24x36 Frame design

[Epmhush]

Hey guys/gals I hope everyone's 2021 is going better than last year so far, quite a ride its been for me personally but I'm doing my best to stay focused on this frame build and take it one bite at a time, but I digress. This frame started out as a 20x24 saltbox but after doing some floor plans up and considering the extra work it would be to increase the size of this frame I've decided to go with a 24x36 frame. There are 3 main items that are holding me up in particular:

1: Roof design - From my research on google and on this forum I'm leaning towards a common rafter roofing system due to the importance that ease of install will have on the build (it will only be my father and I building this frame for the most part) and being able to sit the rafter pairs on the 2nd floor queen post purlin plates and roll the rafters into place will help with that. Which brings me to the 2nd issue

2: Overhang design - I would like to have a 3 foot overhang because the insulation framing and sheathing will eat up a few inches but I cant find a definitive answer on how to design this, I see people say if you use the common rafter to cantilever out over the plate it can leave open spaces for critters and such to enter after the frame dries and I would like to avoid this, but i want to avoid secondary roofing structure if possible and I don't like the look of having the angle change after the eave.

3: Timber sizing - This has been the most challenging part for me as it is quite a bite of information. I can understand the basic methods of splitting loads based on spans etc. but when it comes time to apply this to my frame there are too many unknowns such as how to calculate the additional weight of the insulation, metal roofing etc. for the rafter sizing and calculating the area a load is spread onto with an uneven span and the effect roof pitch has on all of these calculations...

If you guys are looking to get a better idea of what I want to build you can refer to the tedd benson 24x36 timber frame or the 24x36 "barn home" on timberframeHQ, these are very close to what I would like my frame to be.

Any info would be greatly appreciated, thanks.

[Don P]

For the overhang, either attach blocking to the rafters that the bird blocking attaches to or groove the rafters to drop in the blocking. You might be able to bury them in the plate as well, although I kind of doubt it at 3'.  Attaching the overhangs after the fact is not as strong and stands a good chance of showing a slight angle change at the joint. If you just try to friction fit blocking in between green timber rafters it stands a good chance of coming back to bite you.

For timber sizing post a sketch. That will probably generate some comments and options. Include species and snow load and I can walk you through what I know to get you started.

[Brad_bb]

What is this building going to be?  I'd recommend you design the frame on paper with timber sizes, then submit it to the engineer to analyze it and see if any sizes need to be changed. Include your foundation details too and how the frame will attach to foundation. The cost is not bad.  You shouldn't be intimidated either.  Whether you need engineered plans for your county or not, it's still a good idea to make sure your design is appropriate.  I've used and a lot of timberframers use Firetower.
Engineering | Fire Tower

[Epmhush]

Good morning fellas, thanks for the quick reply. So first I would like to say that the building will be a cottage and some day a retirement home for me, which is the reason for going with the larger 24x36 as I would much rather have the room and not need it than the reverse. 

It will be queen post style truss with the "perlin plates" that contact the rafters higher than the truss tie beams, the second floor will have a 3 foot knee wall and the "living room bay" will be open to the floor above. It will be 4 bents 3 bays sitting on a concrete pier foundation (1 pier at each post) and I plan on building it with a 10/12 pitch with a metal roof. For the insulation/sheathing I plan on framing it out myself to be more cost effective than SIPs, most likely going with ridged polystyrene or similar. Thats all I can think about for design at the moment but I will work on a sketchup model using the dimensions in the tedd benson book just for a starting point. 

To answer some of your questions:
 
@brad I do plan on having an engineer go through my design but I would like to come to my own conclusions first before having them check it and for some reason I love to torture myself with new knowledge

@Don P You mention "bird blocking" i'm not sure what you mean by this? Is that blocking installed at the birdsmouth between the rafters above the eave? So is your suggestion to the build a frame off of the bottom edge of the rafters?

I plan on building this frame with balsam fir/spruce from the surrounding forest and milling it myself.

I do have further questions but ill try and get the model finished by the end of this coming week to give us a starting point to discuss from.

[Epmhush]

Hey guys, I finished up a rough model of what I'm looking to do minus the joints. The knee braces aren't offset yet and the first floor joists also need to be staggered but you get the idea. If you don't have sketchup take a look at the photos.. let me know if you would like to see anything specific.

So basically as I said before I'm looking for help sizing the timbers because I don't want to cut timbers larger than they need to be. Also some help with the rafter as we discussed.

Thanks.



