Ridge Cap Calculator

How the ridge cap calculator works

Ridges and hips are capped with the same pieces: cut or pre-formed ridge cap shingles on an asphalt roof, ridge tiles on a tiled one. Each piece covers a set length along the ridge, so the count is the total length divided by that cover, with a little extra for cuts and breakages, rounded up to whole pieces and then to whole packs:

N = ⌈L × (1 + w) ÷ c⌉, packs = ⌈N ÷ p⌉

L is the total length of ridges and hips, w the waste as a decimal, c the length each cap covers and p the pieces in a pack or bundle. The one input that matters most is c, and it is not the length of the piece.

Cover per cap: exposure, not length

Ridge cap shingles overlap, and only the exposed part of each one counts. GAF's Seal-A-Ridge sell sheet gives an exposure of 6 2/3 in (169 mm), about 45 pieces to a bundle, and four bundles to roughly 100 lineal feet; its IR version is 5 in (127 mm) with about 60 pieces a bundle. The two agree: 45 × 6 2/3 in and 60 × 5 in both come to 300 in, or 25 ft per bundle. The US default uses 6 2/3 in, and the calculator gives exactly four bundles for 100 ft with no waste.

Ridge tiles on a tiled roof are laid end to end rather than lapped, so the cover is close to the tile length. BMI's Redland half-round ridge tile is 450 mm long, which is the metric default. Other profiles differ; check the length or the cover per metre on the product sheet for yours.

Worked through

With the US defaults, 33 ft of ridge at 6 2/3 in exposure and 5% waste is 33 × 12 × 1.05 ÷ 6 2/3 = 62.4, so 63 caps, or two bundles of 45 with 27 to spare. In metric, a 10 m ridge of 450 mm tiles with 5% waste is 10,000 × 1.05 ÷ 450 = 23.3, so 24 tiles. A hipped roof with an 8 m ridge and four 5 m hips has 28 m to cover: 66 tiles at the same cover and waste.

Measure along the ridge and each hip, not across the roof plan. On a hipped roof the hips are longer than they look from the ground, because they slope down to the corners.

If the roof is still on paper, a hip can be worked out from the plan. On a regular hipped roof, where every slope has the same pitch, each hip runs at 45° across the plan, so its length on plan is the half-span times √2, and its true length adds the rise: hip = √(2r² + h²), where r is half the span and h the rise. For a building 8 m across with a 35° pitch, r is 4 m and the rise is 4 × tan 35° = 2.80 m, so each hip is √(32 + 7.84) = 6.31 m. Four of them add over 25 m of capping to the ridge, which is why hipped roofs need so many more ridge tiles than gables.

Fixing the ridge

In the UK, the code of practice for slating and tiling, BS 5534, no longer accepts mortar alone. Marley's guide to its fixing requirements explains that since the 2015 amendments, hip and ridge tiles have had to be mechanically fixed whether or not mortar is used, and the Roof Tile Association's ridge details note that this is usually done with a dry-fix ridge system, which also lets the roof space ventilate under the ridge. Marley adds that BS 5534 now points to BS 8612 for dry-fix ridge and hip systems, and that its recommendations may not suit heritage roofs, where the approach is agreed with the local planning authority and conservation advisers. Elsewhere, fix ridge caps and tiles as the manufacturer and your local code require. GAF calls for about two nails per Seal-A-Ridge piece, and the calculator counts them; where a ridge vent is fitted, follow both the vent and the cap maker's fixing instructions.

Choosing the waste allowance

The default is 5%. Ridge caps are cut at the ends of the ridge, where hips meet the ridge and at any change of direction, and a few crack or chip in handling. A simple gable ridge can go lower; a hipped roof with several junctions justifies more. The pack rounding usually adds some spare as well, and the result shows how many.

What this tool does not do

Apart from the nails for shingle caps, it does not count end caps, hip irons, ridge vent, dry-fix kits or mortar, which are ordered separately, and it does not work out the roof slopes or the length of hips from a plan. Working on a roof ridge is work at height; use proper access and follow the safety rules where you are.

Using this calculator with other BuildMetricLab tools

The hip roof calculator works out the hip lengths, the roof pitch calculator the slope, and the roof ventilation calculator the free area a ridge vent needs to provide. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.