I am new to this site and to timber framing in general. I have built a few small traditional stick frame structures and lots of remodels. I am planning to build small cabin as a guest house and just a fun project. My question is I want to have a loft at one end and I will have to clear span 15’. What size 4x8, 4x10 ect should I use for this. I have access to 3x12 which can handle the task but would like to keep the joist spacing as far apart as possible and to keep the beam thickness at short as possible. I’m looking for that sweet spot I guess. The loft will be roughly 15x20 and used for a bedroom. I want to put tongue and grove on top to be visible from below. I’ve looked for sites that help with calculating this but have had little luck. Looking for good advice not criticizing comments. Thanks for the help.

Hi Alex,

First welcome to the forum...

As for size recommendations, I don't really give those out anymore for liability reasons via forums and/or only having basic information about a project. There is way too many complexities to a timber frame (unknown load parameters, wood species, grade, Timber Framer's experience, future load parameters not shared for a floor diaphragm, etc...) to make such advice viable or "good practice" professionally within this craft...

What help I can offer is that even professionals (myself included) always have our projects (at minimum) checked by qualified PE that not only have the requisite credentials, but timber framing experience as well. I have been working with (and friends of) the same firm for over 30 years now (Fire Tower Engineered Timbers) and could not recommend them more highly for any project, commercial or simple cabin structure or play house. IF!!!...I don't use an Engineer on a project, that is by the clients choice and I still won't go outside of scope for timber sizes of those found in previous projects that have already been approved and/or I require a "hold harmless" contract with those clients.

As for your project, it reads as exciting, but the size (no wood species or grade was offered?) seems out of proportion as "4x stock" is not often found as major carrying beams and usually relegated to joist (4x6, 4x8, 4x4, or their metric equivalent) for most timber frames in that size range you have shared. "Tidewater Capes," as one example, had these size timbers (aka narrow and deep) but each frame design in that style is specific to a given project and/or regional vernacular style and is out of context for a "first project," (in my opinion) for a novice Timberwright. There are recommendations and charts in several of the timber framing books, and on line, yet again, these are "basic" in nature or follow only a set design that has already been engineered.

From an aesthetic perspective, I would suggest either the timbers being closer together with a heavier T&G structural floor diaphragm, or wider "live edge" timbers that are at minimum a 8" deep and no further apart than 24" on center, though other configuration are possible depending on wood species, grade and loads...

Good Luck and would love to see pictures shared here when you get this all figured out. Others here may be of more help than I have been with their recommendations...

Thanks for the information. The 3x12’s I can get are southern yellow pine not sure of the grade and I can get ahold of #1 and better Douglas fir timbers that can range from 4x8 to 6x12. Would one species be better to go with than another?

Hi Alex,

Both species would work...but again, it's about many other factors. This link can offer many of the consideration that need to be evaluated and understood...as well as other factors:

http://www.timbertoolbox.com/Calcs/beamindex.html

This really isn't a realm of "plug in" information regarding size/species/grade...without knowing many other pieces of vital information ranging from your skill sets to what type of joinery/housing these timbers will have, is this a traditional "green wood" (aka rough lumber) project...etc...etc.