There is more information about installing slate
The Slate Roof Bible, 3rd Edition, by Joseph Jenkins.
Common in USA. All slates in field of roof are same length and width. For more information about the fundamentals of slate roof installation styles read this article.
Slates are same length, but vary in width.
Slates may vary in length, or be laid on the roof to appear varied in length. In either case, the bottom exposed edge is staggered. The degree of stagger can vary from roof to roof. The photo above shows a 1 inch stagger on two Vermont sea green roofs about 80 years old. The photo below shows a staggered butt roof with 3 lengths of slates (16 inch, 18 inch and 20 inch).
The roof above is made of random width, mixed color, new and salvaged slates in a staggered butt style. The slates include Vermont unfading purple, Vermont mottled green and purple, Vermont sea green (new and salvaged), Vermont unfading green, Virginia Buckingham (salvaged), and New York red (salvaged). Lengths range from 16 inches to 20 inches and widths range from 8 inches to 14 inches.
The photo below shows a staggered butt roof with 3 lengths (16", 18" and 20") with 4 colors of new slate: VT sea green (3/8 of roof surface), VT unfading purple (1/4), VT black (1/4), and VT unfading green (1/8), using 6 widths (7", 8", 9", 10", 11" and 12"). This slate was donated to the Natural Building Colloquium East (2005) by Camara Slate Company.
The photo below shows a staggered butt slate roof with all the slates rounded by hand prior to installation. Lengths are 18 inch, 20 inch and 22 inch with a mix of widths including 9 inch, 10 inch, 11 inch, 12 inch and 14 inch. The slates are about 50% new and 50% salvaged. Colors include VT unfading purple (new), VT unfading green (new), VT sea green (new and salvaged). Half of the roof is a mix of salvaged VT sea green slates. The remainder are new slates.
The photo below shows a staggered butt style slate roof, with 3/8 Vermont unfading green slate. 3/8 Vermont "sea green" (semi-weathering green), 1/8 Vermont purple and 1/8 Vermont black, in three lengths (14 inch, 16 inch and 18 inch) and seven widths (7 inch, 8 inch, 9 inch, 10 inch, 11 inch, 12 inch and 14inch). The slate was attached with 1.5 inch copper slating nails. The ridge will be finished in a saddle slate ridge using 8 inch by 16 inch Vermont green slates and 20 ounce copper step flashings. The slates were donated for this demonstration roof installed at the Ohio University East Campus for the International Preservation Trades Workshop, 2005, by Camara Slate Company.
Other staggered butt slate roofs (above and below) using Vermont slates with multi-colors.
Same principle as staggered butt, but the exposed edge is cut into random shapes or angles. Roof above is made from a mix of recycled slates.
Above is a random width slate roof (PA black slates). The textural effect is created by periodically inserting two slates on top of each other where one slate would normally be. The lower slate in the pair is dropped down a couple inches and the slate on top is raised up a little. This provides thickness texture to the roof as well as staggers the exposed butt creating a unique and tasteful architectural style.
Above is another textured PA black slate roof installed in a very unique style. Longer slates are used in diagonal rows with thicker slates used nearer the bottom of the roof. The same roof also utilizes thicker slates at random to enhance the textured look (below).
Traditional style of slating probably originating in the UK, Wales or Scotland. The slates vary in both length and width. Typically, the slates also diminish in thickness as they approach the top of the roof. The roof above includes a mix of Vermont slates. More about graduated slate roofs.
The unique roof above mixes VT unfading green, sea green and purple slates with NY red. Note the unusual ragged butt style.
Another mix of VT sea green and purple slates with NY red (above).
It is common to mix slates of the same length and width, but of different shapes. Above is a common style, installed with recycled Vermont "sea green" slates; below is a less common style, but quite dramatic nonetheless.
One of the unique characteristics of slate roofs is that slates can be used of different colors to create unique patterns, thereby creating landmark roofs.
The above set of roofs were installed by Brent Ulisky and Orion Jenkins, formerly of Joseph Jenkins, Inc. The pattern was determined by the homeowners from a blank schematic supplied to them by the contractor. The slate is VT unfading green and VT unfading purple attached to rough-sawn hemlock roof decking with copper nails. All flashings are 20 ounce copper. The ridges are saddle ridges of VT purple slate with 20 ounce copper step flashings.
Traditional German (below):
Traditional Spanish (below):
There is more information about installing slate roofs in the Slate Roof Bible, 3rd Edition, by Joseph Jenkins.