Replacing a chimney cap? Choose the right scaffold.

At some point in this job two plates are up there together: the old one you have just freed and the new one still to be tried for fit. The choice therefore lands on the Altrex RS52, 135 cm between the guardrails with a boarded length of 245 cm. That gives you somewhere to put the old cap down while the head is swept off and the holes are checked. Hiring a tower for replacing a chimney cap is really a question of temporary storage at height, halfway between the ground and the head.

A narrow RS51 at 75 cm just about copes with bolting a light plate on, but it grates on a replacement. You are forced to send the old cap down before you can carry on, and that means an extra run with a line at the worst point of the day. Go narrow only where the way through to the back forces it, and measure the tightest pinch point first. Both widths appear in the range of towers you can hire.

Smaller models are nowhere near a head on a roof. A room scaffold, the RS44, gets you 3 to 4 metres and is built for inside the house, slim enough from around 70 cm to pass an internal doorway. The MiTower takes a single person as far as 6.2 metres, which is not enough to work safely around a heavy concrete plate. Only the RS52, which goes on up to 14.2 metres as a model, is a serious candidate for a head on a roof.

Do not measure to the top of the old cap; measure to the topmost brick of the head beneath it. That plate is coming off, and afterwards you have to clean the rim, inspect the holes and set the replacement true. Two storeys puts you in the 8 to 12 metre band for working height, and the platform then hangs at 6 to 10 metres. The difference between those two numbers is reach: a platform height plus something like 2 metres is what working height means.

On this job, take the extra half lift rather than the one below. Freeing the old fixings means being able to look in under the rim of the plate, and you cannot do that squinting up at it from underneath. Stand a little higher and you see straight away whether the legs are bedded into mortar, whether brackets run round the outside and whether anyone has used sealant. The build that goes with ten metres of working height is set out with its part count.

The tape begins on the ground where the wheels will sit, not on the threshold or the terrace. Where a back garden lies below street level, count that drop into the build you order. On a house whose chimney breaks out above the second floor you are almost always in the upper part of the band. Fall short and the old plate hangs at chest height while you need to tilt it, which is not a position to be in with concrete.

Where the build goes depends on where the chimney comes through the roof. If you can get at it over the flank wall, the tower stands at the side; on a mid terrace it is normally the back garden. Where the head pushes up out of the middle of a pitched slope, no amount of building up will bring you to it and hiring a tower achieves nothing at all. Study that from a distance first, from across the road or from a window on the far side.

Underneath you want level, load bearing ground with clearance on every side for the outriggers. Then set aside somewhere for the old cap to end up once it is down on the ground: a trailer, a barrow, or a sheet spread out on the grass. Without that spot a heavy, dirty slab lands in the middle of your terrace or up against the wall, and you finish up dragging it about later. Give some thought to disposal as well, since a plate like that rarely fits in a car boot.

Keep the zone below completely empty for as long as the old cap is still fixed. Lumps of mortar come away the moment you start freeing the fittings, and a piece of cement occasionally travels well beyond the base of the build. No bicycles, no garden chairs, no children playing, and a word to the neighbours where a garden or a narrow path is shared. If a leg lands on the footway, ask the council what is permitted.

Two to three hire days is a sensible commitment here, and it is worth setting them out in order. The build fills a half day; freeing the old plate takes an hour when the bolts cooperate and half a day when everything has seized. After that comes cleaning the head, making good whatever you find, and only at the very end the setting and levelling of the new plate. That sequence compresses badly and hurries even worse, particularly ten metres above the garden.

The uncertainty sits in what is waiting under the old cap. Loose bricks, a weathered flaunching or a hole that has been letting water in all mean pointing or rendering before the replacement can go on. Fresh mortar also wants to set before anything heavy sits on it, and that costs a night at least. Reckon on one spare day beyond your programme, then, even if you are convinced the job is a morning's work.

That slack is affordable because every day beyond the first twenty four hours is 95 per cent cheaper. Adding a date behind your planning costs far less than a second delivery and a second build because the old plate turned out worse than expected. Put the collection day after the day you expect to finish, then, rather than on that day itself. Every figure appears with VAT alongside it in the rental prices per model.

Begin by looking rather than pulling. Walk the four faces and work out how the cap is held: on legs with bolts, on brackets clamped round the head, or on legs bedded into the brickwork. Undo the bolts instead of levering at them, and cut them off when they will not shift at all. Where legs are mortared in, chop the mortar away around them. Hauling on a seized cap brings the top bricks with it, and then you have a masonry job on your hands as well.

Once the plate is loose it goes down under control, not over the edge. Pass a line round it, tilt it gently onto the boards and lower it along the outside while somebody at the bottom steadies it and watches the zone. A concrete slab weighs tens of kilos and shatters into flying pieces if it is dropped from that sort of height. Take your time over that one moment, because it is comfortably the most dangerous quarter of an hour in the entire job.

Then comes the part nobody can plan for in advance: the cleaning and the judging. Brush the head off, see whether the top course still holds, check whether water has been running down into the flues, and make good anything that has worked loose. That inspection is precisely where replacing a chimney cap differs from putting a first cap onto a bare head, where the brickwork can be assessed before you order. Here you know nothing for certain until the old plate is standing on the ground.

What turns up is a complete, counted heap of separate components; nothing arrives ready built. Pick a place for that heap which is not the place the old cap is going to land, because those two things on one drive do not work. Orders that reach us on a weekday before the clock turns 15:00 go out for the next working day. Ideally schedule the delivery after the replacement cap has been dropped off, so nothing needs storing.

Erecting it stays your job, with the manual beside you and the build video within reach. Two people need a half day for that, outriggers set and guardrails checked all the way round the platform. Do not run the build into the same morning you mean to free the old cap, or you begin the heaviest part of the whole job with half a day already in your arms. A separate build day here is planning rather than luxury.

Afterwards you take the tower apart, count the components and stack everything at the agreed spot, and we come for it. Before that, check that no grinding dust or cement residue has been left on the boards from freeing the old plate. A bucket of water and a stiff brush deal with it easily enough, as long as none of it has been left to dry on. What the journey costs in each province is listed under the delivery charges, VAT included.

Load class 3 under EN 1004 permits 200 kg on every square metre, and that is the class these boards belong to. On a replacement it matters more than on other work, because at some point an old concrete slab lies next to a new cap with two people and a tool case in between. Send the old plate down before you bring the new one up, then, rather than letting the pair of them wait on the boards together.

Wind counts twice over when there are two loose plates. Old or new, the cap catches like a sail the instant you tilt it, so put off both the freeing and the fixing in a strong blow and leave the plates flat until then. The rest is familiar: brakes on the wheels before anyone goes up, nobody and nothing on the deck while the tower is being moved, and the outriggers reset and checked after each shift.

The greatest danger in this work is the head itself. A cap that has sat tight for twenty years can come away with the brickwork attached if you lever at it, and then rather more than a plate falls. Work by freeing things off, never by pulling at them, and keep the ground beneath clear the entire time you are busy with the fixings. Guarding against a fall is covered under fall protection on a mobile tower.