Roof Trusses

[jasepaterson]

Hi, I'm looking for some ideas on building roof trusses. I am building a 24 x 24 foot cabin in the bush here in Northern Ontario Canada and am wondering what sort of patter to use for the trusses, a web, or ridge pole assembley or what, I am not an excellent carpenter but am quite capable if I have help.

Thanks

[mjpliv]

Some rules of thumb -

Do not exceed 6' clear span with a 2x4 top cord and do not exceed 8' clearspan with a 2x4 bottom cord. I would suggest that your purchase the metal truss plates with the holes drilled for the nails instead of plywood gussets. Use every nail hole that's over lumber and put the plates on both sides.

Your 24 foot trusses would require a minimum of 3 botom cord spans and 4 top cord spans. If in doubt or you are expecting some serious roof loads then go for 6 top cord spans and 4 bottom cord spans.

If you sweet talk your local truss manufacturer, they may design one for you. At the very least, ask an engineer to take a look at it for you. Its a little peace of mind.

[jasepaterson]

So it is feasible to use trusses over a 24 foot span without installing a center brace, or braces on the wall if I follow a 2-4 foot spacing with the truss design you posted?? I was thinking of going 5/12 pitch instead of 4/12 due to the length. as for gussets, you mentioned metal ones, I intended to use 3/4in plywood gussets at an average of 6''x20''. Let me know if I am on the right track..

Thanks Again

[mjpliv]

Unless you talk to a qualified engineer, building your own roof trusses is a crap shoot - plain and simple. All I can do is offer a "rule" of thumb. This rule of thumb comes from designing roof trusses for a living for two years.

The process to design trusses involves calculating the loads for the entire roof, applying those loads to the individual components of the roof and then migrating those loads to the bearing points at the end of the trusses though a series of triangulated webs. In the course of migrating these loads, there are both compression and tension loads created. The shape, thickness and size of the plywood gussets as well the length and quanity of nails is determined by the load direction and value.

I took the liberty of called a freind who designs trusses for a local plant and asked him to calculate one force for me. He ran your 24' 5/12 truss with our meager Nova Scotia composite snow/rain load (35.5 lbs) and then told me the tension load on the bottom cord is in excess of 1500 pounds. When you build these trusses you will have to join the bottom cord to achieve a 24' span. I am not sure about you but I would not feel comfortable sleeping under that roof in the winter time unless I was confident that those plywood gussets and nails were calculated properly.

I have seen roofs fail and it ain't pretty!

[mjpliv]

Is there a reason you are not considering a traditional "stick framed" roof with ceiling joists, rafters and a ridge board??

[jasepaterson]

No actually I am not too sure of any design or how to accomplish stick framing. I was considering going with a cathederal style roof with a ridge pole, with two supports on either end and one in the middle of the cabin. The cabin is for moose hunting and fishing, I have bought all the plywood and 2 x4's and 2 x6's, the floor I have all framed with 4 x 8, and 2 x 6 joists, I picked up from the mill here. To tell you the truth I am lost when it comes to the roof becasue it is such a large span, please let me know about stick framing.

Thank A Million

[mjpliv]

Give me a bit of time I may be able to find a "canned" tutorial for you. If I can't find one I will type one out and upload to one of my domains and post a link for it here.

It is actually not all that difficult. The hardest part is being precise in your measurements and skillsaw work. Your rafters (a quick glace) will be 2x6.

It would be helpfull if you could find out the snow/rain load for the area you will be building.

[jasepaterson]

For rain in my area, we do not get too much, as for snow we get more snow than any other place I know, last year we had about 6 feet. other than that I really appreciate your advice, I practically building this cabin solo, my gramps gave the land to me but I received permission to build it closer to the river. Last year I hauled all the lumber in my 17.5 boat and my 16 foot boat. The only access as of now is by boat, so I only want to have to bring the materials I need If you know what I mean.

Thanks
Again
Jason

[doyle]

Jason, wished I was there to help....this sounds like a fun project and something that is right up my alley. Here's what I would do given your logistics circumstances: The ridge beam would be 2x8's, joined in the center of the building. The length would be 12' or 14', depending on wether you are going to have overhangs on the gable end. The rafters would be 2x6x(14 or 16 length) with birds-mouths cut out of the end resting on the outside wall. I would sister-up 2 joists in the center where the ridge beam joins together for safety's sake. The ridge beam could be supported on the gable end by a 53" board on top of your wall. The 53" (approx) dimension would give you the 5/12 pitch you are aiming for. I would definitely go with collar ties (2x6) to tie one side of the roof together with the other and to keep the side walls from spreading out. The collar ties could be left exposed to the room, or you could cover them over to give your room a vaulted ceiling, but not a cathedral ceiling like you wanted.

