The Collaborative International Dictionary of English v.0.48:

Summer \Sum"mer\, n. [F. sommier a rafter, the same word as
   sommier a beast of burden. See Sumpter.] (Arch.)
   A large stone or beam placed horizontally on columns, piers,
   posts, or the like, serving for various uses. Specifically:
   (a) The lintel of a door or window.
   (b) The commencement of a cross vault.
   (c) A central floor timber, as a girder, or a piece reaching
       from a wall to a girder. Called also summertree.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Summertree \Sum"mer*tree`\, n. [Summer a beam + tree.] (Arch.)
   A summer. See 2d Summer.
   [1913 Webster]