Most homes built before 1970 have a board roof deck instead of plywood. This does not usually present a problem but a few points are worth noting.
First, as the boards age, they may bend or twist but are otherwise sound. The new shingles will not completely hide these irregularities. Architectural shingles will disguise more than 3-tab shingles. The only way to eliminate this problem is to re-sheet the entire roof deck. Unfortunately, the added cost could exceed the re-roofing estimate. Second, thousands of nails will be used to install your new shingles. Some of them will, inevitably, penetrate between the boards. In turn, these nails will not hold the shingles as well as the nails that penetrate the wood. Third, on some very old "heritage" homes, rough planks may have been used to sheet the roof deck. Over time, these planks may shrink and the gaps between the boards may become too large for shingle applications. In this rare case, the entire roof deck will need to be re-sheeted with plywood. This will result in an extra charge.