Are roofs in Manchester, England, especially suitable for moss growth? Cities tend to be warmer and drier than the surrounding countryside. High summer temperature, reduced rainfall, and increased evaporation should facilitate growth and favour drier, less fertile species. In contrast, other factors may increase growth: human activity, changes in land use, increased roof surfaces, other changes in local climatic conditions, and changes in the biota. Manchester is experiencing a shift in land use within reach of the city, with a large area of improved pastureland being converted into a sporting destination for basketball and other summer sports.
Local climate conditions may thus become more favourable for growth. Mosses tend to grow on wet surfaces and are little affected, at least directly, by temperatures above 0 °C. Rooves are 40–80 °C hotter than air temperature in summer, and conditions may thus be more favourable for growth. Heat, drought, and nutrient accumulation should negatively impact development.
Historic climate data and current climate modelling suggest that the leaves of some plants, especially cereals and other annual crops, will become thinner and that soils will dry out sooner in the summer growing season. Atmospheric nitrogen deposition has accelerated the conversion of the heathland in the Peak District National Park to a forest monoculture.
What affects moss growth
Mosses like wet places. They can go through periods of dryness, but being too dry for too long can stop them growing. Other essential conditions are the presence of rain, light and nutrients. In particular, different species need different amounts of these to develop their best.
Rain might seem like a good predictor of moss growth, but this can be misleading. If a place is either very wet or very dry, this can limit growth. Instead, total moss growth is better related to the number of days with rain, rather than how much rain falls. Light is also essential. Moss growth is limited by too much shade, but can also be limited by too much light. Mosses usually grow fastest when there is little shade but also some cloud cover.
Another critical factor is the presence of nutrients. Mosses typically use nutrients from rainfall, particularly nitrogen and phosphorus. Roofs normally receive little nitrogen from external sources, so the nutrient content of rainwater is essential. In general, more nutrients mean more moss.
Climate and Manchester factors
Moss prefers wetter, cooler climates, so it grows better in Northwestern Britain and Northwestern and Western Ireland than elsewhere in Britain. In winter, the warm Gulf Stream pushes moist, mild air towards Britain, but when it cools and drops rain in North West Britain, the dry area on the east coast often sees snow. On clear winter nights when the wind drops, frost rapidly covers surfaces. Near Manchester, the number of frost days is low, giving moss more time to grow.
Moss declines when summers are dry. Near Manchester, intense, dry winds from the east and south-east in autumn and winter rapidly reduce moisture levels in the sandy soil, cutting fresh growth and spore production. Moss growth on rooftops and trees appears to be limited by moisture. Although Manchester is drier than the North West as a whole, moss should grow more rapidly on roofs than in other cities because factors—heat, darkness at night, and polluted air—favour faster growth and lower death-rates.
Roof conditions and moss growth
Mosses can grow anywhere in suitable conditions. On roofs, they are often unwanted but can grow quickly, not only in size but also in numbers. Several roof conditions can increase moss growth.
Mosses tolerate shade and can therefore grow on roofs and walls where trees provide shade and moisture. On flat roofs, debris can trap nutrients, substrates can hold water and sediment, and shade can be provided. Moss is more abundant in these conditions. Mosses can also grow on roofs where birds defecate, since their droppings offer nutrients.
Why roofs in Manchester may see more moss
So far, we have found that moss growth depends on many factors, including the weather and the location where it grows. Manchester has the right conditions for moss, but not every part of the city has the same conditions. Roofs are not the same either. Some roofs are covered in tiles, others in slats, some are plain, while others are dotted with chimneys and other structures. The conditions on different roofs may result in varying amounts of moss. For these reasons, roofs in Manchester may be getting more moss than roofs in other cities.
The weather in Manchester is ideal for testing moss growth on roofs. The city gets plenty of rain, but it is not always windy. The roofs in and around Manchester are also different: some are plain, others are hilly, some are covered with slats, and some with tiles. Moss likes a bit of shadow to grow well. It may be that the roofs that get less sun grow more moss, but it is hard to tell. Roofs with chimneys and other structures may cast shadows on the roof, making it messy. In some places, moss can get food from birds and other animals.
How to reduce moss growth
You can make your roof less hospitable to moss in a few ways: keep air moving, reduce rainfall so the roof stays on the dry side, reduce wind that brings in rain, and lower the temperature or reduce light. None of these is easy or practical.
Air movement helps dry the moss out. Air can easily pass over and under slats or tiles, allowing them to dry quickly. Moss thrives where rainwater stays for days, not where it dries out. Growth slows during dry spells but speeds back up as soon as rain returns.
Hedges, trees, walls, or fences can prevent rain from falling on roofs. Slope helps reduce the time it takes for rainwater to flow down and off. Where the moss remains dry for long enough, growth slows relative to the surrounding areas.
Trees or hills can block the wind, reducing the amount of rain blowing onto the roof. Wind reduces moss growth by drying it out. Lichens tolerate wind, but moss does not, so growth drops in windy places.
Conclusion
Moss usually grows faster in wet conditions due to the high moisture content. Moss is more abundant on shaded roofs, especially when the roof surface temperature is below 15 °C. Moss growth is higher on roofs that receive little or no wind.
Meteorological conditions in a given area will affect the rate of moss growth on roofs, as well as the condition of the roofs themselves. These latter conditions result from the roofs' exposure or orientation, or from terrain above that casts shadows on the roofs. Therefore, if roofs on the southern side of a valley remain under the shadow of hills or mountains during most of the day, the absence of solar radiation probably reduces the rate of moss growth. These relationships should occur in Manchester, where it is very cloudy, and most roofs are well exposed to the prevailing rainy wind. Since roofs have shallow slopes, raindrops remain on them for a long time, and the humidity is often 100%.
As all these conditions can be more favourable to moss growth in Manchester than in other regions, assessing the rate of moss growth on roofs there adds scientifically interesting insights. Furthermore, although moss growth may enhance the semi-natural character of the city landscape, its growth on roofs can also pose serious problems for maintenance and preservation, contributing to the degradation of substratum materials and wood structures.
