Replacing slate roof - whether to vent new asphalt one or not

Our 1940s slate roof is unfortunately at the end of its life and we are replacing it with architectural asphalt shingles. The relatively low slope roof attic space accessed through a hole above the stairs has always been very hot in our DC summers. There is one gable vent in the brick house. There is no room for soffit vents so the question is whether to bother installing a ridge vent and if so whether the gable vent needs closing. I have read that you can get rain infiltrating into ridge vents and if there is no gain from venting we may be better just redoing the roof with the current system. Any advice very welcome. Thanks

This is a very popular ridge vent system here in hurricane country due to the high velocity horizontal wind and rain typically experienced. The old, metal vents do leak but they are poorly designed and constructed. Typically found on most spec homes, when the high winds hit, they peel off like a banana peel and become missile hazards. When I re-roofed my own home last summer, I used this and have been very happy.