First of all working on any pitched rood can be simply dangerous, but especially tile or slate roofs. These roofing materials become more than exceptionally slippery when wet or even accumulate a bit of moisture, rain, or winter snow or ice. If your roof is steeply pitched, or if you are deathly afraid of heights, then play it safe and call in a professional or skilled trained professional with the proper tools and training.
Second, slate roofs no doubt do deteriorate and wear over time and ages. Some even become “crumbly”. If you walk onto a roof in this condition, in an effort to fix one, more or several visibly or apparently broken slates, you can well cause very expensive and costly damage to other areas or even the rest of the roof. Stepping in the wrong or incorrect spot or spots even on brand new freshly installed slate tiles could cause this tiling to break or crack.
If your roof slates are ringed in white, flaking or turning darker in color or coloration it is more than wise to take care and caution and stay off your roof and roofing. Again get on the blower and call a professional roofing company with a good, known and established reputation to make necessary repairs. Go for a premium experienced service company. It is false economy to seek out a novice. Better to leave it un-repaired
If you do decide to fix your own roof never ever work in wet, windy or cold weather. On brittle roofing materials such as slate or tile use a “chicken ladder”. These hook over the ridge of the roof and provide secure hand and foot holds while properly and safely distributing your weight evenly over fragile roofing surfaces.