[Ron6519]

You all seem to miss the point. You never walk directly on a slate roof. Never. The options are to set up a system of a minimum of 24" wide plywood or use a ladder system with hooks that wrap the ridge or a combination. Walking on the roof puts pressure on the individual slates that can put stress cracks on every slate you step on. You won't know there's a problem immediately because the issue will develope over time. Slates will start to fail and slide down the roof.
You need to displace the pressure over as wide an area as possible to not do damage to individual slate pieces.
Ron