do 18" perfections and 18" tapersawn shakes have the same recommended maximum exposure
There is a maximum exposure for wood shingles. 18" shingles will have the same amount, whether tapered sawn handsplits or perfections. Maximum exposure is based on the remainder of the exposed part of the shingle to extend up under 2 more courses that way, where the nails are that hold the shingle in place, which by the way is always 2 nails regardless of how wide the shingle is. That way the water passing around the edges of the shingle onto the shingle under it has another shingle to get around before it causes a leak. The other guide line is that the shingle gaps are to be no closer than 2" apart from the shingle above or below.

On a 4/12 pitch, the rule may change there you may need to tighten up the exposure where the shingle top is covered by 3 shingles and not 2.

For example, maximum exposure for an 18" shingle will be 8" for a roof with a higher pitch than 4/12. On a 4/12 pitch the exposure would need to drop to 5 1/2".



With a lower pitched roof the rain water will not move off fast and would have a chance to migrate deeper around the shingles, therefore more chance of a leak.

The butt thickness only adds to the longevity of the shingle.