The job
Modelled, not recorded. Every figure on this page is re-run through the calculator it names whenever the site is built; the estimate is worked, but no job was carried out, so the last section gives the mechanism of each likely error rather than a measured overrun.
A flat roof takeoff looks like an area calculation and is really a drainage problem with an area attached. The roof is not flat: it falls to its outlets, and where those outlets are decides the tapered insulation scheme, which decides the insulation volume, which decides the upstand heights, which decides the membrane area.
So the first decision on this job is not a quantity at all. It is whether the existing outlets stay where they are. Keeping them is cheaper and constrains the scheme; moving one can halve the tapered insulation on a roof of this size. That decision is made in the first ten minutes and it dominates the cost.
The build-up here is a WARM roof: deck, vapour control layer, insulation, membrane — insulation above the structure, so the deck stays warm and there is no ventilated void to detail. That is the common arrangement for a recover, and it is why the upstands, the outlets and the thresholds all have to be checked for height before anything is ordered: adding insulation raises the roof surface, and every upstand has to still be high enough above the finished level afterwards.
The area is where estimators under-order, and not because the arithmetic is hard. A flat roof's membrane quantity is the plan area PLUS every upstand, every return into an outlet, every overlap and every seam — and on a small roof with a parapet, the upstands can be a substantial fraction of the plan area.
What was measured, and how
Plan area, corner to corner in both directions
About 8.0 × 5.0 m (26 ft 3 in × 16 ft 5 in), checked on both diagonals
Measured at deck level rather than from a drawing, because a recover works to what is there. Diagonals taken as well — a roof that is out of square changes how sheets lay out and how much is wasted at the edges.
Outlet positions and their invert levels
Two outlets, both on the long parapet side
Located and levelled before anything else, because they are the low points the falls run to. The invert — the level water actually leaves at — matters more than the outlet's position on plan.
Upstand heights available at every edge
Parapet gives ample height; the door threshold is the tight one
Measured from the existing deck to the top of each parapet, threshold, door sill and abutment. This is the constraint that decides how much insulation can be added, not a preference.
The existing build-up, by opening up
Timber deck, existing insulation dry at the opening
A small opening cut to see what is actually there: deck type, whether there is existing insulation, whether it is wet. A recover over wet insulation traps the water permanently.
Every penetration and abutment
One soil vent, one wall abutment along the 8 m (26 ft) side
Counted and measured individually — soil vents, flues, handrail bases, an abutment to the house wall. Each is a detail with its own membrane, and collectively they are where the time goes.
The takeoff, in order
Each step needs something from the one before it, which is why the order is part of the answer.
Set the tapered insulation scheme from the outlets
- Needs
- The outlet positions and inverts, the plan dimensions, and the minimum fall required — all from the measurements above.
- Produces
- The taper layout and, crucially, the minimum and maximum build-up depths. Everything downstream depends on this, which is why it is first. On this roof the fall crosses the 5.0 m (16 ft 5 in) width, from the house-wall abutment down to the outlets on the parapet side. At the 1 in 40 design fall that BS 6229 is commonly read as pairing with a 1 in 80 finished fall, that run adds 125 mm (4.9 in) to the starter at the outlets, so the thickest point lands against the house wall; the 50 mm (2.0 in) starter is an assumption until the board schedule fixes it.
Result175 mm (6.9 in) at the house wall, from a 50 mm (2.0 in) starter at the outlets
Tapered Roof Insulation Slope CalculatorCheck the build-up against the upstand heights
- Needs
- The maximum build-up depth from the taper scheme, against the measured upstand and threshold heights.
- Produces
- Whether the scheme fits. If the threshold is too low for the thickest point, the scheme changes here — before anything is ordered rather than after it arrives.
Quantify the insulation and the cover board
- Needs
- The plan area and the taper scheme's depths, since a tapered layout consumes a volume rather than an area times one thickness.
- Produces
- The board quantities by thickness, and the cover board that protects the insulation from the membrane's fixing and foot traffic. Unlike the tapered boards, the cover board is one thickness over the whole plan, so it really is an area: 14 whole sheets over the 40 m² (431 sq ft), before the cuts at the edges and round the soil vent.
Result14 sheets of 2.88 m² (31 sq ft) board over 40 m² (431 sq ft) of plan
Roof Insulation Cover Board CalculatorQuantify the membrane
- Needs
- The plan area PLUS every upstand, return and overlap — the extents from the measurements, not the plan area alone.
- Produces
- The roll count and, from the roll width against the roof's dimensions, how many seams there will be.
Quantify the seam treatment
- Needs
- The seam count and length from the roll layout in the step above.
- Produces
- The tape and primer. Seams are where a single-ply roof fails, so the layout that minimises them is worth choosing deliberately.
Detail the outlets
- Needs
- The outlet positions and the finished build-up depth, because the outlet has to be dressed down into the falls.
- Produces
- The sump and outlet details. An outlet left proud of the finished falls is a roof that ponds around its own drain.
Check the overflow provision
- Needs
- The roof area, the rainfall intensity and the primary outlet capacity.
- Produces
- The overflow size and position. On a parapet roof this is not optional — a blocked outlet with no overflow puts the water inside the building.
