Hi Tim,
In very simple terms my experience with Douglas fir is that you are alright to use centrally located boxed heart timbers up to 6 x 6 without much fear of checking. Using timbers above that size is bound to result in checks opening up. With larger timbers where its a requirement to limit checking then it would be better to use free of heart or halved timbers, this way the wood can shrink back upon itself without checking about the heart. Keep in mind that other drying distortion patterns will take place depending on where the cut is positioned in the cross section of the tree. Checks run back to the heart and the juvenille wood and so when checks open up then this forms a perfect path for beetles to enter a timber and do their worst so they don't call timber framed buildings in England "half [halved]timber buildings" for no good reason.
Regards
Ken Hume
