28'x36' barn with attic room and dormers

[misterpetro]

I am new to the forum, the info is fantastic! I am planning a 28'x32' barn with an attic room and 1 large shed dormer (maybe 2). I will be building in Southern Michigan. I would like a free span floor and I have been researching Weyerhaeuser I-joist data, and I see an I-joist with a 28' free span spec. I am sharing the chart and looking for input to verify that I am reading this chart correctly. I would prefer 16"OC or wider, the chart claims a 19'2"OC is possible but I am not sure about that deflection spec. Thanks in advance for knowledge shared and all the great info on this forum. Might fumble the Pic posting, patience appreciated

 

[misterpetro]

Sorry, tried to insert a photobucket link and it did not work. Any advice on posting from photobucket? I would like the pic to show in the posting window.

 

[LB-1911]

Sorry, tried to insert a photobucket link and it did not work. Any advice on posting from photobucket? I would like the pic to show in the posting window.

Your not the first.......


https://www.google.com/#q=photobucket+garage+journal+site:www.garagejournal.com

 

[69gp]

You could use room in an attic trusses or steel floor joists. Steel floor joist you can clear span up to 33' and at that length the joist is only 14" deep. I am looking into using the steel on my garage.

http://www.clarkdietrich.com/support-tools/catalogs

http://www.clarkdietrich.com/sites/...talogs/FullLine/CD_FullLineCatalog_9-1-14.pdf

 

[misterpetro]

Here's a mockup of what I want my barn to look like (and a second try at photobucket)

 

[misterpetro]

I have the understanding that you can't cut attic room trusses without compromising the floor load, is that correct? Large dormers require lots of stick built roof rafters. Therefore, a 28' free-span I-joist is my preferred choice, trying to avoid steel and support posts.

 

[james.work.90]

I would definitely go with attic storage trusses or beefier joists for the attic. That table you have the l/360 standard circled it looks like and that's gonna be real bouncy if you put tools or store things up there. I build a lot of houses and I am all for not over building and wasting money, but the 360 standard is the bare minimum AND it assumes normal loading like an average room with some couches tables etc. Put a bunch of heavy tools or a ton of storage things up there and you might be disappointed. It won't fall foen but 20 years from now you'll be happy if you did beef those up.

 

[james.work.90]

I would also look at floor trusses. Menard sells trimmable ones and any local lumber yard or truss manufacturer should be able to give you a price with your description above.

 

[misterpetro]

I would also look at floor trusses. Menard sells trimmable ones and any local lumber yard or truss manufacturer should be able to give you a price with your description above.

Thanks James, I will have another look at those floor trusses.

 

[MushCreek]

The other thing people don't plan on is the trampoline effect of a big floor. At 1/360 deflection, that works out to 15/16" over 28 feet. When somebody goes walking by, you can really feel the floor bounce.

 

[JCByrd24]

MushCreek and others are right on, when you have a 28' span the deflection turns into bounce. If you are stuck on I-joist go with 16" deep 560s at 16" OC. I bet you can do a little better with open web trusses.

 

[dfiler2]

You can control the bounce.

I used 28' 14" I-joist 16" O.C. with 3/4" T&G Strong Floor product on the top and panel liner fastened on every joist on the bottom, there is no bounce.

The panel liner acts like bridging, you can also add 1x4 or 2x4 to the bottom of the floor joists to spread the load to multiple joists.

I also had the under side between the joists sprayed with 1.5 inches of spray foam as the loft is storage only and not heated.

 

[james.work.90]

Dfiler continous bridging or better yet drywall or plywood certainly gives you a bump in rigidity but nowhere near that muxh. We evaluate spans in a program called Strucalc all the time and adding the option modifies the adequacy of a span by 1-2 percent. 360 vs 480 you would be 20-30 percent inadequate....upsize them. You can't rely on bridging or sheeting to get the span you need. Cetainly not a bad idea to do it though.

