Input needed for roof framing design

[Boyd]

I'm (still) in the planning stages of an upcoming 20x40 detached garage build. I'm limited to the narrow width due to lot setbacks. In order to fit as much square footage as possible into the limited footprint, I'd like to frame the roof in such a way that maximizes the open/useable space on the second level.

I've done a ton of online research about loads and I've looked at so many span tables it's made me cross-eyed, but I'm still not sure about my roof framing plan. The sketch below is the best I could come up with, having to keep to an 8/12 roof pitch to match the house. Does it look kosher?

(Apologies for the pic quality, my scanner got confiscated for a school project so I snapped a pic of the sketch with my phone)

 

[Boyd]

And I'm completely open to suggestions if anyone has any other ideas on how best to utilize/maximize the space, irrespective of the framing.

 

[Jeremy77]

I would notch those rafters with a "birds mouth" so that they rested on the joists.

 

[Jeremy77]

Are you stick framing or having trusses built? Not sure if you have to worry about a snow load ( I spent time in Borger, Tx. And we got a lot of snow.) but if not, 2x10 joists and 2x6 rafters should be more than sufficient to span 20'. How far on center are you placing them?

 

[little d]

Boyd,
first off, outstanding job on doing your homework!

A few things I see; you dont have a ridge board, not saying you need one but if you are doing it by yourself, it will be a lot easier to nail the rafters in as apposed to holding one up while setting the other one (unless you are going to assemble them on the ground first). On the collar ties, knee walls and ceiling joist's, if you are not planning to cut them to fit in the structure (just nailing them to the side), do yourself a favor and cut them to fit in the structure and use plywood to sandwich, glued and screwed to secure them, this will make for a much stronger build and you could replace the 2by6 collar ties with just plywood. If you don't do this day in and out and can't do it on the run (I can't anymore), you can snap chalk lines on your floor for a pattern, make sure the first one fits and then make all of the cuts the same. On the knee wall/joist/rafter, 20' is a pretty good span if you plan on any weight up there but, if you add another 2by4 from where the bottom of the knee wall joins the ceiling joist at 45 degrees up to the rafter, you have webbing like a truss, making that part of it rigid and the only sag will be from the top of the knee wall to the bottom of the collar tie and no farther then it is, it wont be much. Oh, and use hurricane ties to attach to the walls, toe nailing is a lazy mans way of doing it, do it right.

I hope you can use some of this, good luck and keep us posted,
D.

 

[finn]

A truss company can provide engineered attic trusses to your requirements with a certification stamp for not much more money.

Makes the building go up a lot quicker, and probably straighter.

 

[Firebird 1]

Get Trusses. If youre doing the job yourself hire a contractor for the day to set and brace them and put on roof sheathing. They would not charge that much and you will save a months worth of work. (what is a months time worth to you?) Go to a GOOD lumber yard and they will design them for you any way you want them. You can have a very nice storage loft. Think outside the box, trusses do not have to have all the diagonal braces wasting space. I am a 30+ year master carpenter and a test I have always given new hires was their ability to lay out a rafter, which I believe no one should be called a carpenter if they don't know how to read a framing square. However in this day for a simple barn structure I wouldn't use anything but a pre engineered truss.

 

[NUTTSGT]

Like the last couple of replies, I'd have a set of trusses built for the garage. Take your drawing to the lumberyard or Menard's and have them use your drawing as the template as to "what you want."

 

[kpete]

Agree 100% with the last few replies, get trusses. The cost difference plus strength far outweighs and time spent on rafters/joists. Also, if you design the truss with a bottom cord overhang rather than the top cord, it will give you more headroom, since you're not only gaining the height of the bottom cord, but also the width of the overhang as well.

 

[kpete]

 

[Boyd]

I would notch those rafters with a "birds mouth" so that they rested on the joists.

Thanks, I will. The sketch was just something I put on paper to verify the storage are dimensions.

Are you stick framing or having trusses built? Not sure if you have to worry about a snow load ( I spent time in Borger, Tx. And we got a lot of snow.) but if not, 2x10 joists and 2x6 rafters should be more than sufficient to span 20'. How far on center are you placing them?

Borger, huh? Small world. I'm originally from the panhandle area. As a kid I lived in Plainview, Floydada, and Lockney. You are correct. In my area of the state I don't need to worry about snow load. My calculations say I can go 24" o.c. with the rafters. Structurally, I'd feel more comfortable at 16" o.c., but I'm also concerned about thermal transfer through the rafters. Our Texas summers are brutal and with a dark roof I'll need to use every trick I can to minimize heat transfer.

I looked at trusses, but there aren't many "ready made" options for my specs, and custom trusses were cost prohibitive. Stick framing a simple gable roof with a relatively small span shouldn't be too difficult (I hope).

 

[Boyd]

Boyd,
first off, outstanding job on doing your homework!

A few things I see; you dont have a ridge board, not saying you need one but if you are doing it by yourself, it will be a lot easier to nail the rafters in as apposed to holding one up while setting the other one (unless you are going to assemble them on the ground first). On the collar ties, knee walls and ceiling joist's, if you are not planning to cut them to fit in the structure (just nailing them to the side), do yourself a favor and cut them to fit in the structure and use plywood to sandwich, glued and screwed to secure them, this will make for a much stronger build and you could replace the 2by6 collar ties with just plywood. If you don't do this day in and out and can't do it on the run (I can't anymore), you can snap chalk lines on your floor for a pattern, make sure the first one fits and then make all of the cuts the same. On the knee wall/joist/rafter, 20' is a pretty good span if you plan on any weight up there but, if you add another 2by4 from where the bottom of the knee wall joins the ceiling joist at 45 degrees up to the rafter, you have webbing like a truss, making that part of it rigid and the only sag will be from the top of the knee wall to the bottom of the collar tie and no farther then it is, it wont be much. Oh, and use hurricane ties to attach to the walls, toe nailing is a lazy mans way of doing it, do it right.

