Rumors have him marrying a woman named Sarah in 1755. This is supported by the following:
The book A Thousand and a Half Sutherlands has the deposition of a Sarah Sutherland giving up her rights to the administration of her husband, Joseph R. Sutherland's, estate to her son Roger Sutherland. It is signed by Nathan Canfield and Tracy Sutherland, dated 16 February. At the end of the document, William R Sutherland affirms that in January of 1799 he attended the funeral of Joseph R. Southerland, his late brother.
I'm curious if Joseph and Sarah parented a Sally (possibly another Sarah) Sutherland, b. 1760, who later married Israel Root.