Due to its beautiful looks, a turf roof fits the Finnish landscape. A correctly implemented structure is a tight, durable and soundless water roof. A timber-constructed summer house, a sauna at the shore of a lake or a holiday apartment in the middle of nature can all be crowned by a protective turf roof.
Due to its thickness and porosity, a turf roof offers good heat exchange balance, so that the fast weather variation and the differences in night and day temperatures are not felt particularly rapidly in the below spaces.
The upper bottom structures meant for annual use or randomly heated during cold periods must be insulated and aired correctly. The recommended airing gap is at least 100 mm. The lower eaves need replacement ventilation holes and the ridge needs sufficient removal holes.
The ventilation air needs to move under the roof, also parallel to the ridge. When measuring the roof chairs and the underlay sheets, the pressure of the roof must be noted. The pressure is quite high, especially when wet. The understructures are protected with water insulation, but in the edge structures (eaves), the wooden structures preventing the leaking of the turf and the soil must be protected from moisture and rotting.
Split logs and sawed planks may be used at the eaves to support the turf layers. The supports of the eave logs can be made from sawed wood, but for a particular look, crooked branches or parts of roots, obtained from nature, can also be used. The turfside edges of the eave logs or planks should be protected from moisture when used at membrane or plinth lanes. This also includes confirming that there's a sufficient gap for water removal between the lower edge of the supports and the water insulation of the roof.
The most suitable slope of a turf root is 1:3–1:10. On roofs steeper than 1:3, the problem is the maintenance of peat on the roof without leaking (the leaking may be prevented with, for instance, a wooden grate under the turf or in it) and the possible underground drain pebbles. At roofs less sloped than 1:10, the turf roof is not very visible unless the landscape conditions are favourable. The technical minimum steepness is 1:80. The ventilation hole must be larger the less steep the roof is, as, on a steep roof, the ventilation does not work as well as on a steep roof, regarding gravity.
The ventilation of the turf roof must work from between the heat insulations and the underlay sheet, just as on other roofs.
If the heat insulation is installed between the roof chairs, the roof ridge will have a roof-direction ventilation channel created. The air is removed from the ends of the channel or using underpressure fans.
As water insulation, under the turf roof, SBS elastomer bitumen membranes are used. The simplest option is using self-adhesive Katepal Balcony Membranes. Balcony Underlay (TL2) and Balcony Top Sheet (TL2) form a VE80 use class water insulation, without hot work (See the Work Method 1 image)
If those performing the insulation work have a valid roof and water insulation hot work card (black), the insulation can also include professional products, such as welded Katepal elastomer bitumen membranes (see Work Method 2 image).
In the work method 2, the underlay is a membrane of the TL2 class: K-MS 170/3000, K-TMS 170/3300 or K-MS 170/4000, depending on the ceiling's lower material and the selection of the work method. On steep roofs, you may also use a TL3 class membrane.
The top sheet, in the way of operating 2, there is always a TL2 membrane, usually K-PS 170/5000. The top sheet may also be a K-MS 170/4000 underlay, if the membrane has been protected from ultraviolet radiation at all places – including at border areas.
On top of water insulation, there needs to be a root prevention layer and an underwater drain layer. This is most easily achieved by installing a stubbled Katepal Plinth on top of membranes, with the stubbles downwards. The Plinth replaces the previously commonly used aluminium isolation membrane + pebbles + filter cloth. The stubbles of the plinths must be at least 10mm high.
Turf may be light forest turf, such as heather and blackberry turf, which is detached in 40 x 40 cm plates and carried with the root side touching root side. Heavier field turf is also usable and is detached in 30-40 cm wide strips, which are rolled up when carried.
The turf is set on the roof in two layers, with the lower one being upside down. If there is no growing turf available, a field can be raised on the roof by placing the seeds on top of a growing turf, on a light layer of topsoil.