Roof Truss question

Sun Mar 27, 2011 8:43 pm

I must re-roof my storage shed style workshop soon, which will include having to replace some of the rafters damaged by water.

The workshop is 14ft x 16ft and currently has a gable style roof with 1ft overhang on the side with the door. The gable is on the 14ft end. When you look at it, it looks like it was meant to overhang 6" on either side, but was instead installed flush with the back side and overhung 12" on the front. The pitch is even on either side of the ridge board, but it is off-center to the building. The roof system is a simple rafter and ridge board design made of 2x4's. There is one collar beam in the center of the 16ft run. There is much of it that needs replaced as well as all the sheathing.
To quell some related concerns, all the exterior sheathing is going to be replaced also.

This leads me to my roof truss question!

I'd like to "change" the entire configuration of the roof, to include a 2ft overhang on one side and an 8ft overhang on the other side to provide shelter from rain and snow for my Cub and implements. This means no 4x4 posts at the ends of the 8ft overhang. Yes, I am going to take advantage of the townships own stupid zoning law. They tell me I CANNOT add a lean-to that has new posts touching the ground...but nothing says I can't re-roof it in this manner....GO FIGURE.

8ft is all I need to cover the tractor, the building stands between the tractor and the direction the weather shows up...the 2ft overhang on the other side makes the lumber an even 24ft!

I've already priced out all my material needs that I can think of.

So, now the engineering aspect collides with the "want".

I wish to construct W-type trusses (9 of them) to do this. This equates to 24" spacing.

If I use a (1piece) 24ft lower chord out of 2x6 stock, will it handle the 8ft overhang with 1/2" OSB sheathing and asphalt shingles? There will be NO gutter mounted off the 8ft overhang end either.

Will I need to use 2x6 for the upper chords and web members as well?, or will 2x4's suffice for these truss parts?

The upper chords will NOT extend beyond the ends of the lower chord.

Plywood or galvanized gussets?

Anyone have a roof pitch concern? I was going to go with 4/12.

Re: Roof Truss question

Sun Mar 27, 2011 10:25 pm

Marion:

Local Building Codes may take precedence as well of course as the US NBC. I am familiar with our Cdn NBC which is similar to the US code. Here is some design information that might be of help as well... Building Your Own Roof Trusses. Another useful pub would be this one - Housing Notes No. 12 It has been what ???? 37 years since I left my Architectural Design Technician course back in the mid 70's, so I am not all that letter perfect on code or design considerations.

What follows is just going to be my experience from building my own trusses for my New Home For My Cubs project as well as building a number of custom truss designs over the years as well as some cantilevering solutions.

My trusses are 32 feet from facia to the truss extension to go to the peak of my pre-existing roof. I built these trusses from 2x6 for the top chord and 2x4 for the much shorter bottom chord. I used 1/2" plywood gussets mostly because I had the stock, I could cut it to the exact size I needed and because the plywood gussets become part of the union, it becomes a little extra load bearing surface for the strapping. An extra inch on each truss .. worked out to be a bonus. As well you can see the gussets are full span gussets running from top to bottom chord. I also used my pneumatic nailer with 2-3/8" 28 degree nails.

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There are almost 8 foot extensions added to the left hand end of the trusses as seen above. Based on the winter we had here this year I am glad I used the 2x6's for the top chord as my roof did not move at all and we had a lot of snow :!: Most asked why I used 2x6 stock instead of the standard 2x4. I do like overbuilding. Safe beats sorry. That was my answer. Over build if you have the room in the budget. It takes the worry out of stuff.

I am trying to picture your roof in my mind.. and by your description, since you are currently framed with 2x4's proper trusses would definitely be a worthwhile upgrade. What you are considering here is a cantilevered truss and when cantilevering trusses you get into a whole new area of load stresses that is definitely going to change the truss dynamic as well as it's load reaction. To my mind without getting into specifics, I would think that there would have to be some lateral connections within the truss itself aside from the purlins/strapping on the top chord. I can from experience state that cantilevering can be a really cool way to gain space without investing in additional load bearing structures. 16 or 17 years ago I designed my buddy's renovation and the hexagonal kitchen was cantilevered 8 feet. Was challenged by our building inspector so I had to have an engineer check it out. Didn't require a redesign. I guess what I am trying to say is probably going to be best to see if you can find an engineer or architectural design course where you can pose the question on this truss design as a class design solution. I know when we were learning our prof actively looked for novel/innovative design needs in the local community. Here are some ideas on Truss Configurations

Designing by consensus on a forum is not the best way to go. I know it is a bit much for not much of an answer, but I had to walk myself through this explanation so it made sense to me in print. Do a floor plan of your storage shed, then do a truss plan showing where you want to place the trusses and the load points. Also show where the cantilever will begin (at the load bearing wall) and dimension it out the desired distance. Then take the plans/sketches to your local college and see what they can do as a hypothetical. As long as it is a hypothetical you can stay away from possible confilcts. I hope some of this might be useful.