I hope you can use some of this, good luck and keep us posted,
D.

I plan to have a ridge board, but I do like your idea of building my own trusses. Currently, I have more time than money, and I'm always up for learning new things and expanding my skill set. Thanks for your input! I can definitely use it.

 

[joe_padavano]

I plan to have a ridge board, but I do like your idea of building my own trusses. Currently, I have more time than money, and I'm always up for learning new things and expanding my skill set. Thanks for your input! I can definitely use it.

In my experience, it's cheaper to buy commercial trusses than it is to build your own. I can't buy the wood for what I can get trusses delivered to my site. Home Depot and others are a very reasonably-priced source. If you care, these trusses also come with complete engineering documentation for an inspector. It just isn't cost-effective to do it yourself.

 

[ddawg16]

Or do like me and go with a ridge beam that will support the roof rafters. Then you end up with a clear space.

Click on the garage link in my signature.

Basically, I have full use of the 20x27 area above my garage.

Here is one view of my upstairs

 

[Boyd]

A truss company can provide engineered attic trusses to your requirements with a certification stamp for not much more money.

Makes the building go up a lot quicker, and probably straighter.

The truss company I got a quote from seemed awful high vs. what I could stick frame for. But I do like the idea of quicker and straighter.

Get Trusses...I am a 30+ year master carpenter and a test I have always given new hires was their ability to lay out a rafter, which I believe no one should be called a carpenter if they don't know how to read a framing square. However in this day for a simple barn structure I wouldn't use anything but a pre engineered truss.

Thank you for replying. I like getting advice from very experienced folks.

Like the last couple of replies, I'd have a set of trusses built for the garage. Take your drawing to the lumberyard or Menard's and have them use your drawing as the template as to "what you want."

I didn't think of going directly to a lumber yard for a truss quote. Thanks.

Agree 100% with the last few replies, get trusses. The cost difference plus strength far outweighs and time spent on rafters/joists. Also, if you design the truss with a bottom cord overhang rather than the top cord, it will give you more headroom, since you're not only gaining the height of the bottom cord, but also the width of the overhang as well.

I didn't think about that! Great idea! Seriously.

Well, it looks like the consensus is to revisit the idea of trusses. Admittedly, I only got one quote, but it was from a truss manufacturer that caters to the commercial sector. I didn't think to get quotes from actual lumber yards (obviously suffering from "home improvement store syndrome". )

I'll do some more info gathering and update the thread with the results. Thank you all for the input.

Here's a couple of renderings of what I hope to accomplish: (new garage is on the right)

 

[Boyd]

Or do like me and go with a ridge beam that will support the roof rafters. Then you end up with a clear space. Click on the garage link in my signature.Basically, I have full use of the 20x27 area above my garage.

This is exactly the kind of spit-balling I was hoping for. LOVE this idea. Hmmm...

 

[tkbowman]

I singularly built my 24 x 40 garage some years ago and built my own roof assembly with 2x6 construction and a ridge beam. It's similar to your drawing except I have an 8 foot ceiling for most of the width and 7 ' knee walls. In the triangle area I use that for closet space.


Tim in Burien, WA

 

[joe_padavano]

The more fundamental question I have is, why are you even considering trusses for this 20ft wide building if you want to use the attic for storage? I built a 20x24 garage about ten years ago and since I wanted a completely clear upstairs for storage, I used 2x12x20 ceiling joists and simply laid out 2x8 rafters. Your 20x40 building would be exactly the same, just with more rafters. That IS cheaper than trusses.

 

[JerryB]

I've done both: Engineered / manufactured commercial trusses and home built.

My 48' clear span hanger (now my garage) was a fully engineered structure, but built by a good friend (Jeff) who is a lifetime builder. I was his helper. We bought the trusses from a semi-local truss supplier. Engineering specified 2x4 & 2x6 construction using commercial nail plates on all joints. The 48' span would have been very difficult (impossible?) to deal with on our own. It was a pleasure that the trusses were affordable and arrived exactly on schedule. Watching them 'fly' them into place was really interesting!

My carport recently got a new 4 in 12 pitched roof, replacing a very old, very leaky flat roof. Same friend / builder. The clear span is 22' and the structure is ~26' long, requiring 14 trusses. Jeff 'designed' the trusses, and it only took the two of us (along with my wife handing us stuff) less than 3 days to layout and build a simple jig (using plywood that was for the roof sheeting), cut all the materials, and assemble the 14 trusses. All joints were cut to fit and gusseted on both sides with 3/4" plywood that was glued and nailed. The only commercial fasteners are the nails & screws.

I was really amazed at how fast this went, but fully realize that Jeff's knowledge and work ethic were critical to getting this done so quickly! I would probably still be out there trying to figure the angle and length of some component!

My workshop has an attic space over about half of the building that is constructed of 2x6 joists and 2x6 rafters. It has a 4x6 ridge beam that the rafters attach to. All decked with 3/4" plywood and has a 4'x~5' access door at one end. Lots of open storage space there, and lowest cost construction.

 

[theoldwizard1]

Either buy engineered "attic trusses" or hirer an engineer and let him design your attic space with no verticals ! I can be done, but all of the lumber will have to be upsized. That is why you need a certified engineer !!

P.S. A collar tie (that short horizontal piece connecting the left and right rafter) is typically located 1/3 of the way down from the ridge.

 

[rburke65]

I know you said you had more time than money, but time is money......

 

[ddawg16]

I know you said you had more time than money, but time is money......

But time is only money if you're being paid for your time.....or having to pay someone else for their time.