An astragal boot is mounted onto a lower end of an astragal disposed on a free end of a semi-active door in a double-door set disposed over a threshold. The astragal boot includes a platform and a pair of lateral fins spaced from one another and being flexible and projecting downwardly from the platform to seal against the threshold. Primary fins are flexible and project downwardly from the platform a greater distance than the lateral fins and are joined to and extend between the lateral fins. The lateral fins are joined to the primary fins and hold the primary fins into sealing engagement with the threshold to prevent the penetration of water between the astragal boot and the threshold. The lateral fins hold the primary fins against the threshold because the primary fins are thinner thereby minimizing the effort required to close the semi-active door over the threshold.