This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
42. Timber is often used for footing courses where a large bearing surface can be obtained and is necessary; providing, always, the timber can be kept from rotting. In some cases the timber is charred on the outside, and in other cases it is coated with asphalt. If the ground is continually wet, there is little to fear, as timber will not decay when kept saturated with water; but when alternately-wet and dry, unprepared timber cannot be depended upon.
The best method of placing plank under walls for footings is to use 3"x12" plank cut in short lengths and laid crosswise in the trench. A layer of plank of the same size is then laid lengthwise, followed by a third layer again placed transversely. As shown in Fig. 7, b is the stone footing resting on the footing planks a, and carrying the stone foundation wall c between the walls d, d of the trench.