Dendrochronology - Its use in dating structures

Inevitably one of the first questions that owners of period structures want to know is, "How old is it?". And as a professional restorationist/historian, that is one of the first questions you'd like to answer. However, it is rarely that simple. By being well versed in regional styles and patterns of development, the professional can usually come up with a broad timeframe of say 30-50 years that it might have been built in. This can be based on a thorough survey of the structure and its construction techniques and materials. Certainly, short of a conclusive time of construction through documentation this is probably the most accurate method that has been used for many, many years. Many buildings have been so well documented from their earliest days, that coming up with accurate dates is easy. However, for structures older that 1800, it can be very difficult. Research in town records is the logical place to start, but mention of structures in early deeds is quite rare pre 1800.
If structures have been added onto or altered, as most have, there is the further confusion of what came first. Close examination of structures can give clues but is rarely conclusive. It was therefore with great excitement that starting in the 1970's a new scientific method of dating buildings was developed - dendrochronology. Dendrochronology has been used since that time to date period structures throughout England. It is just in the last 10 or so years that it is gaining in popularity here is the US. Dendrochronology is a method by which test bores are taken from structural members of a structure and matched against a data base of known samples taken from existing trees and well documented timber samples. By matching up growth ring patterns, it is possible to pin point to within a single year and even the season of that year that the the timber was cut. As most early structures were constructed using green timbers, we can establish a very highly probable date of construction.
I will not try to go into the full details of the process, but will rather refer you to go to the web site of Oxford Dendrochronology Laboratory, www.dendrochronology.com, in England and read Basic Dendrochronology. Oxford has been instrumental in getting the use of dendrochronology established over here. Anne Grady from Boston and Bill Flynt of Deerfield, MA have been working to establish data bases of samples for the various regions of New England.
This is highly interesting stuff and to be able to positively assign dates to a structure and its various additions only broadens our understanding of early building.