How to Take Off a Timber Roof: Trusses, Battens and Covering

Why roof orders go wrong

If you ask ten builders which part of a takeoff they get wrong most often, most will say the roof. It is not that the arithmetic is hard. It is that a roof is measured on plan and built on slope, and every single quantity in it depends on a pitch angle that someone reads off a drawing and quietly rounds.

Two degrees of error on a 35 degree roof changes the slope length by about 3 percent. That sounds harmless until you apply it to battens, membrane, tiles and every metre of verge and ridge, and then multiply by a house.

This guide takes a roof apart in the order it is built: structure, membrane, battens, covering, edges. It gives the multipliers you need, a full worked example, and the three or four places where an order usually falls short.

Measure the plan, then convert to slope

You measure plan dimensions because that is what drawings give you. You order materials in slope length because that is what gets nailed on.

The conversion is slope length = plan length divided by the cosine of the pitch. Rather than reach for a calculator every time, learn the multipliers:

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A 30 degree roof needs 15.5 percent more material in the slope direction than its plan length suggests. On a 7 m deep roof that is an extra 1.09 m of every rafter, batten course and tile course.

For a lean-to or a mono-pitch, measure the actual rafter length directly if the drawing gives it, and only use the multiplier when it does not.

Rafters and trusses are priced differently

A cut roof is built on site from loose timber: rafters at 400 or 600 mm centres, a ridge board, purlins and ceiling joists. It is measured in linear metres by section, and it needs a carpenter on site for days.

A trussed roof arrives as factory made units, each a triangular frame. You count them.

Truss count = (length of roof divided by spacing) plus one, and then you add the gable frames, which are different units again, and any specials such as a raised tie or an attic truss.

Spacing is normally 600 mm. A 9.0 m long roof at 600 mm centres needs (9.0 / 0.6) + 1 = 16 trusses, plus two gable ends if the supplier does not include them in the run, plus a girder where the design needs one.

Two practical points that affect the order. Trusses are delivered as a bundle and lifted, so access decides whether they arrive whole or in sections. And trusses are engineer designed, so you cannot substitute a different spacing to save money without going back to the designer.

Wall plate, ridge and purlins

The wall plate is the timber bed the roof sits on, measured as the perimeter of the supporting walls, in linear metres, at the specified section (often 100 x 50 mm).

The ridge is one length along the apex, plus a little for the ends. On a hipped roof there is no ridge as such, and instead you have hip rafters or hip trusses, each counted.

Purlins run horizontally to support rafters where spans are long. They are measured in linear metres, and they are easy to forget because they sit inside the roof and never appear in a photograph.

A quick sanity check: on a simple gable roof of about 120 m² of slope, structure usually lands between 0.05 and 0.08 cubic metres of timber per square metre of slope. If your figure is three times that, you have probably counted rafters as if they were 600 mm apart along the slope rather than along the eaves.

Membrane, battens and counter battens

The breather membrane covers the whole slope, plus a lap. Rolls are usually 1.5 m wide by 50 m long, so 75 m² per roll, and laps of 100 mm horizontal and 150 mm vertical eat 8 to 10 percent.

Battens are the part of the order that surprises people. Battens do not follow the tile count, they follow the tile gauge, which is the exposed height of each course. Gauge comes from the tile manufacturer's data and it is always less than the tile length because of the headlap.

Batten linear metres = (slope length / gauge) x number of battens per course x width of roof. In practice, for a 35 degree roof with a 265 mm gauge and a 7 m wide slope:

• Courses = 7.0 x 1.221 / 0.265 = 32.3, so 33 courses
• Batten per course runs the width of the roof, so 33 x 9.0 = 297 m
• Add 10 percent for cutting and the ridge: 327 m

That is 33 lengths of 10 m batten, or roughly 66 lengths of 5 m, and it is the single line most often underestimated on a roof order.

Counter battens, used where the membrane must drain behind the batten, run up the slope at the same spacing as the rafters. They add another 8 to 12 m of batten per square metre of wide roof, so price them separately and do not fold them into the batten figure.

Covering: count the cover, not the tile

Tiles and sheets are ordered by the square metre of finished roof, and the number of units per square metre comes from the manufacturer's cover figure, not from the nominal size of the unit.

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For 120 m² of slope with concrete interlocking tiles at 10 per square metre, that is 1,200 tiles, plus 5 percent for cutting at hips and valleys, verges and the ridge.

Metal sheeting is measured differently again. A sheet of 1.0 m cover and 3.0 m length covers 3.0 m², but the side lap means the effective cover is 0.95 m, and end laps on long slopes take 150 to 200 mm per sheet. Work from effective cover, or you will order one sheet short on every run.

Ridge, hip, verge and valley

These are linear items and they are where a roof order quietly goes wrong, because they are cheap per metre and nobody checks them.

