Abstract: This invention relates to a kiln for drying timber comprising:an elongated housing having a base, two side walls and a top wall and two end walls defining a timber treatment zone;a gas circulation passageway having a top passageway portion extending over said timber treatment zone, a side passageway portion extending down one side of said timber treatment zone, a treatment passageway portion extending through said timber treatment zone, and a side passageway portion extending up the other side of said timber treatment zone merging with said top passageway portion, each said side wall being sloped whereby each of said side passageway portions is broad at the top of the kiln and narrower at the bottom of the kiln thereby to provide a substantially even rate of flow of gases at various levels within said treatment zone; and,reversible gas circulation means within said top passageway for maintaining the circulation of treatment gas in said other direction through said gas passageway thereby the direction of gas f