Dendrochronology is the oldest method, having been introduced over a century ago by an American astronomer, Professor A E Douglass.
He wanted to know whether the number of sunspots affected weather on Earth.
Year by year the trees throughout the region produce a similar pattern of wide and narrow rings in response to the weather changes.
It is this pattern that allows the accurate dating.
If research into other sources of information also fails to throw light on the building's history, resort may be made to the various scientific methods of dating.
This article outlines three of the most important methods currently used for dating buildings or, in a complex situation, the order of construction within the building.
In a 'poor' growing season the trees all respond so that only a very narrow growth ring is formed.
Progressively older patterns are obtained from trees in recent buildings, older buildings, archaeological sites and ancient bog oaks.Because of local, non-climatic causes of change of growth width, the chronologies around the country vary somewhat, and the best dating match is always obtained from a local regional master chronology.The dendro-date is thus the year in which the final ring of the specimen grew (the year in which the tree was felled, but not necessarily the year in which the building was constructed).None is infallible and before embarking on an extensive dating survey, due thought must be given to what might be achieved and which methods might be the more successful. Whilst earlier types of wooden joints may be copied in later buildings and earlier styles may be reintroduced in later periods to confound the conservationist or historian, any reuse of older materials should become obvious by the use of the chronometrical methods described here.
The incorporation of ancient bog oak into a building, no matter how intricately carved or jointed, would immediately become obvious to the chronologist, as would timber renovations.
Oak is the species of prime interest and it is possible to date wood back to over 7,000 years with a precision, in appropriate circumstances, of a single year.