An elongate web is attached to the root end of a spar of a wind turbine rotor blade to provide additional support along the width of the blade. The root end is formed by a winding operation, and a recess is then cut into the surface of the spar. The recess is defined by a relatively large first, cylindrical surface, which is coaxial with the longitudinal axis of the root end, and a relatively small second, conical surface. A tapered end of the elongate web is attached within the recess of the root end using a layer of suitable adhesive and an array of pins. Resilient spacer elements are arranged within the recess so as to surround the pins. The large area of the cylindrical surface causes the tensile and compressive stresses which arise along the elongate web in use to be transmitted to the spar as shear stresses.