For an overview on changing the roof or roof covering on a house and special information on Mansard roof shingles -- follow this link.
The starter shingle
If you put the first row of shingles on the edge of the roof just like all the rest, you will see the bare roof deck through the cracks between the shingles. Many self-taught roofers follow the wrong technique of installing the first row backwards, with the tabs pointing up the roof to get a continuous strip of roofing under the first set of tabs. There are two problems with this technique.
First this creates a bump on the bottom of th...