La rosca de un arco

[lpwe]

I'm having trouble opening the document. Essentially the sentence just says that the decoration crosses the "rosca" of the arch. I think it could be either "extrados" or "intrados". "intrados" is the interior curve of an arch and "extrados" is the exterior curve. But I can't find a confirmation any where that this is the translation. When I can get the document to open, I'll post the sentence.