We convert a P-shaped conservatory by panelling the long lean-to part and the projecting Victorian or Edwardian part separately, then sealing both to the box gutter where they meet. A 28 m² P-shape roof is usually £6,300 to £7,500 fitted and takes our team 2 to 3 days. Panels suit both halves of the roof.
Two roofs and a box gutter
Seen from an upstairs window, the plan of a P-shape matches its name. Along the back of the house runs a lean-to. Off one end of it, a second section pushes out into the garden, finished with either a bay in the Victorian style or the hipped front of an Edwardian. Most owners use it as two spaces: a dining table in the projection and a sofa, or simply a route through to the garden, along the lean-to.
The lean-to slope meets the flank of the projecting roof along a valley. On most P-shapes that valley is formed by a box gutter: a channel with a flat base, built into the roof. It gathers rain from both roofs and leads it to a downpipe.
Why we look hardest at the box gutter
When a P-shape lets water in, the box gutter is the usual culprit. It has almost no fall, it drains both sections together, and it fills up with leaves and moss. With age its joints open up, and water then gets into the frame or drips into the room.
At survey we inspect it for splits, sagging, pools of standing water and marks on the bars underneath. Whatever repair it needs goes into your fixed written price, so nothing new appears on the day we fit. If you want to look before we arrive, our guide on finding a conservatory roof leak shows the signs of a failing box gutter.
Once the new roof is on, upkeep is easy. Look at the box gutter every autumn after the leaves have come down, and clean it out from a ladder at the edge. Never walk across the panels to reach it. Whatever a roof is made from, a blocked box gutter is the main way water gets into a P-shape.
Panel layout across the two sections
- The lean-to run: long straight panels from the house wall to the front gutter, laid out as on our lean-to conservatory roof page.
- The projection: rectangles on its side slopes, with tapered or angled panels across the bay or hipped front, the same as a Victorian conservatory roof or an Edwardian.
- Along the box gutter: panels from each section stop at the gutter edge and are sealed there, so rain falls into the channel and cannot run back under them.
- Caps and flashing: fresh caps over the projection's ridge and hip bars, plus a new flashing chased in along the full length of the house wall.
Two roof types joined by a box gutter give us a wider mix of panel cuts, trims and seals than any other style we fit. As a result, a P-shape costs roughly 16 percent more per square metre of roof than a lean-to.
Size and price of a typical P-shape
Nearly all P-shapes sit in our large band of 25 m² of roof and over. A common layout we see is a lean-to roughly 6 m wide across the house, with a projection around 3.5 m square on the end. Altogether that layout has roughly 28 m² to cover.
| Style | Small (0 to 12 m²) | Medium (12 to 25 m²) | Large (25 m² and over) |
|---|---|---|---|
| P-Shape | £3,500 to £4,150 | £4,600 to £5,450 | £6,300 to £7,500 |
Our large roofs start at £5,000, and that 28 m² example normally works out at £6,300 to £7,500. The price takes in removing the old roof, panels in the colour you choose, fitting, and our 10 year insurance backed warranty through HomePro. On top you can add glazed roof lights (£395 each), LED spotlights (£345) or new gutters and fascia (£295). Our conservatory roof panel prices page breaks down how each part of the cost is made up.
What a 28 m² roof costs to heat
The larger the roof, the more heat it lets out. With 20°C inside and 0°C outside, 28 m² of 16 mm polycarbonate loses about 1,288 W, close to what a small fan heater puts out. Through our 75 mm panels the same area loses about 241 W.
| Measure for 28 m² of roof | 16 mm polycarbonate | Eco Tech 75 mm panels |
|---|---|---|
| Heat escaping on a 0°C night | about 1,288 W | about 241 W |
| Energy lost over an English heating season | about 3,323 kWh | about 621 kWh |
If you kept the room warm all winter, the gap is worth about £711 a year with electric heating at 26.32p per kWh. With gas at 7.97p it is nearer £215. To run the sum for your own room, use our guide to what a conservatory costs to heat.
How we fit a P-shape
Expect us on site for 2 to 3 days. Our fitters usually complete one section before opening the other, so by the end of day one at least one half is roofed and watertight. We deal with the box gutter and the valley where the sections meet while both sides of that edge are open, which lets us seal it properly from each side.
Most single storey P-shapes are fitted without any scaffold. If the lean-to part sits beneath bedroom windows, we measure the clearance at survey, exactly as we would on a stand-alone lean-to.
Daylight across a large room
A room this size has corners well away from any window, most of all at the back of the lean-to against the house. That is where a roof light earns its place, and plenty of P-shape owners pick two or three. The projection normally has glass on three sides, so it rarely needs a roof light. Our roof lights and spotlights page gives sizes and suggested positions.
Planning permission
You will not normally need planning permission, because the conservatory keeps its footprint and height. Check with the council beforehand if your house is listed or sits in a conservation area.
Questions people ask
What does the box gutter on a P-shape do?
It is the channel with a flat base that sits in the valley between the lean-to and the projection. It drains both roofs, so we inspect it closely at survey and price any repair into your quote.
Why is a P-shape a longer job than other roofs?
It is bigger and combines two roof types with a box gutter between them. Most take us 2 to 3 days, and we complete one section before opening the next.
What will a large P-shaped roof cost?
Our large roofs start at £5,000. A typical P-shape with 28 m² of roof usually comes to between £6,300 and £7,500 fitted, with roof lights and other extras on top.
How many roof lights suit a P-shaped conservatory?
Most owners choose two or three, mainly in the lean-to part by the house wall, since it is the furthest point from the windows.
