Replacing a window sill? Choose the right scaffold.

Sill work asks for the lightest model in the fleet, so the MiTower is where you land. It measures 75 cm across, takes one user at a time, and tops out anywhere between 4.2 and 6.2 metres. That suits the job: you pause briefly at each window sill, carry very little up, then roll on by a window width. Across eight openings, a tower one person shifts unaided saves the best part of a morning. That class of stand sits on the page about the one-person tower we let out.

A deck of 75 cm sounds mean, yet on this job your kit simply stands beside your boots. Up with you go a breaker with a wide chisel, a bucket of mortar, two spirit levels, a handful of packers and the replacement sill itself. Beyond that you want nothing at all up there. The moment you tilt the old stone towards you, the whole width of the deck is in use, and anything lying loose gets shoved aside with your foot. Keep the broken pieces on the side furthest from the opening.

Where the run of sills sits higher than a first floor, or where you want a mate up there to pass the stone across, you can instead hire a mobile scaffold with more reach and a deck of 245 cm. On an ordinary family house that is money spent on nothing: you pay for height and floor area sill work never touches. Walk the row of windows before you decide and count how often you will be shifting, because that number tells you more about the right hire than the height of the wall does.

The highest point of this job sits lower than on almost any other bit of wall work. You need to be at the underside of the opening, and on a typical mid terrace that sill hangs somewhere between 3 and 4 metres above the footway on the first floor. A working height of 4 to 6 metres goes with that. Take the height of your deck and add about 2 metres for the distance your arms and upper body cover together, and you know at once where you come out on the wall.

On sill work you deliberately do not measure to above the opening. You want to finish beside the stone, with the brickwork below it at eye level, because that is where the work actually is. Hire a build that puts you too high and you look down on the window sill, losing sight of the front face, the outward fall and the drip groove. The chiselling suffers too: you drive the blade downwards into the bed instead of horizontally under it. Measure to the bottom of the frame and add half a metre at most.

Your zero point when replacing a window sill is the paving the castors will rest on, which is not the doorstep you instinctively measure from. At a house with a raised entrance, or a garden that falls away towards the boundary, that difference is easily half a metre. Pitch it too tight and you spend the last window up on your toes. The other way round costs you nothing, since nobody forces you to use the top deck. The page about a build reaching six metres shows what such a stand looks like.

The ground below a window is seldom clear. There will be a planted border, a water butt against the brickwork or a gravel margin along the wall, and that is exactly the strip where the four stabilisers have to splay outwards. Empty it before the driver pulls up, because shifting a planter while half a sill hangs out of the wall is not an option. On soft ground put levelling plates or a couple of paving slabs under the castors, or the whole build settles crooked the minute you start swinging a chisel.

Then think about what is coming down. Cutting out produces old pointing, lumps of bedding mortar and now and then half a stone, and all of it falls in a straight line beneath you. Nothing should be parked there: no garden chairs, no bicycles, no glazed back door. A tarpaulin or two boards laid over the border catches the debris and you shovel it up in minutes afterwards. Where the elevation gives onto a public footway, keep a strip open for pedestrians and let the neighbours know there is a morning of hammering coming.

Look at the route between windows as well. While replacing a window sill you will nudge the build along ten to twenty times, a few metres each time, and that only goes briskly if the strip beside the wall is flat and clear. One sunken slab, a gully cover or a cellar window sticking out means levelling up afresh at every opening. How to take up a difference in the ground under the castors is set out in the article about levelling up on uneven paving.

Cutting one loose, offering it up, bedding it and pointing round it takes an hour to an hour and a half per window sill, and that stretches the moment the old stone is set in rock hard mortar. Four to six of them will fill a working day for you. Another day follows, because the bed under a new sill has to go off before you can seal the joint against frame and brickwork. On a front elevation with a handful of stones you therefore reach two to three days with the tower standing.

Weather weighs more heavily on replacing a window sill than on decorating. In a frost no mortar goes under a stone at all. In steady rain the fresh pointing washes out before it grabs, and sealant sticks badly to soaked brickwork. Those are waiting days you cannot claw back by pushing harder. So run your hire period from the day you are certain of starting to the day after the final bead of sealant, rather than to the date on which, at best, it might just be finished.

Booking generously costs you very little here. Payment is arranged so that the first twenty four hours carry the full charge and every day you add after that comes with ninety five per cent taken off, which makes a day spent waiting on a mortar bed almost free. Cutting it fine is what hurts: the van turns up while a fresh stone still sits unsealed in the wall. What a sensible span looks like is covered in the explanation of hire periods.

Getting the old stone out is the moment your hired platform earns its keep, because everything you do here pushes sideways. You chisel the bedding out from the middle of the sill towards the ends, and the deck under you has to take that without creeping. So brake all four castors, set the stabilisers down hard and stand square to the opening rather than reaching in from one corner. Anyone who levers from an end brings bricks down with the stone, and on a tower that shifts you will not feel the difference coming.

Bedding the new stone is where the height you hired shows itself. A window sill falls outwards so that water runs off the frame, and you set that fall on packers with a level while the mortar is still soft. From a platform you can drop to one knee and sight along the front face, which on a ladder is simply not possible and is the whole reason people hire a tower for an afternoon of stone work. Build a shade low rather than high, so the bed stays in view.

One stone per position is the rule, two at most where the openings sit close together. Before you climb down to shift along, take away anything that can roll or fall: chisels, packers, the sealant gun and the bucket. Carry up what you need for the next opening in one trip instead of walking up and down three times. If you are taking on repairs to the frame above it in the same round, allow a good hour extra per window.

No assembled tower comes round the corner. What the driver drops off is a counted package of loose components that you turn into a scaffold yourself, and that stack has to go somewhere. Not under the windows, since that lane is reserved for rubble and a barrow. A corner of the drive, or the path beside the front door, works far better. Order on a weekday, before three in the afternoon, and everything reaches your address the next working day. You pin both the drop off date and the collection date yourself.

Building it is on your side of the arrangement. Printed instructions travel with the set, and a video online runs through the order the components go up in, so with a tower this low and this few parts a pair of people are done quickly. Have the load land the day before you start, then the first morning opens with a blade going round the sealant on stone number one. Glance at the forecast while you are at it, since building in driving rain takes twice as long.

Once the last bead is run we come back round to take everything away. You strike the tower and pile the components somewhere a van can pull up alongside, preferably not beside the heap of rubble. Tick off the components list as you break it down and there is nothing to discuss afterwards. Every province has its own fixed sum for the journey there and back, listed under the cost of delivery, so the final day holds no surprises.

Each deck carries up to 200 kg per square metre, and that is what load class 3 means in EN 1004. On sill work you get nearer that figure than on other wall jobs: a natural stone window sill two metres long weighs tens of kilos, and you, a bucket of mortar and a breaker all count on top. So do not park a stone in the middle of the platform but lay it hard against an upright, and take it off the deck the moment it is bedded in the wall.

The ordinary rules apply here just as firmly. Lock the castors before your first foot goes on the step, never move the build with a person or tools aboard, and stop working when the wind gets up. That last one feels excessive on sill work, right up until a gust knocks over a loose stone propped against the guard rail. After every move, walk round the four stabilisers and set them out firmly again before any kit goes back up.

The real danger on this job travels downwards. Chiselling sends chips flying and drops lumps of old bedding exactly where people walk along the wall. Cordon that strip off with tape or a few crates and keep children out of the zone beneath you. Wear safety glasses yourself, because a splinter off a weathered stone carries further than you expect. Everything else that applies is gathered on the page with rules for working at height.