I see "root layer" come up quite a bit for this, including here:

http://www.fiz-informationsdienste.de/de/FG/EnergUmw/Termino...

And quite a few hits for this in combination with "welding" or variants in Google.

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Note added at 42 mins (2006-02-26 21:59:29 GMT)
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Under "Schweißsteg" at the link above.

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Note added at 13 hrs (2006-02-27 11:05:28 GMT)
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Here's a different company that also likes "root":

http://www.industrie.de/industrie/live/infothek/woerterbuch/...

Schweißsteg = weld root face

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Note added at 15 hrs (2006-02-27 12:48:21 GMT)
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Here's yet another one (root face for Steg) from the TU Chemnitz:

http://dict.tu-chemnitz.de/dings.cgi?o=3003;iservice=en-de;q...

I think I'm leaning towards "root face" now, there are also thousands of hits for it on Google with regard to welding.