Penrhyn Welsh slate is the best material for roofing because of its durability and beauty. Here are installation instructions for installing a penrhyn welsh slate roof: 

1. Begin by measuring the area where the slate will be installed. Make sure to allow for an overhang of at least two inches.

2. Cut the necessary number of slates to size using a wet saw with a diamond blade. Be sure to wear eye and ear protection when operating the saw.

3. Install a layer of underlayment over the sheathing, using either nails or staples. This will provide additional protection from water damage.

4. Start at the bottom edge of the roof and install the first course of the slate, making sure that each piece is properly overlapped. Nail each piece in place, using either copper or aluminum nails.

5. Continue installing courses of slate until you reach the top of the roof. Make sure that the final course is properly trimmed and nailed in place before moving on to step six.

6. Install flashing around any penetrations in the roof, such as chimneys or skylights, using either lead or galvanized steel flashing material. 

7. Finish up by applying a layer of sealant around all edges and penetrations to further protect your roof from water damage."