We consider the free additive convolution of two probability measures $μ$ and $ν$ on the real line and show that $μ\boxplusν$ is supported on a single interval if $μ$ and $ν$ each has single interval support. Moreover, the density of $μ\boxplusν$ is proven to vanish as a square root near the edges of its support if both $μ$ and $ν$ have power law behavior with exponents between $-1$ and $1$ near their edges. In particular, these results show the ubiquity of the conditions in our recent work on optimal local law at the spectral edges for addition of random matrices [4].