Amen Jim. I was taught recently "Perfection and Grace puts a smile on your face", corny but true for a craftsman. You do want precise, tight joints as shrinkage will cause a gap to open up even if the joint is perfectly tight when done with green wood. 1/32" of shrinkage gap you can live with in your home. 1/16th or more looks like poor crafstmanship. And if it's your home, you will notice. That is why I said (as I was taught), leave 1/16th when using the saw, then chisel and pare to your scoreline and use your combination square to make those tennon shoulders and mortises dead nuts. It's amazing how much precision you can get with good layout/score lines and a sharp chisel. You should spend as much time surveying and laying out your timbers as you do cutting. That will save you lots of problems in the end.

I would never suggest leaving a exposed mortise with a 16th on either side, nor would I leave excess room above and below the tenon in a frame, however a tie beam is wider than a two inch tenon and the sides of the mortise will not be seen, don't let my question convolute my thinking on precision, Precision is the way to go or you will pay the price ten fold down the line. That being said, again what is the problem in a hypothetical bent where all critical points are on then noses and bearing points not the side faces where plates, joists and purlins leave that tie beam in the right place?

I pose this question because most people who work in a shop do not all have the same degree of precision, so.. if the shop takes the attitude of having a more narrow tenon than fit up will proceed more efficient. To some up long winded post to explain reasoning I still ask the question.

Are there any problems with having room on each side of the tenon in a mortise (when it is concealed, therefore disregarding the aesthetic debate)?

Mo:
If you feel your using too much time in fit up then the joints cut that need extra work during fit up aren't being cut right in the first place.
Making sloppy joints to save time during fit up isn't the answer. Making correct joints in the first place is. If the joint is cut correctly then fit up shouldn't take a huge amount of time.
As far as your exact question about a beam with hidden shoulders, my first thought was the possibility of some rotation, although very little, I would imagine. But also, load goes to stiffness, if a joint isn't stiff then the load shifts to where it can and this could put added stress or load to another area.
Of course this is my opinion.

If you are having fit up problems (more than a quick parring touchup on a percentage of the joints), perhaps your layout technique is not sufficient. What is your layout technique? Do you score as I described in an earlier post? Do you do your layout referenced from one side of the bent, outside faces, basically two reference surfaces? how square are the beams you are starting with?

I saw to the line as well, with hand tools and power tools. I only score in hardwoods. If you look at the shoulders of old frames you don't generally see chisel marks (because they were sawn). One advantage of square rule framing is the housings (girts, braces, studs, all non-through housings that is) hide potential shrinkage. Drawboring takes care of some as well, and then you don't need 15 comealongs (or any really). I'm still practicing with the chain saw on the end of tenon cuts.

thanks for you all's insight. Personally I do not like to or do leave room on any joinery. Just wanted to see the consequences if any that people might think of with said areas.

BB, my lay-out technique depends upon the wood and/or design. Sometimes Mill Rule, sometimes datum line square rule, sometimes edge rule square rule, sometimes french scribe, sometimes sauterelle lay-out (bevel), mechanical pencil, framing square, ruler from combination square, tape measure, 20/16 vision, always attempting dead on replication of design, spans and heights within 1/16th of an inch.

mo

that's an interesting concept to use chainsaws for cutting joints. my background is log building, but i set aside the saw after felling and hewing because i thought it wasn't appropriate for timber framing. but the skil saw isn't seeming that accurate with the uneven surfaces, and i'm ending up chiseling all the cut offs anyway. i wonder if it would be easier just to rough everything out with the chain saw and make a clean cut surface with a slick and chisel.

a friend mentioned using a plywood jig to support a skil saw on a hewn timber. does anyone have experience with this?

we use olive oil as bar oil when quartering moose. only COLD PRESSED extra virgin though. ;>)

use the tool that works the best!
probably safer to use a chainsaw on hewn timber, rather than have the circular saw baseplate hanging up in the air most of the time.
sharp chainsaw and good eye can be an awesome combo.
olive oil is actually good for bar oil, doesn't dry out. I also use canola, bit cheaper.

I'm with Mark - cut to the line with your saw! Life's too short, oak's too hard, and there are too many variables beyond my control to imagine that all of my joints are going to end up with less than 1/16th gap after assembly and drying. House all of your joinery if you don't want any visible gaps.