 

[theoldwizard1]

The most important thing is defining the load that has to be carried. My buddy stores hay "upstairs". He uses 18"-24"wood floor joist 16" O.C.

Traditional rafters above, give him a completely open space.

 

[theoldwizard1]

Start thinking NOW about heat and cooling requirements and therefore insulation. If any of the space will be conditioned, your building would be a prefect candidate for Structured Insulated Panels (SIP).

Unless you have natural gas available and NO requirement for cooling, a mini-split heat pump (2-3 ton) with 2-4 air handlers would work well.

"Piss poor planing prevents proper performance !"

 

[misterpetro]

You can control the bounce.

I used 28' 14" I-joist 16" O.C. with 3/4" T&G Strong Floor product on the top and panel liner fastened on every joist on the bottom, there is no bounce.

The panel liner acts like bridging, you can also add 1x4 or 2x4 to the bottom of the floor joists to spread the load to multiple joists.

I also had the under side between the joists sprayed with 1.5 inches of spray foam as the loft is storage only and not heated.

Thanks Dfiler2. I have been in houses with I Joists and they are rock solid. I eill also end up with a few small partition walls which will help. The T&G floor and bracing will help for sure!

 

[misterpetro]

Start thinking NOW about heat and cooling requirements and therefore insulation. If any of the space will be conditioned, your building would be a prefect candidate for Structured Insulated Panels (SIP).

Unless you have natural gas available and NO requirement for cooling, a mini-split heat pump (2-3 ton) with 2-4 air handlers would work well.

"Piss poor planing prevents proper performance !"

Thanks, I will look into a mini heat pump. I will not have natural gas available, propane or electric only, and lots of firewood.

 

[misterpetro]

Thanks for all the advice! I'm adding a lean-toto my plan. Was going to do it later, but might as well do it now. Here's my rough design.

 

[dfiler2]

Dfiler continous bridging or better yet drywall or plywood certainly gives you a bump in rigidity but nowhere near that muxh. We evaluate spans in a program called Strucalc all the time and adding the option modifies the adequacy of a span by 1-2 percent. 360 vs 480 you would be 20-30 percent inadequate....upsize them. You can't rely on bridging or sheeting to get the span you need. Cetainly not a bad idea to do it though.

Right, I'm only suggesting that to reduce the bounce. You can build a floor that meets all the codes and span table requirements but people are uncomfortable with the bounce.

 

[rburke65]

Floor trusses are rock solid. Nice looking garage you have going there. Update your location in your profile. Good luck.

 

[misterpetro]

Thanks Guys, after more research, I am really liking the floor truss idea. Next question is if I can eliminate the 12/12 pitch with a small knee wall on the 2nd floor. I will send a sketch later.

 

[fastjohnny]

I spanned 26' with 14" I joists, 16"OC. Nice joists, with 3.5" LVL flanges, but way to bouncy for my liking. I added a center beam perpendicular to the I joists, making the floor rock solid. I would definitely consider the floor trusses 16" or greater.

 

[misterpetro]

I found this sketch. I'm not sure about this. The knee walls are not tied in to the floor joists. Can this be done? I'm trying to reduce my pitch from 12/12 to 10/12 and a small wall on the second story would help with ceiling height.

 

[59bones]

I built a garage with a very similar section to yours (minus the lean-to). Your 12:12 pitch should carry completely on the header in the exterior wall and transfer no loads, zero, nada, nothing, zilch to the floor joist framing. You show a wall or stud in your diagram.

I tripled up my rafters on each side of the dormer and built the dormer on the rafters thus transfering load to the outside wall and not to the floor joists. I built the wall a little short and even used a Simpson slip clip to anchor the wall. This way, even when the roof deflects under snow load, it is again not transfered to the floor joists.

This all applies regardless of your floor joist system.

If using a TJI chart, stick with the L/480 for deflection and stick with the 40/20 loading. Avoid 19.2" o.c cause it is a pain with sheathing and drywall.

