Question 1

Why do roofs fail faster in the villages above Macclesfield?

Accepted Answer

Frost and wind, both of which increase sharply with height. The Met Office records around 20 air-frost days a year on the North West coast against over 75 in the higher Pennines, with about half a degree of cooling per hundred metres — and freeze-thaw is precisely what destroys mortar bedding and spalls porous tiles. There is a genuine and measurable gradient across this patch, and it explains a lot of the difference between two apparently identical roofs. Altitude first. Macclesfield town centre sits low; the ground east of it rises quickly. Kerridge Hill is 313 metres, Tegg's Nose 380, Shutlingsloe 506 and Shining Tor 559. Temperature falls roughly half a degree for every hundred metres of that, and the Met Office North West figures show air frost ranging from about 20 days a year on the coast to over 75 in the higher Pennines. Their station averages make the point locally: Buxton 51.5 air-frost days a year against 40.0 at low-lying Woodford. Freeze-thaw is what that does to a roof. Water gets into a hairline crack in mortar bedding or into the body of a porous tile, freezes, expands, and opens the crack a little further. Repeat that thirty extra times a winter and the mortar at a ridge or a verge fails years sooner, and a porous concrete or underfired clay tile loses its face in flakes. Wind is the second factor and it compounds the first. Exposure rises with height and with how open the site is, and wind lifts a roof from its edges — the verges, the ridge, the eaves — rather than from the middle. So the details that frost has already weakened are exactly the ones the wind then tests. That is why storm damage in Rainow, Kerridge, Langley and Wincle so often turns out to be a ridge whose bedding cracked months earlier. Driven rain is the third. An exposed roof takes rain horizontally as well as vertically, which pushes water up under laps and past headlaps that would be perfectly adequate on a sheltered slope. It is why the headlap and the batten gauge are calculated against exposure rather than set by habit. The practical consequences are straightforward. Roofs up the hill want a fixing specification worked out against the actual site rather than copied from a job in town. Dry-fixed ridges and verges are a better answer than mortar there than they are anywhere else. And a pre-winter check earns its keep in Sutton Lane Ends, Macclesfield Forest and Wildboarclough in a way it does not in Broken Cross.

Question 2

Does altitude affect a roof?

Accepted Answer

Frost and wind, both of which increase sharply with height. The Met Office records around 20 air-frost days a year on the North West coast against over 75 in the higher Pennines, with about half a degree of cooling per hundred metres — and freeze-thaw is precisely what destroys mortar bedding and spalls porous tiles. There is a genuine and measurable gradient across this patch, and it explains a lot of the difference between two apparently identical roofs. Altitude first. Macclesfield town centre sits low; the ground east of it rises quickly. Kerridge Hill is 313 metres, Tegg's Nose 380, Shutlingsloe 506 and Shining Tor 559. Temperature falls roughly half a degree for every hundred metres of that, and the Met Office North West figures show air frost ranging from about 20 days a year on the coast to over 75 in the higher Pennines. Their station averages make the point locally: Buxton 51.5 air-frost days a year against 40.0 at low-lying Woodford. Freeze-thaw is what that does to a roof. Water gets into a hairline crack in mortar bedding or into the body of a porous tile, freezes, expands, and opens the crack a little further. Repeat that thirty extra times a winter and the mortar at a ridge or a verge fails years sooner, and a porous concrete or underfired clay tile loses its face in flakes. Wind is the second factor and it compounds the first. Exposure rises with height and with how open the site is, and wind lifts a roof from its edges — the verges, the ridge, the eaves — rather than from the middle. So the details that frost has already weakened are exactly the ones the wind then tests. That is why storm damage in Rainow, Kerridge, Langley and Wincle so often turns out to be a ridge whose bedding cracked months earlier. Driven rain is the third. An exposed roof takes rain horizontally as well as vertically, which pushes water up under laps and past headlaps that would be perfectly adequate on a sheltered slope. It is why the headlap and the batten gauge are calculated against exposure rather than set by habit. The practical consequences are straightforward. Roofs up the hill want a fixing specification worked out against the actual site rather than copied from a job in town. Dry-fixed ridges and verges are a better answer than mortar there than they are anywhere else. And a pre-winter check earns its keep in Sutton Lane Ends, Macclesfield Forest and Wildboarclough in a way it does not in Broken Cross.

Question 3

Why is my roof worse than my friend's in town?

Accepted Answer

Frost and wind, both of which increase sharply with height. The Met Office records around 20 air-frost days a year on the North West coast against over 75 in the higher Pennines, with about half a degree of cooling per hundred metres — and freeze-thaw is precisely what destroys mortar bedding and spalls porous tiles. There is a genuine and measurable gradient across this patch, and it explains a lot of the difference between two apparently identical roofs. Altitude first. Macclesfield town centre sits low; the ground east of it rises quickly. Kerridge Hill is 313 metres, Tegg's Nose 380, Shutlingsloe 506 and Shining Tor 559. Temperature falls roughly half a degree for every hundred metres of that, and the Met Office North West figures show air frost ranging from about 20 days a year on the coast to over 75 in the higher Pennines. Their station averages make the point locally: Buxton 51.5 air-frost days a year against 40.0 at low-lying Woodford. Freeze-thaw is what that does to a roof. Water gets into a hairline crack in mortar bedding or into the body of a porous tile, freezes, expands, and opens the crack a little further. Repeat that thirty extra times a winter and the mortar at a ridge or a verge fails years sooner, and a porous concrete or underfired clay tile loses its face in flakes. Wind is the second factor and it compounds the first. Exposure rises with height and with how open the site is, and wind lifts a roof from its edges — the verges, the ridge, the eaves — rather than from the middle. So the details that frost has already weakened are exactly the ones the wind then tests. That is why storm damage in Rainow, Kerridge, Langley and Wincle so often turns out to be a ridge whose bedding cracked months earlier. Driven rain is the third. An exposed roof takes rain horizontally as well as vertically, which pushes water up under laps and past headlaps that would be perfectly adequate on a sheltered slope. It is why the headlap and the batten gauge are calculated against exposure rather than set by habit. The practical consequences are straightforward. Roofs up the hill want a fixing specification worked out against the actual site rather than copied from a job in town. Dry-fixed ridges and verges are a better answer than mortar there than they are anywhere else. And a pre-winter check earns its keep in Sutton Lane Ends, Macclesfield Forest and Wildboarclough in a way it does not in Broken Cross.

Why do roofs fail faster in the villages above Macclesfield?

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