Terrace Roofs In Oldham And Chadderton

Key takeaways

• Height above sea level changes the weather a roof sees: more wind, more rain, more freeze and thaw cycles.
• Freeze and thaw is the main mechanism, and it attacks mortar and porous materials rather than sound slate.
• Wind exposure means edges matter more here: ridges, verges and eaves fail before the middle of a slope.
• Stone and slate both cope well with the conditions. Poor mortar and cheap repairs do not.
• The same roof specification does not perform identically across Greater Manchester, and that is worth knowing before choosing materials.

Why altitude changes the roof

As ground rises, several things happen at once. Air temperature drops, so the number of nights each winter that pass through freezing increases. Rainfall generally increases, because air forced upward over rising ground cools and drops its moisture. Wind speeds increase, because there is less shelter and less friction from surrounding buildings and terrain. None of these is dramatic on any given day. Cumulatively they mean that a roof on high ground goes through more cycles of wetting, freezing and wind loading each year than an identical roof a few miles away and a hundred metres lower, and roofs fail by accumulation rather than by single events.

Freeze and thaw, the main mechanism

Water expands when it freezes, and if it is sitting inside a porous material or in a crack, that expansion pushes the material apart. Repeat it a few dozen times a winter and the effect is cumulative: mortar joints erode back, brick and tile faces flake away, and hairline cracks open into gaps. This is why mortar bedded ridges and verges are the first casualties on high ground, and why a repair carried out in a hard cement mix that cannot flex tends to crack sooner here than it would lower down. Sound slate is largely immune, because it absorbs very little water in the first place.

Wind, and why edges matter more

Wind lifts a roof rather than pushing on it, and that lift concentrates at the perimeter. On an exposed site the effect is stronger and more frequent, which is why the ridge, the gable verge and the eaves are where trouble starts on terraces in higher districts. It also means the restraint provided by mortar or by mechanical fixings matters more than it does on a sheltered street. A verge that has lost its mortar in a sheltered position may sit undisturbed for years. The same verge on an exposed row will lose tiles in the first serious gale.

What the housing stock is

Much of Oldham and the surrounding districts is terraced housing built during the town's industrial expansion, in the same broad pattern as elsewhere in the region: pitched roofs, natural slate or stone, mortar bedded ridges and verges, chimney stacks on party walls, and rear additions. What differs is not so much the construction as the conditions it has endured, and the fact that stone was more readily available locally, so it appears on roofs and in walls more often than it does further into the plain. Both stone and slate are durable materials that cope well with the weather here, which is why so many of these roofs are still in service.

Where the mortar mix genuinely matters

Older buildings were constructed with lime based mortars, which are softer than the brick or stone around them, flex slightly with movement, and allow moisture to escape. Modern cement mortar is harder, more rigid and less permeable. Used on an old building it forces moisture and frost damage into the surrounding masonry instead of the joint, which is exactly the wrong outcome anywhere and a faster wrong outcome somewhere with more freezing nights. On a high exposed terrace, asking what mix is being used for repointing or rebedding is not pedantry, it is the difference between a repair that lasts and one that accelerates the decay it was meant to stop.

Moss, and why there is more of it

Higher rainfall, more days of dampness and often more shade mean moss establishes readily on roofs in this part of the region. Moss on a slope is largely cosmetic until it is not: it holds water against the covering, it breaks down into a fibrous material that travels into valleys, gutters and downpipes and packs tightly at bends, and in quantity it adds weight. The judgement about whether to remove it depends on how much there is and where it has got to. Scraping moss off aggressively can damage the covering, and roof coatings sold as a permanent answer to it deserve considerable scepticism.

What to look at, and when

The best time to assess a roof in an exposed position is after winter rather than before it, because winter is what does the damage and spring is when the results are visible. Start at the apex and work down, checking whether the bedding along the top of the roof is continuous or broken. Look at the gable verge, which on an exposed row is the most likely place to find deterioration. Look for slates or tiles that have moved, and for fragments of mortar on the ground, which are the clearest early evidence. Look in valleys and gutters for accumulated moss. And compare with the rest of the row, since houses built together give you a reference for what normal looks like.

Specifying for the conditions

The practical implication of all this is that the standard answer is not always the right answer here. Mechanical fixing at ridges and verges rather than mortar alone, appropriate nail specification, attention to how the eaves and verges are restrained, and a mortar mix suited to the building all matter more on an exposed high site than they do on a sheltered one. It is worth asking a roofer directly how the exposure of the property has influenced what they are proposing. A quote that would be identical for a sheltered street and an exposed hillside has probably not considered the difference.

What the neighbours' roofs tell you

On a row built in one go, the other houses are a free source of information, and on an exposed site that information is unusually good. Roofs on the same row have had identical weather and identical construction, so differences between them reflect either maintenance history or position within the row. The end property takes considerably more weather than a mid terrace, since it has an exposed gable, so its verge and gable wall will generally deteriorate first. Houses at the top of a sloping street are more exposed than those at the bottom. Looking along the row and noting which roofs have been done, which are shedding mortar and where the losses cluster tells you more about your own roof's likely condition and probable timescale than any amount of general guidance written for the country as a whole.

The end of terrace premium

One property in every row takes considerably more weather than the rest, and that is the end house. A mid terrace has neighbours on both sides sheltering it and roofs continuing in both directions, so it presents very little exposed edge. An end of terrace has a full gable standing proud, taking driven rain on a large wall and wind on an exposed verge, with nothing beyond it to slow either down. That means the end property in a row of identical houses will generally need its verge, its gable pointing and its rainwater goods attended to sooner than its neighbours, and it is worth budgeting on that basis rather than assuming the row will age uniformly.

Frequently Asked Questions

Do roofs really wear out faster on higher ground?

They tend to, because they go through more cycles of the things that cause wear: more wind loading, more rainfall and more nights passing through freezing. The damage is cumulative rather than dramatic, so the same roof specification generally needs attention sooner on an exposed high site than on a sheltered low one.

What does freeze and thaw actually damage?

Porous materials and anything holding water in a crack. Mortar joints erode, brick and tile faces flake, and hairline cracks open into gaps. Sound natural slate is largely unaffected because it absorbs very little water, which is one reason it has lasted so well on roofs in this part of the region.

Why does my ridge mortar keep failing?

Exposure accelerates it, and the mix may be wrong for the building. Hard modern cement on an older roof is more rigid than the material around it and cracks sooner, particularly where there are many freezing nights. Mechanical fixing rather than relying on mortar alone is often the more durable answer on an exposed site.

Should I remove the moss from my roof?

It depends how much there is and where it has reached. Moss on a slope is largely cosmetic until it blocks valleys, gutters or downpipes, at which point it causes real problems. Aggressive scraping can damage the covering, and roof coatings marketed as a permanent solution deserve scepticism.

When is the best time to have the roof checked?

After winter rather than before it, because winter causes the damage and spring is when the evidence is visible and the weather suits repair work. Mortar fragments on the ground and displaced tiles at ridges and verges are the things to look for once the worst weather has passed.

Is stone a good roofing material for this area?

It has served for a very long time locally, which is the strongest evidence available. Stone and natural slate both cope well with wet, cold and windy conditions because they absorb little water. What tends to fail around them is the mortar, the fixings and the timber, rather than the covering itself.