• Ridge: the length of the apex, plus one unit for the end
• Hips: measured down the slope on each hip line, allowing for the angle of the cut
• Verge: along each gable edge, both sides of the roof
• Valley: along each intersection, plus a valley liner or tray
• Eaves: along the gutter line for ventilation and tray

For a hipped roof, hip length = plan hip length multiplied by the same slope multiplier. A hip that is 4.0 m on plan on a 35 degree roof is 4.9 m on slope, and it needs dry hip units or mortar, plus a hip batten.

Fascias, soffits, gutters and downpipes

The eaves detail is a set of linear items that people either price comprehensively or badly:

• Fascia board: eaves length plus gable length
• Soffit: eaves length multiplied by the soffit width
• Bargeboard: gable length, both sides if both are exposed
• Gutter: eaves length plus 5 percent for outlets and joints
• Downpipes: from gutter to drain, counted per elevation
• Gutter brackets at 600 mm centres, so eaves length divided by 0.6

On a small house this group is often 8 to 12 percent of the roof cost, and it is invisible in a per square metre rate.

A worked example

Take a simple gable roof. House 9.0 m x 7.0 m on plan, pitch 35 degrees, concrete interlocking tiles, trussed, with a 400 mm overhang at the eaves and 250 mm at the verges.

• Slope length = 7.0 x 1.221 = 8.55 m, plus the eaves overhang 0.40 m = 8.95 m
• Slope area one side = 8.95 x (9.0 + 2 x 0.25) = 8.95 x 9.5 = 85.0 m²
• Total slope area = 170 m²

From there:

• Tiles at 10 per m² = 1,700, plus 5 percent = 1,785
• Membrane: 170 m² plus 10 percent laps = 187 m², so three 75 m² rolls
• Battens: courses = 8.55 / 0.265 = 32.3, so 33 courses. Batten length = 33 x 9.5 = 313.5 m, plus 10 percent = 345 m, which is 35 lengths of 10 m
• Trusses: 9.5 m / 0.6 = 15.8, so 17 trusses
• Ridge: 9.5 m of ridge units, at the manufacturer's rate of about 3 units per metre plus one
• Verge: 2 x 8.95 = 17.9 m of dry verge
• Eaves: 2 x 9.5 = 19 m of eaves tray and ventilation
• Fascia: 19 m of eaves plus 17.9 m of gable = 36.9 m
• Gutter: 19 m plus 5 percent = 20 m, with 33 brackets
• Downpipes: two, one per elevation, each about 5.5 m to the drain

In metric that is a complete order. Run the same roof in imperial and the numbers move by less than a percent, which is the check worth doing when a merchant quotes in one unit and you measured in the other.

Waste, deliveries and the awkward bits

Allow 5 percent on tiles for a simple gable, 10 percent on a hipped or valley roof, and 15 percent where there are dormers. Allow 10 percent on battens and membrane, and 1 extra truss for damage and for the one that gets cut wrong.

Roof materials arrive on a lorry with a hiab or a forklift, and tiles arrive on pallets that cannot be broken. Plan where the pallets land, because a pallet of 1,800 tiles is close to a tonne and it is not moving again by hand.

The errors worth checking for

1. Using plan area instead of slope area, which under-orders everything by 15 to 55 percent depending on pitch
2. Ordering battens from tile count instead of gauge
3. Forgetting counter battens on a ventilated cold roof
4. Counting hips as plan length
5. Ignoring the eaves overhang, which adds half a metre to every rafter and every batten course
6. Missing the dry fix items at ridge, hip and verge, which are now specified in most markets
7. No allowance for the valley liner

Check those seven and the order is usually right. They are the same seven mistakes that have been made on roofs for decades.

Frequently asked questions

How many trusses do I need for a 10 m roof? At 600 mm centres, 18, including the gable frames. Check whether the supplier counts the gables separately.

How do I work out slope length from plan length? Divide the plan length by the cosine of the pitch, or multiply by the figure in the table above. At 35 degrees, multiply by 1.221.

How many battens per square metre of roof? It depends entirely on the tile gauge. At a 265 mm gauge on a 7 m slope, you need roughly 1.7 linear metres of batten per square metre of slope.

How many tiles per square metre? About 10 for concrete interlocking tiles, 60 for plain tiles, and 17 to 20 for natural slate at a 500 x 300 size.

Should I allow for scaffold in the roof estimate? Yes, as a preliminaries item. Roof work above first floor level cannot be done safely without it.

Do I need a vapour barrier under the tiles? You need a breather membrane on a cold roof and a vapour control layer on the warm side of the insulation. They are different products serving different purposes.

Run the numbers in FRELUX

The FRELUX roofing calculator takes plan dimensions and pitch, converts to slope, and returns tiles, battens, membrane, trusses and the linear items at the edges, priced from your market's price book. Change the pitch and the whole order updates, which is the fastest way to see where a two degree error lands.