The exterior details are the main considerations in designing for weathering, particularly as they affect water flow, but the type of finish, the spacing of joints, and the amount of steel cover are also important. In designing, there is a need to trace the anticipated flow of water over the whole surface. The flow of water, if it runs only short distances, may have a cleansing action. The greater likelihood is that with the usual detailing it will be permitted to flow farther than it can carry its load of dirt or than it can wash uniformly. In working out details the architect should consider where water will land. Horizontal details should be designed in such a way that any dirt that collects will settle in shadowed areas where it will accentuate the design, not detract from it. The type of finish, or surface texture, influences the self-cleaning ability of the concrete. Vertical ribbed tooled surfaces weather reasonably well. Exposed aggregate finishes with hard aggregates may be self-cleaning in nonindustrial atmospheres, but it is becoming standard practice to coat them with thin films of clear lacquers with very low gloss. Although board marked surfaces can be made to weather well when carefully detailed and made of quality concrete, all too frequently neither of these requirements is met. Control joints should be provided in sufficient numbers to accommodate drying shrinkage and thermal movement and to confine major cracking to predicted locations. Wherever there are unavoidable notches control joints should be installed to relieve stresses. Joints should ordinarily be featured, not concealed. The purpose of adequate concrete cover is to provide an alkaline medium for the steel as protection against corrosion. As carbonation occurs in concrete, due to reaction with atmospheric carbon dioxide, its alkalinity is reduced. In dense, good quality concrete, carbonation is likely to occur to a depth of only a small fraction of an inch, but in highly porous concrete carbonation may eventually occur all the way through. In good quality weathering concrete the depth of carbonation would not be expected to be very deep.