Can someone explain the difference please?

[kkcomp]

I am planning to build a cabin out of White Oak. After a few other posts and advice given in them I thought I would be going with 8x8 D shape logs. 

I keep seeing don't use the pith but I also see don't make beams out of sap wood. What is the difference and the reason for the two seemingly opposite schools of thought? Am I misunderstanding terms? I don't want to waste a bunch of lumber or find out I did wrong just after the last peg/ nail/ screw is in. 

[biggkidd]

I can't believe no one has answered this yet so I will try even though I am probably the least knowledgeable person here. 

Neither the sap wood or pith should bother log construction to my understanding. When cutting boards from a log you don't want the pith on one side of the board as it will cause it to warp. If the pith is evenly contained in a board or beam I would think that would be okay as well. Same sort of thing with sap wood and heart wood in a board it will warp or twist.  I may not have this exactly correct so hopefully someone with more hands on will speak up.

[KenMac]

I have no experience with log building but I do know a little about white oak. In my usage of white oak it is used due to its' rot resistance. This only pertains to the heart wood as the sap wood will rot if outside. This may not apply to your use but it's all I know. Good luck with your endeavor.

[Magicman]

I use White Oak as bridge decking on my property and I can assure you that the sapwood will rot in a couple of years.

[Don P]

If you saw a cant with the heart centered within it the stresses that want to bow the timber are as balanced as you can do. Since the timber does contain the heart, the drying stresses pretty much guarantee there will be a large drying check as the timber seasons. Following the shortest path from heart to edge it will usually check on an exposed face in a rectangular timber.

If you are proposing sawing 3 sides of the cant to create a D log the heart will be off center so there will likely be bowing to deal with. The exposed face will be sapwood so the least decay resistant wood is exposed to the weather.

If you look at old white oak cabins here, they were hewn on the two exposed faces and left in the round on top and bottom. This balanced the bowing tendency, removed the sapwood from the weather face but did expose it on the edges, which is where rot often begins.

[kkcomp]

OK I think I got it. The sap wood has a higher moisture content so more prone to rot, insects and uneven drying. So do the bends, warps, twists etc. show at milling time or over time? Does kiln drying make any difference? Should the sap wood be used mostly for furniture/ projects or firewood? 

[metalspinner]

The rot resistance in white oak comes from the tyloses in the heart wood. White oak sap wood does not have tyloses in it.

The sap wood is still good wood for furniture, etc and isn't considered a defect by the NHLA. Though some furniture makers don't like the look of mixed sap/heart in boards.