dismantled window above you. That counts for more on a swap than on other roof work, because the slope lies open between taking out and setting in. With fitting a completely new roof window it is different, since there you still cut the opening.

The slim 75 cm version is a fallback. It passes a gate the wide one cannot, but then you have no room to stand the old and the new unit beside each other. If there is no alternative, let the old window go all the way down before the new one comes up, and allow half a session extra, because with the narrow build you walk up and down far more often than you would like on a day like that.

The measuring point on this job is easy to pick out: the sill of the window that has to come out. You can see that point from the ground, so nothing has to be worked back from inside sizes in the loft. Standing level with that sill lets you tilt the unit towards you instead of wrestling it up over the rail. On a terraced house with two storeys and a gable roof that point usually sits between 8 and 12 metres above the paving.

From measuring point to build is one further step. The height of your top deck plus roughly 2 metres of reach together make the working height you state when you hire. If the sill of the roof window sits at ten metres, you order a build that puts you at about eight. Write both figures down before you finish the booking. What that takes in sections you will find under ten metres of working height, decks and guard rails included.

Standing too low is more awkward on a swap than on painting work. You have to unscrew the old frame, pull the lead free and tilt the whole thing outwards, and that only goes smoothly with your hip at about sill height. A metre too low and you are pulling everything towards you above your head with glass in your hands. So take one section more rather than too few; dropping down a deck during the job is easy, adding one is not.

Put the tower square in front of the existing roof window, because that window is your aiming point. From the garden or the street you can see exactly where the build has to come, so measuring indoors is not needed. Do allow for the pitch: the steeper the roof, the further the opening sits from the wall and the more it matters that your build stands half a metre off the brickwork rather than tight against it.

The ground has to be flat and load bearing and the four stabilisers have to come out on all sides. With a roof window you are often at the back of the house, where terrace slabs give way to grass and a raised patio catches just one leg. Solve that with levelling plates or by shifting the build a metre, and check with a spirit level before you build higher. For sloping terrain there is guidance in the piece on stabilising and anchoring.

Below you keep a strip clear for the glass. The old unit does not simply go against the wall: put a blanket or a couple of battens under it and stand it where nobody walks past. You use the same spot later for the new window, just before it goes up. Keep the route from your van to the foot of the build clear as well, because you would rather not carry a complete roof window thirty metres down a narrow garden path.

Replacing a roof window comes to two or three hire days in practice. The first morning goes on building; after that you measure the existing opening and check that the new window matches what you ordered. The swap itself, so taking out, setting in, dressing the lead and laying the tiles back, is a single day when everything is to hand and it stays dry. That only works when nothing still has to be fetched from a merchant.

The third day is your margin. You need it if the flashing turns out to be a different type than expected, if a batten under the lead has rotted away, or if the sealant has to cure overnight before you finish the inside. The weather counts too: you do not want to open the slope on a morning when rain is due by midday. So put the swap on the driest day of your hire period and leave the day after free.

Building in that margin is not expensive. The full tariff covers no more than the first 24 hours; from the second day onwards 95 per cent of it falls away. Taking a third day into the booking therefore costs far less than a second delivery and another build when you have booked too tightly. What a longer period means for roof work is explained under setting your hire period, with examples of jobs that run in phases.

Everything on this job happens from the same deck of 245 cm. Start by taking out the glazing, because on nearly every roof window the glass unit comes free of the frame, and that alone saves you about half of the weight you have to handle. Pass that unit straight down, on a rope or through a second person, rather than leaving it standing beside you on the tower, where the first gust of wind can tip it against an upright.

Then the old frame comes loose. The tiles around it come off, the lead and the flashing come free, and the screws come out of the rafters. Work from the platform and do not stand on the slope, because the tiles beside the opening are loose at that moment. Lay everything you take out down in the order it has to go back: first the tiles, then the lead, then the screws and the brackets.

The new window travels up in reverse order: the empty frame first, the glass after it. Make sure somebody is in the loft when you set it, because from the tower you push and from inside they pull. What people misjudge is the time between taking out and setting in. Reckon on at least an hour with the slope open, so do not start that late in the afternoon when the light is already going.

The tower arrives in parts, counted and complete, with the van up to the door or the drive. Book on a working day before three in the afternoon and the set is standing the next working day; which day that is you choose yourself. Have it come the day before the swap, so the build is up when the new roof window is dropped off. The rates for delivery and collection differ per province and include VAT.

Assembly is your own job, and on this one it is quite manageable. With two people and the manual supplied you are at height in a couple of hours, while the instruction video shows the order of the sections. Set aside a whole morning for it and do not start taking your roof window out the same day. Lay the parts somewhere they are not in the way when the glazier or the builders merchant unloads, so not right in front of the drive.

At the end we collect the set again on the date in your booking. You strip it down, count it through and lay everything ready where the van can reach. Do that only once the sealant around the new roof window has cured and you are sure you need not go up once more. Sealant, paint and mud you wipe off the deck panels before handing them back, because they dry hard and are awkward later.

The decks of these towers fall in load class 3 of EN 1004: 200 kg per square metre may stand on each one. That looks generous for a roof window swap, but add up what is up there during the exchange: two people, a complete frame, a glass unit and a tool box. Stand the glass upright against the rail rather than flat across the deck, and keep the middle of the platform free so the weight stays spread.

The standard rules apply in full. The wheels go on the brake before anyone climbs on, nothing and nobody is on the deck while you move, and as soon as it blows hard you stop the swap. Wind is extra awkward here, because a loose glass unit or an empty frame catches far more than you expect and can pull straight out of your hands. Check at the start of every hire day that the four stabilisers are still tight.

The risk that belongs specifically to this swap is the glass itself. A unit that tips from a platform ten metres up is lethal for anyone walking below, and the shards spread across the whole garden. So fence off the zone at ground level, let nobody pass under the build while there is glass above, and work with two people the moment a unit starts to move. There is more in the agreements on working safely at height.