Quantify the deck repairs and the vapour control layer
- Needs
- The deck area, plus whatever the opening-up found needed replacing.
- Produces
- The vapour control layer area including laps and upstands, and the deck material for repairs. On the 40 m² (431 sq ft) deck the lap allowance brings the sheet to 44.0 m² (474 sq ft), inside one of the 93 m² (1,000 sq ft) rolls the calculator counts in; the turn-ups at the parapet and the abutment are extra area rather than lap, and the repair material stays provisional until the strip.
Result1 roll — 44.0 m² (474 sq ft) with laps, at 93 m² (1,000 sq ft) a roll
Vapor Barrier Calculator
The figures
| Step | Calculator | Figure |
|---|---|---|
| Set the tapered insulation scheme from the outlets | Tapered Roof Insulation Slope Calculator | 175 mm (6.9 in) at the house wall, from a 50 mm (2.0 in) starter at the outlets |
| Quantify the insulation and the cover board | Roof Insulation Cover Board Calculator | 14 sheets of 2.88 m² (31 sq ft) board over 40 m² (431 sq ft) of plan |
| Quantify the deck repairs and the vapour control layer | Vapor Barrier Calculator | 1 roll — 44.0 m² (474 sq ft) with laps, at 93 m² (1,000 sq ft) a roll |
The waste factors, and why these ends of the ranges
| Material | Applied | Why |
|---|---|---|
| Membrane | Extents plus laps, then rounded to whole rolls | The roll width against the roof's dimensions dominates. A roof a little wider than a roll means a second roll mostly wasted, and the offcut is rarely usable on a small job. Choosing the roll direction before ordering is worth more than any percentage. |
| Tapered insulation | The scheme's own quantities, with an allowance for cutting at edges | Tapered boards are supplied to a designed layout rather than as a stock item, so the quantity is the scheme's rather than an area times a thickness. The waste is at the perimeter and around penetrations, where boards are cut to the falls. |
| Seam tape and primer | The seam length with a generous allowance | Running short mid-seam is not the same as running short of a board. Seams are the failure point of a single-ply roof and a joined-up seam made in two sessions is a weakness — this is one to over-order. |
| Deck repair material | A provisional allowance, confirmed after the strip | What the deck needs cannot be known from one opening. The honest approach is a provisional quantity and a decision on the day rather than a confident figure taken off an unopened roof. |
Where this estimate is most likely to be wrong
Membrane area
usually underThe plan area is the easy part. Upstands at parapets and abutments, returns into outlets, the dressing around every penetration and the laps between sheets all consume membrane, and on a small roof with a parapet on one side those extents are a substantial fraction of the plan area rather than a rounding.
Narrow it by: Measuring every upstand height and every penetration before ordering, and adding them to the plan area rather than applying a percentage to it.
Deck repairs
usually underNobody knows what the deck is like until the old covering is off. One inspection opening shows one spot, and the areas most likely to have failed are the ones under the old outlets and at the abutments — which is exactly where an opening is least likely to be cut.
Narrow it by: Carrying it as a provisional item with a rate agreed in advance, rather than as a fixed quantity that will have to be varied.
Tapered insulation
either wayThe scheme follows the outlets. Keeping existing outlets constrains the layout and can force a much thicker maximum build-up; moving one outlet can change the volume substantially on a roof this size. The decision is made before any quantity exists, and it moves this line more than any allowance.
Narrow it by: Pricing the taper scheme both ways — outlets as they are, and outlets relocated — before deciding, since the insulation saving can exceed the drainage work.
Detailing time rather than material
usually underThe estimate counts area and the work is in the details. A soil vent, a wall abutment, two outlets and a door threshold are each a separate hand-formed detail, and on a 40 square metre roof they can take longer than the field membrane they interrupt.
Narrow it by: Counting the details as items with their own allowance rather than treating the roof as an area with some edges.
Tools this job needs
Frequently asked questions
- How thick does tapered insulation get on a small flat roof?
- On this roof the water runs 5.0 m (16 ft 5 in) from the house wall to the outlets on the parapet side. At a 1 in 40 design fall the taper rises 125 mm (4.9 in) over that run, so on an assumed 50 mm (2.0 in) starter board at the outlets the build-up reaches 175 mm (6.9 in) at the wall. Any flat base board the U-value needs, plus the cover board and the membrane, adds to that depth.
- How many cover boards does a 40 m² flat roof need?
- Fourteen for this 8.0 × 5.0 m (26 ft 3 in × 16 ft 5 in) roof: 40 m² (431 sq ft) divided by a 1.2 × 2.4 m board of 2.88 m² (31 sq ft), rounded up to whole sheets. A 4 × 8 ft board of 32 sq ft (2.97 m²) also comes to 14. That covers the plan only; the cuts at the parapet, the abutment and round the soil vent come on top.
- How much vapour control layer does a 40 m² warm roof need?
- One roll of the 93 m² (1,000 sq ft) sheet the calculator counts in: the 40 m² (431 sq ft) deck plus a lap allowance of a tenth comes to 44.0 m² (474 sq ft). The turn-ups at the parapet and the house-wall abutment are extra area rather than lap, so they are measured and added, and on a roof this size they sit well inside the same roll. A system VCL sold in another roll size divides into 44.0 m² instead.