Spotting the P
Look down on the floor plan and the letter shows itself. The long straight leg hugs the rear wall, often as a lean-to or a hipped run, and at one end it swells into a rounded, faceted bay: the bowl of the P. The bay has its own ridge and hub, fanned like a Victorian.
Some P-shapes wrap round a corner of the house instead, giving an L with a bay on the end. Either way, wherever two roof planes meet in a trough rather than a peak, there is a box gutter carrying water out between them. That gutter is the heart of this page.
The bowl
- Faceted bay with wedge panels
- Radial bars meeting at a hub
- Cresting and finial if fitted
The leg
- Long straight run of square panels
- Single slope or hipped ends
- Joined to the bowl by a box gutter
Cutting and joining a P-shaped roof
The bay is built exactly as a Victorian would be: wedge panels cut to each facet, sealed at the hub, capped. The leg is built like a lean-to, with square ended panels between parallel bars and a flashing into the brick where it meets the house.
Where they meet, panels from both sides finish over the box gutter, cut to the angle of the trough. Each edge overhangs into the gutter so water drips in rather than tracking back under the panel, and the joints along it are sealed on dry metal and covered. The ridge of the bay ties into the leg with its own cap and flashing.
Fitting the leg is quick. The bay and the junction take most of the time, which is why the shape costs more per square metre than either half would alone.
P-shape guide prices
Prices are for the panel roof fitted, before extras. Because a P-shape combines two roofs, most are medium or large; a 6 m wide P is a typical large one. The real figure is fixed in writing at the survey, and nothing is paid before it.
A new box gutter, if the old one is past saving, is priced at the survey like any other repair, along with roof lights, spotlights and frame work. All five shapes are compared on the cost page.
| Roof area | Guide from | Guide range | |
|---|---|---|---|
| Small | up to 12 m² | £3,550 | £3,550 to £4,250 |
| Medium | 12 to 25 m² | £4,700 | £4,700 to £5,650 |
| Large | over 25 m² | £5,900 | £5,900 to £7,100 |
The box gutter decides how the roof behaves in rain
On every other shape, water runs off the outside edges. On a P-shape a share of it runs inwards, off two roof planes at once, into a trough that sits above your room. In a downpour that trough carries more water for its size than any gutter on the house, and if its outlet is blocked or its joints are open, the overflow comes indoors.
That is why so many old P-shapes have a stain along one line of the ceiling. At the survey we look inside the box gutter, at its joints, its fall and its outlet, and tell you whether it can stay. If it cannot, it is replaced while the roof is open, which is the only sensible time to do it.
Heavy rain is not rare. The average town in our Rain Ledger logged the figures below; your own town's numbers are a click away.
- 26
- hours over 4 mm a year, average town
- 6
- downpour nights a year, average town
Two days, closed in overnight
A P-shape is the shape we most often book over two dry days. The order of work is planned so the house is never left open to the night.
A typical two day P-shape
- Day one, morningRoom cleared, the leg stripped bay by bay and panelled, so the long run is closed in early.
- Day one, afternoonThe box gutter is opened, checked or replaced, and the leg panels are finished over it.
- End of day oneAnything still open on the bay is panelled or sheeted over before we leave.
- Day twoThe bay facets go on, the hub is sealed and capped, and the ridge is tied into the leg.
- FinishCaps, cresting and flashings checked, gutters cleared, old roof taken away.
Is a P-shape worth doing?
It is the dearest shape, but it is also usually the biggest room, and big rooms under polycarbonate lose the most heat. The panels hold heat at about 0.43 against roughly 1.6 to 2.4 for old 25 mm sheets. If the box gutter is the only leak and every other part still holds, a gutter repair alone may be the cheaper answer, and we will tell you so.
Read the fitting process for what the two days feel like from inside the house, then start a quote when you are ready.
