When I was at Leesburg, VA and sat in on Bruce's grading talk, he mentioned that NeLMA uses the same grading rules for hardwood as softwood, should you need hardwood timbers grade stamp in the area of the country where NeLMA rules apply.
What I have done, is that I bought a grade rule book. I studied what grade #2 was for timbers 5" x 5" or larger, and try to get all my timbers to pass this rule as I mill them. If the knots are smaller then what the grade #2 rule is then these timbers could be "or better".
You can still have a hardwood 8x8 with a 6" knot in it. So you should at least try and get a grade rule book and learn the requirements and try and make sure all your hardwood timbers meet that grade. Then you'll know that your frame is safe, and you can sleep at night.
Jim Rogers