 

[misterpetro]

I built a garage with a very similar section to yours (minus the lean-to). Your 12:12 pitch should carry completely on the header in the exterior wall and transfer no loads, zero, nada, nothing, zilch to the floor joist framing. You show a wall or stud in your diagram.

I tripled up my rafters on each side of the dormer and built the dormer on the rafters thus transfering load to the outside wall and not to the floor joists. I built the wall a little short and even used a Simpson slip clip to anchor the wall. This way, even when the roof deflects under snow load, it is again not transfered to the floor joists.

This all applies regardless of your floor joist system.

If using a TJI chart, stick with the L/480 for deflection and stick with the 40/20 loading. Avoid 19.2" o.c cause it is a pain with sheathing and drywall.

Thanks for the info 59 bones. Are you speaking of the knee wall diagram? Is that a viable building technique? Can you set the roof rafters on a 2 ft knee wall above the floor trusses?

 

[MushCreek]

AFAIK, to build on a knee wall like that, you've either got to use a ridge beam to support the rafters, or have a collar tie within the bottom third of the rafters. Otherwise, the weight of the roof and any other load, such as snow, will cause the rafters to spread from outward thrust, and the hinge point of the knee wall won't offer enough resistance. A ridge beam is a load-bearing member (as opposed to a ridge board) and has to have support in the middle, or be hefty enough to span the entire length.

Another option is to balloon frame the walls, and hang the floor rafters on the inside so that the walls are continuous without a hinge point.

 

[misterpetro]

AFAIK, to build on a knee wall like that, you've either got to use a ridge beam to support the rafters, or have a collar tie within the bottom third of the rafters. Otherwise, the weight of the roof and any other load, such as snow, will cause the rafters to spread from outward thrust, and the hinge point of the knee wall won't offer enough resistance. A ridge beam is a load-bearing member (as opposed to a ridge board) and has to have support in the middle, or be hefty enough to span the entire length.

Another option is to balloon frame the walls, and hang the floor rafters on the inside so that the walls are continuous without a hinge point.

Thanks MushCreek. I liked the sound of collar ties/ceiling joists for the roof and this small knee wall. I will take another crack at my design with a 10/12 pitch and see what I get for an attic room.

 

[59bones]

Thanks for the info 59 bones. Are you speaking of the knee wall diagram? Is that a viable building technique? Can you set the roof rafters on a 2 ft knee wall above the floor trusses?

I know it is difficult to verbalize but...

I was referring to your diagram showing vertical studs located at the intersection of the 8' high ceiling joists and the 12:12 pitch roof rafters. If you do that without a "slip-clip", the studs become load bearing and then transfer loads to the floor joists/trusses. That is bad with "I-joists. It is not good with conventional lumber. It can be done with open web trusses but your truss designer must be aware of the imposed loads.

I suggest not doing that and let the roof sit on the outside walls and build your dormer on top of some beefed up rafters. This will also give you some neat little triangular spaces on each side of the dormer.

Another poster was right about his comments about collar ties and their locations but installing them will not eliminate roof loads on the wall in question.

If you like, PM me your phone number and I will call you.

 

[Jlbc212]

AFAIK, to build on a knee wall like that, you've either got to use a ridge beam to support the rafters, or have a collar tie within the bottom third of the rafters. Otherwise, the weight of the roof and any other load, such as snow, will cause the rafters to spread from outward thrust, and the hinge point of the knee wall won't offer enough resistance. A ridge beam is a load-bearing member (as opposed to a ridge board) and has to have support in the middle, or be hefty enough to span the entire length.

Another option is to balloon frame the walls, and hang the floor rafters on the inside so that the walls are continuous without a hinge point.

What he said ^^^
A pitched gable roof should form a triangle. The two opposing roof rafters are two legs of the triangle and the ceiling/floor joist are the third leg. Triangles are inherently stable. With three sides and three connecting points only one shape can be formed. Add one or more sides (such as the knee wall the OP questioned) and the new shape becomes unstable. A ridge beam acts like a bearing wall and the roof essentially becomes two lean-to shed roofs.
I have a 10:12 pitch on my 28 ft deep house and garage. It makes for a good size attic room.

