T-structures encode important cohomological information regarding triangulated categories. Therefore the construction of t-structures is an important problem. In this talk we consider a way to construct new t-structures from old by taking the extension closure of aisles. There are connections to the structure of the poset of (bounded) t-structures. The main examples we will consider are tame hereditary algebras, where we give an explicit combinatorial criterion to ensure averaging is possible. We expect our techniques will apply more generally to the bounded derived categories of (tame) weighted projective lines. This is a report on joint work with Nathan Broomhead and David Ploog.