A subsill positioned at least partially under a sill in a sill assembly. The sill assembly positioned under a door or window. Two or more pressure chambers extend lengthwise between the subsill and sill. Pressure chambers can extend one behind another. The subsill can include a backstop with some of the pressure chambers extending lengthwise between the backstop and the sill. Apertures in the pressure chambers are positioned to force water to flow by a circuitous path.