 

[theoldwizard1]

I found this sketch. I'm not sure about this. The knee walls are not tied in to the floor joists. Can this be done? I'm trying to reduce my pitch from 12/12 to 10/12 and a small wall on the second story would help with ceiling height.

MY gut says that knee wall will cost a lot !

 

[theoldwizard1]

Thanks MushCreek. I liked the sound of collar ties/ceiling joists for the roof and this small knee wall. I will take another crack at my design with a 10/12 pitch and see what I get for an attic room.

A collar tie reduces the second floor ceiling height by 1/3 !

 

[theoldwizard1]

If you are even MILDLY interested in SIP construction, you should understand how to optimally design and build with them.

The typical Structurally Insulate Panel (SIP) gable roof uses a structural ridge beam (4x10 - 4x14) LVL. The load from this beam must be transferred directly to the foundation at each end (tricky if you have a garage door on the end, but doable). The panel is one piece,4' wide, from ridge beam down to the side wall overhang and "flown" into place with crane.

For a build in you area, you would probably want a 10-12" thickness of insulation in the roof panel. Typically, a 2by of that thickness is used to "spline" 2 - 4' wide panels together and if becomes the "rafter". Lots of construction adhesive and really long screws.


If you really like the idea of a knee wal on the second floor, you could just add 4' to the wall panels, The ceiling joists for the second floor would rest on 4x4 wood "columns" that stood on the foundation but attached to the inside of the SIP wall panel. Because the floor is not transferring load to the wall, you can put the ceiling anywhere yo want !

 

[misterpetro]

That sounds really good Wizard. I like the last paragraph. I am looking to stick build and I have not yet considered SIP

So, if I continue the stick build method, could I still use 4x4 or 4x6 posts every 9 feet (4 total over 36' length) from the foundation all the way up and into the knee wall structure? I assume I would cap that whole secont story 2x6 knee wall structure, including the posts, with a double top plate and support the roof that way. This way I can still rest my floor trusses on top of my first story wall. Does that make sense?

Would I still need a ridge beam, or would a ridge board be sufficient for the roof construction?

If you are even MILDLY interested in SIP construction, you should understand how to optimally design and build with them.

The typical Structurally Insulate Panel (SIP) gable roof uses a structural ridge beam (4x10 - 4x14) LVL. The load from this beam must be transferred directly to the foundation at each end (tricky if you have a garage door on the end, but doable). The panel is one piece,4' wide, from ridge beam down to the side wall overhang and "flown" into place with crane.

For a build in you area, you would probably want a 10-12" thickness of insulation in the roof panel. Typically, a 2by of that thickness is used to "spline" 2 - 4' wide panels together and if becomes the "rafter". Lots of construction adhesive and really long screws.


If you really like the idea of a knee wal on the second floor, you could just add 4' to the wall panels, The ceiling joists for the second floor would rest on 4x4 wood "columns" that stood on the foundation but attached to the inside of the SIP wall panel. Because the floor is not transferring load to the wall, you can put the ceiling anywhere yo want !

 

[theoldwizard1]

So, if I continue the stick build method, could I still use 4x4 or 4x6 posts every 9 feet (4 total over 36' length) from the foundation all the way up and into the knee wall structure?

First, I am not a structural engineer !

Second, what I was trying to describe was a method of building the second floor such that it was NOT structurally connected (no load transferred) to the outside walls ! I suggested this, because it would save money when building with SIPs to just have a one piece 12-14' wall with the rafters being the only load resting on top. The posts would have to be under each second floor joist, so every 16-24" O.C. (?) depending on your load. Yes this is a lot of extra wood although the columns could be made up of 2 - 2bys. This would eat up a lot (more than 100% ?) of the cost savings of the extra tall one piece SIP wall. ($500-$600 for 72 - 2x6x10'.) Don't forget to factor in the double wide foundation cost !


