Homemade Cottonwood Roof Trusses

[jrokusek]

I have been PM'ing Larry about this idea, but thought I'd post the question to the group.  I'm contemplating cutting enough lumber to build a garage on our family farm.  Not building anything out of the ordinary.....just a 24'x30' garage to store a fishing boat and some small farm equipment in (4 wheeler, sprayers, etc).  I'm going to have someone else do the concrete work, and I'm going to do the rest. 

In an attempt to save some $ to appease my wife, I'm contemplating building my own roof trusses.  I took a look at the free plans from Iowa State University and then ordered the book called "Designs for Glued Trusses" to help me out - although I think I'm on the right track.  I don't have the book yet, but have a few questions.   Here's the Iowa State link:  http://www.public.iastate.edu/~mwps_dis/mwps_web/tr_plans.html

My biggest question is:  because cottonwood has less strength than spruce, pine or fir, should I increase the top chord to 2x6 instead of the standard 2x4?  I don't really care I guess since I have the sawmill and the wood.  Lots of wood to plane smooth though!

The gussets  - Larry used scrap OSB that he was able to get from construction sites.  Anyone else ever use OSB for gussets?   I was planning on using regular plywood and honestly just assumed that OSB couldn't be used.  If I can do that I'd jump for joy since I can get loads of scraps for nothing.  My goal is to cut as much as I can from my own trees, and only buy what I can't make myself. 

Anyone have any words of wisdom here?

Jim

[beenthere]

I'd think you could easily make trusses without planing the lumber. And using OSB should work, just increase the size of the gusset if worried about a difference in strength (more nails and glue).

[tsodak]

You are on the plan that I am on JR.  I am cutting cottonwood for my trusses now and stacking it. I am thinking 24X40 for next year. There is no way I am going to plane it. It looks so good and straight rough sawn that I a m going to build a jig to put the rafters together and just go to town. I think I am going to practice on a storage shed of maybe 10x20 first though. Build it on beams like an old granery and just put it together. That one will be all cottonwood except the beams under the floor that will be in contact with the ground. Hoping to even use board and batens of either ash or cottonwood. You use what you have access to I guess!!!

[TexasTimbers]

Cottonwood makes fantastic framing lumber. I have not used it yet for framing, but my dad has pointed out on numerous occassions how many 100+ year old Cottonwood barns are still standing stron in this area. He says they used to take it to the mill more than SYP because they loved the light weight once dried.
He reminds always me "You HAVE to make sure it never gets wet!".

OSB gussets are perhaps even more preferrable than plywood but only because they withstand moisture much better but, you are using Cottonwood so you aren't going to allow water to ever touch those gussets right?

Titebond II or III will be fine for gluing and screwing. So would a polyurethane glue such as Gorilla but it isn't necessary IMO for this application. Save your money.

bt is right no need to plane, and theoretically (and probably in reality) the glue will bond better to a rough surface than if it is planed.

[jrokusek]

No glue, seriously?  I would think that a smooth gluing surface would be preferable to rough sawn.   We have some old buildings around here that are cottonwood framed and are holding up fine - although they use rafters and not engineered trusses. 

Yup - no water going to touch the cottonwood or gussets.  It's just a run of the mill garage (no pun intended).   I was thinking I was just going to use construction adhesive - something like PL 400 or PL 500. 

For framing I was going to use whatever wood I can get my hands on - most likely more cottonwood and maybe some ash and elm (no softwoods around here).  I'm having a hard time drying elm straight so that's my last choice for framing material.

[TexasTimbers]

No glue, seriously?

I was advising that you do use glue. Titebond II or III would be sufficient.  I was saying that PU was not necessary though to save the difference between the PU and the yellow glue. It is significant.

Having said all that, it is not critical that you use any glue at all if you construct them properly to spread the load and tension and comprression according to proven methods. I just like to glue and screw since it provides a highly cost effective safety net.

Some will say glue is not necessary and I would not argue, I just prefer it.

[blaze83]

This is a very good thread, My dad and I are looking in to doing the same thing here in Ohio on his property, we have a lot of aspen in the 18 -22 DBH range, I'm thinking aspen is similar to cottonwood, we also have a lot of elm and ash and some poplar. We're planning on building a shop, 24 by 36 or so....we need a mill first though

blaze83

[Larry]

Not saying it won’t work but Titebond carries a disclaimer...something to the effect of don’t use this stuff for building construction.  Think it has to due with creep under load.

I’ve always used the PL stuff...can’t remember which one right now.

[TexasTimbers]

Everything has a disclaimer. Consider if you are the manufacturer. You *have* to have that disclaimer. If the glue joint was a critical component I would not use carpenter glue, but there is not going to be much stress on the glue. It's for insurance remember and the trusses should always be built to carry the load with no glue at all.

Still, you bring up a noteworthy point. On my docks I always used Gorilla, but those big headers are under stress at the glue joints with a boat hanging from them, swaying in thunderstorms. They needed the help.

[beenthere]

I'd use a construction adhesive, that has gap-filling characteristics (that will accomodate the rough sawn surface). Also, I'd use at least nails, but likely screws with shear strength in addition to the adhesive. A bolt or two if in doubt could be used too. 