Re: Roof Truss question

Mon Mar 28, 2011 5:35 am

Marion, IF you have a Menards in your area contact their building materials estimator he may be willing to do a site visit and give you a good plan. It sounds like you may want to go with a truss instead of adding on to the ridge beam roof you have depending on what your roof pitch is.

MY estimator makes job site visits all the time so don't let them tell you they don't. after all he gets a commission if he sells you the trusses.

Steve

Re: Roof Truss question

Mon Mar 28, 2011 4:42 pm

Steve,
I will not be adding to the existing ridge board if I go with the change. If I keep it the way it is, I will simply replace the damaged rafters and recover it with new sheet goods.
Trying to add to what's there and make what I want looks like more work than making up the trusses, and most likely will not be as strong or stable.
If I go with the overhang style I want, I will be removing the entire old roof and start from there.

Rudi,
Wanna visit Canton Ohio? :lol:
I understand most of your answer. The last thing I want is to look out one winter morning and see that 4" of snow on the roof tilted it off the end and onto my cub :censored: !
I get this visual...Fred Flintstones "car" tipping over when they put the slab of ribs on the tray at the drive-in...anybody remember that ??? HAHAHAHA

However, one 'could' argue that there would be 4" of snow on the other side of the peak of the roof equalizing the weight distribution. Nonetheless, I would probably 'build safe' as you mentioned and double up the 2x4's under the cantelevered side of the trusses just to sleep better at night! I have little doubt the truss would fail under any conditions we have here whether I use 2x6's all over or even 2x4's for everything that isn't horizontal. I think I just need to make sure that back wall will support the load, and perhaps get creative with SECURE fasteners when attaching the trusses to the building.

Re: Roof Truss question

Mon Mar 28, 2011 8:43 pm

Marion:

Yup, I certainly remember the Brontosaurus Burgers and the Ribs :lol: Course that kinda dates us huh :?: :?: :?:

I don't like being ambiguous but without having a sketch in front of me, my structural design manuals and a calculator there is no way I even want to contemplate designing someone else's trusses. With that said however, when I did mine a lot of it was done whilst sitting on an upturned 20 litre fuel pail and simply studying the problem visually. Once I had that done, the design problem was much easier even though the math was kinda onerous.

You already know what and how you want to do it, I suspect that you are probably close ... best be safe if you can. Do you have access to a College design course that the question could be posed to? I would feel much more comfortable if they ran the design through the truss stress program (we used one called STRESS way back when -- used punch cards -- remember those?? ) to see what the loads would be like, how much deflection the truss will withstand as well as torsion and the compressive forces that will all come to play especially on the cantilevered portion. That is where my concerns would lie.

If you look at the Cantilever options in this document .. Maple Valley Truss, you will see what I mean.

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Your cantilever dimension is going to be about 8 foot. So you are going to have to design a sub truss within your truss as well as use continuous lateral bracing to tie them all together. That sub truss would have to be constructed in the area of the overhand -- as designated by the cantilever dim in the sketch above. You might get away with building another W-truss within that area.

Image

The CLB will reduce torsion and strengthen the framework so that the the amount of deflection occurring from compressive forces will be lessened because of the additional chord and webbing material. This will provide the strength needed to withstand the snow load, as well as the wind load. How you tie your trusses to the load bearing wall abaft of the cantilever will determine how well the truss will withstand some of the wind loading.

You will have to consider how you are going to anchor those trusses. I would think that a continuous saddle bracket would be a definite option which would allow for through bolts and square washers. By using say a saddle bracket you will be adding to the structural integrity when it comes to vertical uplift from lateral wind loading.

Oh an afterthought. I would definitely be using plywood gussets as the plywood gussets will resist torsion from lateral wind loading whereas the galvanized expanded sheet metal gussets will not.

Other question ---- the walls of your shed -- western framing 2x4 walls or cinder block?

That is about all my tired brain can handle at the moment .... I hope I made some sense :roll: