Historically, masonry has been categorized into three basic categories. Each type of masonry has its advantages and disadvantages, and in this series of posts we will explore what they are. Broadly, the different types of stonework are categorized by the precision of the stonework. Unfortunately, this distinction tends to make the boundaries between these classes of masonry rather hazy. There are several old books written on this topic; one such is Structural Engineering by George Swain. It is worth mentioning here that any form of masonry can be done without mortar; traditionally, however, mortarless was considered inferior to mortared work.
Ashlar: Overview
Ashlar is the highest grade of stonework. With tight joints and precision cuts, the highest grades of ashlar utilize stones essentially cut into massive bricks that stack beautifully. Ashlar work is very enduring.
There are several different types of ashlar. Though the most common form uses stones cut essentially into giant bricks and laid accordingly, this is not necessarily a requirement, allowing for some variety among ashlar structures. That said, ashlar is never laid in a totally random fashion; there are indeed courses, even if they are broken at intervals. The bridge shown above is a good example of ashlar laid with broken courses. The key trait of ashlar masonry, however, is that the stones fit in place well. The joints in ashlar should never be wider than one half of an inch. That said, in some ashlar work vertical joints wider than one half inch may be considered acceptable.
The stones in ashlar are cut. The cuts on the surfaces of the stone are referenced to a chisel line cut into the rock. This line marks how far down the subsequent cuts in the stone should be made. Any pieces of rock which project beyond this chisel draft are removed. In some cases, the stones may not be precision-cut all the way through; the ends, for instance, may be left more or less unaltered. However, the bottom and top surfaces should still be leveled off.
The amount of mortar used in ashlar work tends to be low, but the exact amount will depend on stone sizes and how accurate the joints are. In the highest grades of ashlar masonry, only 1 cubic foot of mortar per 1 cubic yard of masonry will be required. There is no reason why mortar must be used with this type of construction; in fact ashlar-style mortarless structures would be quite strong. However, traditional ashlar structures almost invariably used mortar. After all, the expense of the mortar would probably have been small when compared to the expense of the precision stonecutting required.
Ashlar: The Challenege
The challenge of ashlar masonry is in the stonecutting. The stones must be precisely fitted. Traditionally, such cutting was expensive, which prompted builders to resort to lower grades of masonry. For the DIY enthusiast, the precision stonecutting required to build ashlar masonry will take considerable time and practice. However, if one is not up to the challenge, there are several other grades of masonry that are still practical for lasting structures.
Stone Arch Bridges 