 

 

 

 

[everythingwood]

I'm no engineer but for the house sized structure an 8x8 post will typically do what you want.  Wood is very strong in compression and you are typically sizing the posts so you have room for the joinery.  for the larger spanning members you should figure out your loads and size them using the calculators in the "toolbox".  It is in the "extras" tab.  I'm not sure if they have one that combines point loads and uniform loads though.  Again, for the house sized structures I would start with an 8x10 for the main spanning beam.  

I have always been a fan of the Dutch barn style frame with the main posts supporting the anchor beam and the purlin plate.  The joinery is fairly simple.  The only drawback of the design for a house is you now have two posts in the living area at each bent instead of one.

Also, how long of a timber can your saw handle?  If you can make the main spanning beam (anchor beam) one piece you will simplify the joinery as well.    

Also, No basement in Canada?  do you have bedrock issues?  I'm in Wisconsin and I almost never see piers used for a house.  I think it's typically a wash to go with the full basement.  Reinforced floating slabs or slabs with insulated footing walls are the typical "foundation" system used here when there are bedrock or groundwater issues.  Of course you always have the plumbing concerns with slabs.

[swmn]

Got to know your snow load.  Just have to know it.

I fooled round with this some when I was getting my feet wet, there are some pictures at the bottom of page one in this thread:

https://forestryforum.com/board/index.php?topic=107340.msg1674206#msg1674206 

What I am up against is my snow load and tree size points me at 4x8 rafters, but when I put it together like in the pictures, if I put 2x6 scraps or similar vertical between my rafters to fill the "HAD" (height above decking), as the wood dries down many may gaps will open.

[Don P]

@Don P You mention "bird blocking" i'm not sure what you mean by this? Is that blocking installed at the birdsmouth between the rafters above the eave? So is your suggestion to the build a frame off of the bottom edge of the rafters?

Like so, this is done in log construction fairly often where needing to block between round rafters but would do the same thing here. Use dry material for the blocking.



 

Moments are additive. You can figure the uniform moment under the point load add the point load moment and then figure the minimum size needed. This one is easy, as the point load wanders around off center check locations around the point load to make sure there isn't a higher moment nearby.  Can you saw those ties in one piece, this is no place to attempt a scarf. It will structurally work better to extend those "queenposts" all the way down for straight, easy load paths.

Second floor girts and ties can intersect the posts at different levels. This would also allow stacking the joists on the beams rather than dropping them in and reducing beam section (and strength).

A continuous perimeter foundation is a much better idea than piers.

[Epmhush]

Thanks for the replies,

@everythingwood I agree that the dutch style would be strong and it would definitely allow more headroom in the loft but it also adds a bit of extra work on the roof, I'm trying to keep a balance of strength and simplicity in the frame and every little bit of "fat" I can cut helps. As far as a basement goes it would add a lot of extra cost in excavation and masonry work that I don't feel I need for the purpose of this cottage, although it would simplify a lot of things and add an insulative value while also looking great... maybe a future project?  hahaha. I am using a HM126 mill with an extension, so it will cut about a 16 footer

@swmn My snow load is about 40 PSF, ill have to take a look through your post when i have a few mins, looks like there is some great info in there.

@DonP Thank you for throwing that in sketchup, I am definitely a visual learner so that helps. This was the image I had in my mind more or less, but will the blocking have issues as the rafter dries? Or is it a negligible amount of shrinkage in this area?

As for the measurement of loads this reads like french to me, and I'm not from Quebec  :D. I understand what you mean by moment, uniform load and point load but when I read the whole sentence I feel like a gym teacher at a math convention. What do you mean by "center check locations"?

I agree two posts under the queen posts would be stronger than the center-lined single post, although this would add additional cost (an additional pier in each bent - Is this by preference or necessity? Could I increase the center post size instead?)

Are you suggesting I "split" the girt by having a one piece queen post top to bottom? Wouldn't this introduce a similar outward force issue as the scarf joint? My sawmill is also limited to 16 feet ish so I wont be able to cut a full 24' beam

Second floor girts and ties can intersect the posts at different levels. This would also allow stacking the joists on the beams rather than dropping them in and reducing beam section (and strength).

Would this not reduce the headroom on the first floor? its currently at 8'. Or would you suggest lengthening the posts to make up for this reduction in height?

A continuous perimeter foundation is a much better idea than piers.

I agree, the reason for going with piers is cost; less excavation and I would imagine much easier to handle for a guy new to concrete work.

[Don P]

Lack of foundation is a false savings, it will almost always come back to bite you. A closed perimeter of walls provides bracing that piers cannot. Piers are the classic "soft story" that collapses under the rigid box on top of it in a wind or seismic event. The floors will feel much better. Weigh that savings carefully.