This is only my 2 cents worth since we don't have to worry about snow loads way down in the southern U.S.

Perhaps beefing up the collar ties and/or rafters to what I said would work. I would run any final ideas by an engineer. It's worth the money.

[jasepaterson]

Man Doyle, starting to sound easier than I thought now that it is explained to me, now all I need is for you to come down and build this for me! lol! Ok I understand the concept of the ridge beam and the 2 x6 rafters and the center joist, what I do not fully comprehend is what type of board will be used to give me my 53'' for my 5/12 rise and run? Also I am not familiar with collar ties? Oh and the portion of the wall that the ridge beam is going to be resting on, should I not beef up a couple of studs underneath the ridge beam for support?

Thanks a Bunch

[doyle]

Well, the 53" board is really just to support the ridge beam while you install the rafters. The opposite end would be temporarily supported by a 12' long board. Attach the end of the ridge beam that's falls over the center of the room to that 12' board at the location that makes the ridge beam level. The 53" board doesn't necessarily have to become part of the gable wall, but it can be left there as well. You will need to temporarily brace the 53" board in all directions so that is perfectly plumb in the center of your wall. Since I usually do a lot of this kind of thing by myself or with just one helper, I add two smaller pieces of 2x4's (maybe 24" long) onto each side of the 53" board so that the end of the ridge beam is secure and won't fall off or tip over. These smaller pieces will be sticking up above the 53" board, say 5". Once the other end of the ridge beam is secured, you're ready to start marking and cutting rafters.

The studs that you cut on the gable end need to be over the studs in the main wall. That will carry the weight of the end rafters down to the wall.

As for the collar ties, they are placed horizontally, attached on each end to a rafter, perhaps every other one. I would probably try to attach the ends of the collar ties at the mid-point of the rafter length. If you didn't have these, the weight of the roof would cause your side walls to bow out (trust me on this...lol) most likely causing a collpase of your whole structure. That's why I suggested maybe using 2x8's for the collar ties, as you will be able to get more nails (or screws) into each end. The ends of the collar ties will be cut to the same angle as your rafters. I would just buy 16' long boards and just let them fall where the length allows them to fall (allowing for your end cuts, of course).

[jasepaterson]

Great, I now have a plan, now all I have to do is execute it. Thanks again Doyle, if you're ever in Ontario, Canada, and want to hunt or fish, look me up in Kapuskasing, Ontario, thanks again.

Jason
Paterson

[mjpliv]

A quick note on the replies above. Be carefull not to confuse a ridge beam roof with a ridge board roof. Two different things completely. The one described above is a ridge board roof.

I found this short tutorial on roof framing and trusses. Be carefull of those load tables, they are a too light for your application.

http://www.ridoe.net/careerdev/PDFs/Drafting_Roof.pdf

I will keep looking!

[mjpliv]

This one is a little more comprehensive as far as illustrations go http://www.awc.org/pdf/WCD1-300.pdf

[jasepaterson]

Thanks MJ, great info, I understand that a ridge board is not load bearing and that a ridge beam is. I thank you for your time, and consideration.

Thanks
JP

[mjpliv]

Pay dirt!!! :D

http://www.josephfusco.org/Articles/Roof_Cutting/raftercutting.htm

Took a while but here is an article that pretty much covers what you need to know and more. Take a spare battery with you for your calculator.

Seeing as you are building in Canada, let use the National Building Code Of Canada (1995) span tables for your rafter sizes. In northern Ontario I would assume that you will be in the higher end of the scale as far as snow loads go. with a 50-60 lb snow load a 2x8 SPF (2 or better) rafter will span the 12' required to reach your ridge board.

With a 2 ply 1x6 ridge board and a 1' gross overhang on a 5/12 roof your minimum stock length will be 14'-6".

I will follow this post up with a few more notes shortly.

[jasepaterson]

Well I guess I wil have to start calculating, didn't know there was so much involved! Thanks for the info, I will put it to good use.

