turf log

This could be a log, but could be far more elaborate
In this link you need to scroll quite a long way down:

http://en.wikipedia.org/wiki/Sod_roof#Sod_support_along_the_...

To stay in place the sod must be supported by strong beams along the eaves. These logs or beams (Norwegian: torvvol) come in a variety of shapes and sizes, depending on the quality and status of the building, and on local tradition. An equivalent English term would be turf log.

A modern turf roof is descibed here:

http://www.safeguardeurope.com/case_studies/turf_roof_log_ca...
"Timber battens were then fixed to the turf hooks, creating a "lip" around the whole roof area, creating a space for the turf to sit in. 100mm of soil was then applied to the roof, follwed by 50mm of turf."
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There is probably not a lot of traditional terminology in English, though I may be mistaken. (Ignorant is the honest thing to say!)

However, they are catching on again as an environment friendly type of roof, and there are lots of websites on 'green roofs', 'turf roofs' or 'sod roofs' which are interesting to read and may even help you.