Flat roofing in Three Legged Cross

What fails first. On felt it is the creases and the junctions. A three layer built up system with a mineral cap sheet lasts well until the surface goes brittle, then it splits where it has been folded. A single torch on layer laid over an older covering blisters, because the damp underneath had nowhere to go. And a lifted or failed seam at an upstand lets water run under the whole roof rather than through one hole, which is why the stain is never where the fault is.

What we lay. EPDM rubber on a plain rectangle, bonded down in a single sheet with no lap or seam in the open field. GRP fibreglass where the roof has kerbs, corners and upstands, because it is laminated in place and the detail becomes part of the same skin. Single ply, hot air welded, on a long run.

Deck, joists and falls. Plywood, OSB or chipboard decking sits on joists, and the firrings on top are what create the fall to the outlet. Wet chipboard swells, gives underfoot and comes off. A roof laid flat holds standing water, and ponding in the same place every winter will shorten a new covering exactly as it shortened the old one. BS 6229 sets out the falls and the drainage, and where re-firring is not practical we build the fall in tapered insulation.

Garden rooms are where this goes wrong. Any heated space brings the insulation into scope under Part L when the covering is renewed. A warm roof puts the PIR above the deck, with a vapour control layer beneath it and the membrane over the top, so nothing in the structure is ever cold enough to condense on. A cold roof puts the insulation between the joists and leaves the boards below the dew point, which is exactly how interstitial condensation gets into a build up and rots it from above. Building control are notified and inspect it before it closes.

Junctions and openings. A small roof with four junctions has more places to fail than a big plain one, so this is where the care goes. The abutment gets a chase cut in and a lead cover flashing dressed home, with a cavity tray above it. A parapet gets a coping and an apron. Openings sit on kerbs so the covering turns up an upstand: rooflights, a lantern, a Velux style flat roof window, a hatch, a soil and vent pipe. The free edges finish on a drip trim, and the rainwater outlet takes the water to a hopper, a downpipe or the gutter. A garden room or an extension is two or three working days, and we say how long first.

No rate per square metre, because the square metres are the easy part and what is under the covering is what decides the job.

Getting to it. A detached garage, a garden office at the bottom of a plot and a utility roof tucked against the back of a house are three different jobs before anyone gets set up. A garage or a single storey extension will often come off a tower, anything above a two storey wall wants a scaffold, and the platform has to stand while the deck is open rather than for an afternoon. The work at height rules are why it is a priced line rather than something folded into a rate.

The deck under the covering. A sound deck can be re-covered. Chipboard that has been wet for years, or ply that gives underfoot, has to come off and be replaced, and you cannot tell which you have from the ground or from a photograph. We lift a corner before quoting, because the deck is the single biggest swing in the price.

The falls, and where the water is meant to go. Most flat roofs that fail were never laid to a fall, or the fall runs at an outlet that has since been blocked or built over. Correcting it with firrings is more work than laying a membrane and it is the difference between a roof that lasts and one that ponds. Upstands, edge trims and each outlet are priced one at a time, because that is where the hours and the leaks both are.

Comparing quotes? Ask which system is being quoted and how many layers, whether the deck is being renewed or covered over, whether the falls are being corrected with firrings, how the upstands, outlets and edge trims are detailed, and whether the scaffold and the skip are in the price. Two quotes for the same job are usually two different jobs.

The village sits inside the green belt. For a repair that means nothing at all: re-covering the garage you already own is maintenance and nobody needs telling. For a new flat roofed garden room, home office or extension it means the size, the height and where it sits on the plot get looked at before anyone thinks about the covering. Some of it falls inside permitted development and some of it will not, and that is worth settling first.

There is a practical side that catches people out. Garden rooms and offices are often bought as a kit and put up with a roof designed to be cheap rather than to last. The fall is token, the outlet is a hole in a corner, and the insulation sits between the joists in a cold build up that condenses the first winter you use it. If you are having one built, get the roof specified as a warm roof with a real fall and a proper outlet from the start. It is a small part of the cost and it decides whether the room is usable in five years.

We cover Three Legged Cross, Verwood, West Moors, Woodlands and the rest of BH21. You are dealing with Bournemouth Roofing & Building, the trading name of Bournemouth Roofing & Building Ltd, registered in England and Wales, company number 15471719, which you can look up in a minute. Thomas Hannen's business, and he is the one who comes out to price the job, not a salesman. A verified customer's 10 out of 10 on Checkatrade. The visit and the written quote are both free.