Thanks
JP

[mjpliv]

I'm not quite finished yet.

I did a quick little sketch of a roof assembly I would feel comfortable with. I would still suggest that you spent a few dollars and have an engineer review it. The measurements for the rafters are based on using a 2 ply 1x6 ridge board (actual 3/4"x5-1/2") and 1/2" wall sheathing. If you change any of this, the dimensions will have to be altered. If you intend to install the sheathing after the rafters then your bird's mouth cut will have to be adjusted or shimmed.

The DWF file requires a viewer that must be downloaded from Autocad (free!) but it will allow you to get a clean print job out of it and if you have access to a printer that can handle 11x17 you can print it on a single sheet in scale. If not, it can print to scale over 4 sheets of 8-1/2 x 11. If the file will not open in your browser window after downloading the viewer, right click and save as to your computer then open the file with the viewer program (it will be in your start/programs menu.

The other is a straight JPG file.

One detail not showing is the lateral restraint of the ceiling joists. We haven't talked about what your ceiling finish intentions were. If you will be applying a ceiling finish then add 1x4 furring at 16" OC perpedicular to the joists (c/w 2 nails per contact point). If you are leaving the rafter space open, you can use 1x4 furring at 4' OC or 2x4 at 8' centers. Either way you, you should restrain the ceiling joists from lateral movement.

The king post does nothing more than keep the ceiling joists from dropping.

Hope this helps!

Some other tips for you -

Take some mason's twine with you. After you set your ridge board in place with the ends plumb and flush with the gable centers, run the twine between two nails 1/2" above the ridge board. As you install the rafters use the twine to keep the ridge board straight as you go.

Before installing the rafters, plumb all of the exterior walls by installing temporary diagonal braces back to the floor. Leave these braces in place until the rafters, collar ties, king posts, sheeting and any lateral roof bracing are installed.

When you mark out the centers/edges for the rafters on the wall top plate, run your tape measure out as far as it will go (a 1" x 25' tape is best) and mark as many centers/edges as possible with out moving it. Use your quick square or framing square to extend the marks the full width of the plate. Always start your rafter centers from the same end of the building. Repeat this for the ridge board (but extend the center marks the full height of the ridge board on both sides. If you only mark one edge of a rafter and use an "x" or arrow to indicate which side to install the rafter, make sure everybody involved marks the same side. You would be surprised how many times that gets messed up.

Cut one perfect rafter (test it as close to the gable end as possible, on both sides of the ridge board and at both ends of the building (A little hint - if it fits on one side of the ridge board but not the the other, your ridge board is in the wrong place but if it does not fit by the same amount on both sides of the ridge board, the rafter is wrong). When it is perfect, clearly mark it as a pattern and use it to mark out the remaining rafters. Do not try to measure out each rafter separately. I have never seen that work. If your wall plates are level, the walls plumb and the building is square then the rafters will literally slide into place with little or no effort.

If the rafter fit starts to get sloppy, double check the plumb of the walls.

Use a piece of 2x? cut the right length to fit between the ridge board and the floor as a gauge to make sure the ridge stays level as you go.

Toe nail the rafters to the top plates with at least two 3-1/2" nails on both sides.

Make sure you compress the wood with any bolts used. The wood will shrink.

[mjpliv]

The JPG was well over the 400x400 pixel limit to upload here so its on one of my servers.

Try this link for the JPG http://www.buildersmatrix.com/reference/RAFTERS.jpg

And if you have trouble with the DWF file try this link http://www.buildersmatrix.com/reference/RAFTERS.dwf

[mjpliv]

Just saw a mistake in the drawings - the ceiling joist splice bolts should be 5-1/2" or 6" long

[jasepaterson]

This now, is coming very clear to me, and I will surely use the tips that you gave me. I cannot seem to get the dwf file, I don't know what to open it with. Also the jpeg is unclear and hard to read.

Thanks
JP

[mjpliv]

Follow this thread - we had a discussion about the DWF viewer here - the link to the download site and instructions are all listed there.


http://www.construction-resource.com/forum/ftopic734.html

[mjpliv]

After re-reading this thread I notice that at no time did I mention that the rafters are to sized to be installed at 16" OC.

Kind of important to mention, I think! :oops:

[jasepaterson]

Thanks, I was looking at that today, thanks again, I downloaded the wrong autocad and that was the reason I couldn't open the file, all is well, thanks.

JP