When sizing the beams that carry drop in joists, measure the width between beam pockets, that is the effective beam width. An 8x12 can quickly become a 4x12 of useable beam size. I'm not sure you'll lose all that much additional headroom by using an unnotched dropped girder

With the posts as is 3/4 of the roof weight is landing at midspan on the 2nd floor joists which are carrying that floor load as well. Vs letting the roof and second floor loads flow down the posts directly, putting no additional members in bending. That's big, again weigh it carefully. This can be done with the same vertical load paths, one row of columns, by using a ridgebeam. I've built temporary big boxes to slide along the floor to give the same safe working platform you are framing in. Don't let the raising ease of that queen drive it, you can duplicate that scaffold if that is the only reason.

[everythingwood]

I don't have any experience with piers but, like Don, I can see the problems.  You are counting on a dozen piers to be equally stable in the face of settling and frost heaving.  Better to have a continuous foundation.  If you want something you can do with minimal excavating an insulated monolithic slab would be my choice.  I'm using one for my woodshed... of course there is no plumbing to consider.  Careful planning will be needed for plumbing and utilities.  On the plus side, you don't have to worry about floor framing!
 
Here is one of the dozens of frames I sketched up for my woodshed.  I eventually settled on a monitor style building with separate lean-to's instead of the Dutch frame.  I have a post on it somewhere here and will begin a new one once I get to cutting joints (soon if the sawyer gets here next week).  



 

As you can see there are no timbers longer than 16 feet in this frame.  For the rafters there would be a splice of some sort at the purlin plate  The top plates of course will be longer and will have scarf joints, unless you go with an English tying joint or similar where the top plate is interrupted by the post.  I never went through with sizing the timbers for this design and I'm not sure on the extra braces supporting the tall knee walls but I have seen this on photos and drawings of Dutch barns.  I think the joinery is cool... you could leave it exposed or deck the side bays and use it as storage space upstairs.

I will be building my structure on a monolithic slab and planned on 6" concrete piers to keep the posts dry and off the floor  

[Epmhush]

Lack of foundation is a false savings, it will almost always come back to bite you. A closed perimeter of walls provides bracing that piers cannot. Piers are the classic "soft story" that collapses under the rigid box on top of it in a wind or seismic event. The floors will feel much better. Weigh that savings carefully.

We don't have much if any seismic activity and I don't think wind is much of an issue here (There's an old balloon frame house sitting on rocks across the road, 150+ years old that is in the middle of the field still standing) but I don't want to disagree just because it isn't what I want to hear.. What do you suggest for a suitable foundation that I could tackle myself? or if not tackle myself what would the most economical yet feasible option be? 

With the posts as is 3/4 of the roof weight is landing at midspan on the 2nd floor joists which are carrying that floor load as well. Vs letting the roof and second floor loads flow down the posts directly, putting no additional members in bending. That's big, again weigh it carefully. This can be done with the same vertical load paths, one row of columns, by using a ridgebeam. I've built temporary big boxes to slide along the floor to give the same safe working platform you are framing in. Don't let the raising ease of that queen drive it, you can duplicate that scaffold if that is the only reason.

For me its not only the ease of raising but also another 8x10 or larger for the ridgebeam is another large tree (and a high grade one at that) that I will need for the girts and I'm not sure how many of these we have on our land... also the danger of lifting an 8x10+ timber off of a second floor with two people would preferably be avoided.

Here is one of the dozens of frames I sketched up for my woodshed.  I eventually settled on a monitor style building with separate lean-to's instead of the Dutch frame.  I have a post on it somewhere here and will begin a new one once I get to cutting joints (soon if the sawyer gets here next week).  

That's a nice looking bent, I've always enjoyed the look of an anchor beam. How would the second floor framing tie in though?

Also the monolithic slab is an option, but as you said there are some important plans that have to be in place for the plumbing and one thing I liked about the crawl space I would get from piers is the ease of maintenance (and a bit of storage) also would this not make the frame susceptible to water damage from the rain splash on the ground?

[everythingwood]

That's a nice looking bent, I've always enjoyed the look of an anchor beam. How would the second floor framing tie in though?

In most frames the floor joists run from anchor beam to anchor beam with a girt at the same elevation between the posts.  On the smaller "wings" I lowered the tie below the anchor beam to keep from crowding the joints too much.  If I was planning this for living space I think I would enclose the upstairs on the outside (or inside) of the main posts and leave the smaller ties and roof framing exposed from below.  You could also add more small framing members running them from the girt to the plate (perpendicular to the main floor joists) and deck them on top then put in some small doors from upstairs and use the space for storage.

