Planted roofs

Turf roofs have featured in the history of building in Northern Europe as long as can be remembered. Resources have been boundless and laying methods relatively simple, though labour-intensive. The high thermal insulation offered by turf roofing made it a strong competitor against slate, tiles and other materials that have subsequently appeared. The thermal insulation makes it popular even in the tropics; there are houses in Tanzania that have a 40 cm-thick layer of earth with grass on the roof. During the last 20 years there has been a renewed interest in green roofing, especially in heavily polluted towns in central Europe, such as Berlin.

Green roofs are usually associated with folk architecture, with only grass species growing. But many other plants can be chosen, even bushes and trees. The roofs do not necessarily have to be sloping but can be almost flat. However, flat roofs should always have a slight gradient to enable drainage.

The roofs are made up of several layers (Figure 15.12). Topmost are the plants with a soil layer underneath. Under this is a filtering layer which prevents earth from falling through, and beneath this is a further layer for draining away excess water. The waterproofing layer is furthest down and must be robust enough to prevent roots from penetrating and water entering the structure. On a sloping roof of over 15°, the filtering or draining layers are unnecessary, but otherwise the roof is built up in the same way (Figure 15.13).

The plant layer. A wide spectrum of plants can be grown on roofs, some of which strengthen the network of roots and thereby the roof itself. They can stabilize it, retain moisture over a dry period and even reduce fire risk. There are evidently many advantages to a varied flora on the roof (see page 163).

The earth layer (Table 15.4). The usual turf for a roof comprises grass that is well bound by its roots, cut up into pieces 30 x 30 cm and about 10 x 15 cm thick. In Norway it is traditional practice to use two layers of turf, the lower layer turned with the roots upwards, the upper with the grass on top. On the ridge, longer pieces of turf are used. Even loose earth can form a top layer, compressed to the same thickness as the turf. On a sloping roof, it is advantageous to lay wire netting with 23 cm of earth on it before compressing the earth and sowing. For a roof with a slope of more than 27° it is necessary to lay assisting structures of battens to hold the turf in place. These are not fixed through the roof covering but at the ridge, to each other, or resting on the eaves of the

15.13

Principles for using turf coverings on a sloping roof. Source: Norwegian Building Research Institute.

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|Table 15.4 Required depth of soil for various vegetation

Plants

Minimum depth of earth

Type of roof

DC

Grass

10 cm

Flat/pitched

< A

Bushes

25 cm

Flat/low pitched

IL

Small trees

45-80 cm

Flat/low pitched

Vegetables

45-60 cm

Flat

roof. The structures do not have to be of a very durable material, as they lose their function when the system of roots binds together.

The earth should have plenty of humus, which can be increased by mixing in compost or peat. A depth of at least 15 cm of earth is recommended. A thinner layer will dry out or erode easily. For sedum species, which are particularly resistant to dry periods, the depth of earth need only be 6 cm. On a roof with not much of a slope or a flat roof it is possible to use a layer of earth without turf for growing vegetables.

In Berlin around the turn of the century, a method of covering roof gardens with 20 cm building waste mixed with earth was used. It was partly introduced to prevent the spread of town fires. A whole series of such courtyards still exist in the quarter of Neu-Koln.

The filter layer, which is necessary on a roof with a slope of less than 15 °C, can be rough sand or sawdust.

The draining layer, needed on a flat roof, can be rough or fine shingle or loose expanded clay pellets.

The waterproofing layer is necessary to ensure that excess water runs off the roof. There are different ways of achieving this, but the most common is bituminous or plastic-based solutions (Table 15.5). Steel, aluminium and cementitious sheeting has a limited usability since they are most often eaten away by the acidic humus.

Flashing around chimneys and pipes that go through the roof are usually of lead or copper. The use of these materials should be kept

Table 15.5 Alternatives for waterproofing of turf roofs

Material

Amount of work

Lifespan

Areas of use

Steel/aluminium sheeting

Low

Short

Sloping more than 15°

Corrugated cement sheeting

Low

Medium

Sloping more than 15°

Bentonite clay with bituminous felt

Low

Unknown

Flat roofs

Slate/tile roof

Medium

Long

Sloping more than 20°

Bituminous felt

Low

Medium/low

All roofs

Polethylene sheeting with bituminous felt

Low

Unknown

Sloping more than 15°

Polyvinyl chloride sheeting with bituminous felt

Low

Unknown

All roofs

Bark from birch

High

Long (30-100 years)

Sloping more than 22°

15.14

Slates for rain protection around chimneys.

to a minimum for environmental reasons. Slates can be used around chimneys on pitched turf roofs (see Figure 15.14).

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