Sod provided an insulating layer to rooftops and walls in these northerly places, representing the most efficient and sustainable approach to building in the environmental context; a naturally abundant resource and relatively easy to install and maintain. Typically built with the help of the entire community, a sod home could be constructed from start to finish within the short warm season when the ground thawed enough to allow chunks of soil, grass, herbs and wildflowers to be excavated from the surrounding fields.
You may wonder how in the world these homes were made weather-proof with a roof made of sod? These days we use synthetic rubber liners to prevent moisture from damaging the roof structure in green roof systems, which typically have a lifespan of about 30 to 40 years. Incredibly, the sod roofs of the Old World had a similar lifespan, though they used only the bark of native birch trees for weatherproofing. This was collected in spring when the upward flow of sap made the bark easy to peel from the trees. Birches are among the most common trees in these regions and have the ability to give their bark without suffering permanent damage.
First, the frame of the home was built with a combination of stone and logs (from birch or other species that were available), including rough wooden planks that served as roof boards to support the sod. Starting at the eaves of the roof, Nordic homebuilders would lay overlapping sheets of birch bark like large shingles, so water flowing down the roof would run over one sheet and then another, draining off at the bottom.
A layer of sod about 3 inches thick, soil and all, was placed upside down on the bark as it was laid to hold it in place. Having the grassy side against bark helped the water to drain freely down the roof and also prevented the bark from decaying as fast as it would if it were in contact with soil. A second layer was then placed grassy-side up on top of the first, for a total of six inches of insulation. Finally, a log would have been attached horizontally to the base of the roof along each of the eaves to prevent the sod from sliding down. The rough-hewn logs that were typically used as a wall structure in conjunction with sod roofs were compressed by the tremendous weight of the sod, closing up all the drafty little cracks and helping to make sod homes as comfortable as possible in the long winters.
Some of the oldest remaining homes actually have sod walls, as well. This also served the purpose of insulation and typically would have been faced with stone on the inside to prevent moisture from entering the home. Unlike actual hobbits, Nordic people are known for being quite tall, but the photos show, the entrances to their homes were quite modest in size; a reflection of the scarcity of wood needed to build them rather than the size of the people who dwelled within.