When it comes to laying new wood shakes down, it is most commonly laid over solid board sheathing or plywood. It can be laid over spaced sheathing but this is not as common. Whatever way you do it, use a 18 inch wide strips of felt, weighing around 30 pound, to inter-lay between the shakes. Before this is done though, you will have to install the actual board sheathing or plywood. Take care to cover all knotholes and resin pocket that you see, with sheet metal squares. If you are choosing to install spaced sheathing instead, you will have to lay out a well seasoned lumber along the roof. This needs to be laid horizontally and be about 1 by 4 inches. The spaced sheathing should be the same measurements. Nail the boards to the rafters and leave a few millimetres space where the boards meet. Always install furring strips if you are laying wood shakes over asphalt shingles or solid sheathing. I would recommend applying the interlayment after nailing the courses of shakes down.
If you are choosing to use wood shingles, these are most commonly laid over a roof deck with spaced sheathing and no underlayment. The reason for this is to expose the shingles in order to create maximum air circulation. As you know, shingles are smooth on both sides which mean they lie flat and can trap water. Shakes on the other hand, are split with grooves that naturally caries water off them, allowing for air to move in-between. You must use furring strips to allow for the air to move if you are laying wood shingles on either asphalt shingles or solid sheathing. Always check the building code in your area regarding what is allowed and what is not allowed. Always repair or install new flashings if needed. Use 1 by 4s well-seasoned furring strips over asphalt shingles or solid sheathing. The first board should be laid flush with the eave line for best results. From there, space the centre of the boards so they are the same distance apart as the actual exposure for the shingle. This means 5 inches if you have a 5 inch exposure and 7 inches if you have a 7 inch exposure. It should be is easy enough to remember. The boards should be extended by half an inch beyond the rake and just like with wood shakes, allow a few millimetres of space where the boards meet. Put two strips together at the ridge and leave another half an inch between abutting strips at the valleys. Take it easy and good luck!