To do what you are suggesting in a "stick built" structure would require a large "header" (2 - 2x10 or 2x12) to span on top of and in between each post for the joists to either rest on or be connected to with joist hangers. This is not a cost effective way of doing stick built.

 

[theoldwizard1]

Would I still need a ridge beam, or would a ridge board be sufficient for the roof construction?

There is a big difference between a ridge "board" and a ridge "beam" (much hand waving to follow ! )

The board is just to provide a nailing surface for the rafters to prevent them from flopping around until the roof decking is installed. It carries "no load", meaning all the load is carried down the rafters to the wall. A traditional wall would like;y "bow" out without a collar tie or rafter tie.

A ridge beam is a structural member designed to "carry part of the load" from the roof. This load is transferred via structural columns (usually built inside the end walls) to the foundation. Because the ridge beam carries part of the roof load there is less load on the walls and less outward force. Less outward force means you probably will not need a collar or rafter tie, giving you a much more open space.

If you don't need all of that space, attic trusses are MUCH more cost effective !!

 

[oldcpecdr]

On mine I ran the shed dormer ALL the way across..almost no increase in cost and a LOT more sq feet upstairs...you can subdivide it any way you want.

I went for a steeper roof pitch and actually have a third floor loft as well...great for bulky light storage.

Also plan your lift beam and trap door to get heavy stuff upstairs....a lot easier than after building like I did...

Mike B

 

[Jlbc212]

In stick built structures the ceiling/floor joists are an integral part of the roof structure. The joists are under tension and keep the walls from pushing out under the weight of the roof. An alternate method of stick build construction when there is a desire for a cathedral ceiling (or in your case a desire for a knee wall) and the ceiling/floor joists are not tied to the roof rafters would be the installation of a ridge girder. The ridge girder acts like a bearing wall. Each half of the roof now acts like a lean-to shed roof where the outward push of the roof on the walls is eliminated. In your situation the ridge girder would have to be enormous because of the length of its span, 36 feet. IMO your best route would be to either purchase attic trusses with the dormer factored in or to stick build with a 12:12 roof pitch and a 4:12 roof pitch for the dormer.
The sketch you posted looks nice, but it is only a conceptual drawing. I wouldn't build like that.

 

[rustynutts]

I don't think it is structurally possible to design a room truss with a dormer.
I would recommend an engineered floor truss and a stick framed roof system. Working with your second design, I will suggest a few changes. Make the dormer and the shed roof the same length as the building. Frame a 1' overhang at the 12/12 pitch to keep the look of your first design, but give maximum space and structural stability to the building. Keep the ceiling joists as shown, the interior wall is not needed structurally. Frame a knee wall on top of the first floor wall to support the shed roof rafters and help eliminate push on the main rafters. That's my vision of an aesthetically pleasing and structurally sound building.

 

[misterpetro]

That sounds great Oldcpecdr. How did you tie the 2 outside dormer walls and the roof rafters? 28 feet is a long span. I would be interested in hearing more about your double dormer barn. I just sketched one. The attached sketch includes rafter ties across the dormer span and collar ties on the 12/12 pitch wings. Is this the proper engineering for this type of structure?

Note: Gabel end walls are removed to show interior rafter framing.

On mine I ran the shed dormer ALL the way across..almost no increase in cost and a LOT more sq feet upstairs...you can subdivide it any way you want.

I went for a steeper roof pitch and actually have a third floor loft as well...great for bulky light storage.

Also plan your lift beam and trap door to get heavy stuff upstairs....a lot easier than after building like I did...

Mike B

 

[misterpetro]

OK Guys. I found this photo. This is a second story with dormer. The roof rafter collar ties are not set on the top plate of the outside dormer wall. Is this adequate engineering for a 28x36 barn?