I just thought of something... if you widen the center isle to 14 feet but leave the peak height the same you decrease the roof slope a little and your side walls will be a little higher and perhaps you can put the small ties above the anchor beam (still trying to avoid all the joints meeting at one point).  If you did that and had the floor joists sitting on top of the anchor beam you could probably make the floors align from the center isle to the side isles.  Of course, you still have the purlin plate sitting there.  In any frame with a room under the eaves you have these short spaces.  Use them or hide them.

As for protecting the outer posts and sheathing from the rain, I agree.  you need to have at least a short perimeter wall sticking down so you can leave the slab stick up out of the ground, or put a row of block on the slab and leave rebar stick up where the posts sit so you can fill the block and put in one of the steel post bases to tie everything to the concrete.

ETA: Of course with a row of block you have that sticking in your living area :-[.  I may be talking myself around and back to a basement!

This is why it took me so long to finish the plans just for an outbuilding!  My wife asked me after about two months why my plan was taking so long and I told her it wasn't... I was just on my 20th plan :D.

[Epmhush]

This is why it took me so long to finish the plans just for an outbuilding!  My wife asked me after about two months why my plan was taking so long and I told her it wasn't... I was just on my 20th plan .

No kidding. I think I may have to downsize this frame so that I can afford to do a proper crawlspace foundation... and if I need additional space I can lengthen the frame later. I might even change it up to a small 2 story parlor type style building. I'd hate to get halfway through this frame and get posted to the other side of the country so smaller may be better.

[Epmhush]

So I've done a bit of reading on different foundations and I'm wondering what you guys think of a modified pier type foundation. If all of the piers were connected by a footing poured all at once to prevent joins in the concrete, with added "counterforts" to create ridged 45 degree "wings" from the piers. Would this not prevent the piers from moving independently of themselves, while also stabilizing them in the soil? I've been seeing a lot of people have the same comment on floors sagging with a crawlspace foundation so I'm now hesitant to spend the extra money on a foundation that may present similar issues but cost 3x or 4x as much. Thoughts?

[Hilltop366]

the building will be a cottage and some day a retirement home for me

This to me says at a minimum a crawl space would be well worth the extra concrete and expense.

The sagging floor should not make a difference one way or another, no matter how you do it you will need proper support for the frame and floor.

[Don P]

Beams on piers supporting a floor are more likely to sag and transmit vibration than a floor, walls, roof supported on continuous foundation walls.

Piers as wide as they are tall would not be likely to overturn but the floor and beams would need to be sized for the spans between pier tops and would be more likely to sag and transmit vibration. Those size differences figure into the cost offset. The hold downs would need to be stronger and the end product would be less satisfactory. If code matters, a pier foundation is an engineer required foundation. This comes from failures. Something to consider either way. The lightest prescriptive foundation is pier and curtain wall, you can find details in chapter 4 of the IRC. I have built on one and lived in another, and repaired a few more, not worth the small "savings" IMO.

[Epmhush]

So I've done some calling around and I've determined that a full perimeter pour is out of the question, to be able to have a concrete mixing truck get up the driveway would require it to be properly prepped which would cost north of 10K, and then the concrete pour alone would be another 5-10k. So I would be looking at 15-20 grand before I even cut a joint which for a small seasonal cabin is not reasonable. Not to mention I cant find an engineer with time and willingness to certify "owner cut and raised timber" in the area. Would a block foundation work for this? I could haul the blocks myself and do the work myself at an unrushed pace compared to pouring concrete.. I used a cost estimator and it came out to around 3000 dollars which seems reasonable.

[Mike W]

Considering the access limitations as well as resources to set, pour and finish a foundation by yourself, a block foundation set on a continuous footing would be a great solution for you.  You could set and pour the footing by hand mixing (not a lot of fun) but doable.  Install vertical rebar in the footing to tie in the block walls, use bond beam block to run horizontal bar ever couple rows, solid grout (more hand mixing) and could all be done by yourself or with a helper if available to you.  

Would eliminate a lot of the issues Don and others mentioned on the unpredictable behavior of separate piers.

Could even use split face or other deco CMU for the block above grade for a better look, they are usually only a $1 or $2 more than standard CMU block.

Keep the updates coming on which way you decide to go with it.

We are still planning our outdoor pavilion timber frame, think we are on rendition 28 or so, yea like others, a long process for us to get this started, and we already have the monoslab in place waiting.... :

[Don P]

It would work fine. Review chapter 4 of the IRC, or your code which is I believe similar.

I've mixed and poured quite a few footings by hand, and might be next week. I need to do some math to see if I have enough for the minimum for a truck, if so I'll have him pour into the bobcat bucket and run it in that way, otherwise I've got 2 pallets of bag mix stocked that can go for footings or grout pour.