What are flat weft hair extensions? Flat Weft Extensions are a cross between the thin and flat wefts of hand tied but with more flexibility in that the wefts can be cut. The application is a sew in method similar to the hand tied or machine weft application. KmXtend's Flat Weft Extensions are crafted with the same russian hair that our luxury hand tied extensions are made from. We carry 20" in the flat wefts and they come in 50 gram wefts.