Roof Batten and Purlin Calculator

Last reviewed How we calculate

Tile roofs hang on horizontal battens, and many metal roofs are fixed to purlins. Both run across the slope in rows at a set spacing. This counts the rows, the total length and the pieces to order.

Battens or purlins for a roof

Rows, total length and pieces of battens or purlins for tile and metal roofs.

This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.

For tile, the gauge set by the tile and pitch. For metal, the purlin spacing in the fastening schedule.

Around 10% on a simple gable. Allow 15% or more where there are hips, valleys or dormers.

Batten pieces —

Rows on each side
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Total batten length
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Tile battens are set out from the eave and ridge details in the tile maker instructions, so the first and last gauges often differ from the rest. Counter-battens, where used, run up the slope over the underlayment and are counted separately. Batten size and fixing are set by the tile or panel manufacturer for your rafter spacing.

How the calculation works

Divide the slope length from eave to ridge by the spacing and round up, then add one for the row at the eave. Multiply by the length of the eave and the number of roof sides for the total length, add waste, and divide by the length of one piece.

A worked example

A roof with a 16 ft slope length and battens at a 13.5 in gauge needs 16 rows on each side. On a two-sided roof 40 ft long that is 1,280 ft of batten, and with 10 per cent waste it takes 118 pieces of 12 ft.

Setting out

For tile, the batten gauge is the tile exposure, and the tile manufacturer sets it for each pitch. The first batten at the eave and the last one at the ridge are usually set by the eave and ridge details rather than the gauge, so the courses in between are adjusted to fit. Counter-battens, where used, run up the slope over the underlayment and are counted separately. Batten size and fasteners depend on the rafter spacing and the weight of the covering, and the tile or panel manufacturer specifies both.

Frequently asked questions

What is batten gauge?

The spacing between tile battens, which equals the exposure of the tile. The tile manufacturer sets the maximum gauge for each pitch.

What are counter-battens?

Strips that run up the slope over the underlayment, under the tile battens. They lift the battens so water and air can pass beneath them.

What size of batten do I need?

It depends on the rafter spacing and the weight of the covering, and the tile or panel manufacturer specifies it along with the fixings.

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