There is a Titebond construction adhesive;
Link to construction adhesive

Comparing strengths of different species (i.e. cottonwood to spruce-pine-fir), take into consideration that the cottonwood is likely more free of knots and defects than available spf, and likely will have higher strength.

I doubt you can go wrong using the cottonwood, OSB, and the Iowa State truss plans. 

[TexasTimbers]

I always try and err on the side of caution. I sheepishly withdraw my former position and defer to the use of a product designed for the intended use. Why use a glue not rated for construction even if it is most likely not going to be under much stress.

I promise to quit having these mid-day crack-smoking breaks.

[jrokusek]

I always try and err on the side of caution. I sheepishly withdraw my former position and defer to the use of a product designed for the intended use. Why use a glue not rated for construction even if it is most likely not going to be under much stress.

I promise to quit having these mid-day crack-smoking breaks.

Darn fingers get in the way of my mind sometimes.  Meant to say something like "...glue - not construction adhesive?"  I didn't realize that construction adhesive is prone to creep.  Good stuff to know.

When woodworking I like to glue everything whether I need to or not.  It sure makes everythigng stiff and solid. 

Glue, construction adhesive...whatever.  Lots of strong nails and/or screws, Iowa State U. plans (whenever the book gets here), strong back, so-so mind, and man am I looking forward to start cutting some boards!  This will probably be one of thise projects I look back on and say, "there's no way I'm ever going to do a project like that again!"

[Don P]

A coupla things about connections.  Glue, nails, screws, bolts, split rings or shear plates all take load differently. Glue takes load right now as do split rings. "Load goes to stiffness" is one saying you will hear over and over. Everything mushes out of the way as a load is applied until the stiffest thing takes the load. If that thing is glue,  then it will take it until the glue joint handles the load or fails or creeps out from under the load. Then it goes onto your next backup plan. If you put 2 bolts in there for good measure it begins loading them. If it actually needs 3 bolts then the 2 bolts will take load until either they displace enough to let the nails/screws start taking some, or they fail, throwing all the load onto the nails or screws. Nails and screws slip a bit before taking their ultimate load, that's why you can't count on them helping the bolts very much.

That was a long winded way of saying that you should figure out what is the load bearing connection, and make that one connection type capable of withstanding the loads. Don't assume mixed fasteners will share the load, they will more likely fail one after another.

I just stood a nail laminated bent for the shop over da weekend. It does have glue and some bolts that hold the wood in tight contact. Neither of those is a backup plan, they couldn't handle the stress if the nails failed. I figured how many nails were needed, and put in a bunch more  .

I think the polyurethanes are structural, I think any would help make it stiffer. Planing will make the strongest glueline. We experimented with various methods of surface prep in one shop using commercial urea formaldehydes. Fresh planed wood walked away from every other prep method and was more reliable. I ran the destructive testing in another furniture plant for a little bit, we were using titebond type glues. Again fresh planed won out. Alot of those glues weren't worth much at 150*, we were simulating a warm truck trip, an attic can easily see that too.  All that said you cannot make a reliable glue joint in the field, especially in a tunnel or with stuff over peoples heads.

We used to build garages with "trussed rafters", now people order trusses premade. I think you could build it out of balsa if you sized according to the materials strength.

I think we need to confirm for certain whether osb is a good gusset, not sure. Not sayin somebody my wife knows real well doesn't have em in his barn 

[Handy Andy]

  About 30 years ago when I was framing houses, we used truss principles and just nailed the pieces to the sides with 16d nails.  Those roofs have not failed, but after a few years, the building inspectors started telling us to butt the pieces and use gussets.  Now you have to use engineered trusses, nothing can be built on the job, but you can stick frame a roof with no truss principles, just have to brace off walls that go clear down to a footing.  Think most of it is just bs.  I have a truss book, it says you have to use glue, no power nailers allowed.  They like to make hard work out of it.  One other thing, if you start putting up the roof, and don't get it finished and blacked in, there is a chance it will get wet.  Jim

[thurlow]

I've posted these pictures before.............built a 24 x 30 ft shop last year, using home-made trusses, 1/2 inch plywood for gussets.  Put together with construction adhesive and air-gun nails.  Wouldn't hesitate to do the same thing again.

 

 

 

Only difference..........I was using store-bought sticks.

[Corley5]

We built the trusses in my house from green planed 2X6 big tooth aspen.  We used a construction adhesive in caulk tubes might of been Liquid Nails  in addition to #10 nails to attach the 1/2" plywood plates.  Being that the wood was green yet I don't think the glue probably did much good  but the roof line is still straight 

[Don_Papenburg]

I do not use anything but  PL Premium  construction adhesive now.  I think it is the best on the market right now .   Has very good holding and gap fill .  But it is best to have flush fitting joints.

[brdmkr]

There was an article in a recent Fine Woodworking mag that tested different types of glue.  Surprisingly, the titebond type glues were superior in all tests.  The biggest surprise was the polyurethane glue (Gorilla glue) failed on many of the tests.  It was the poorest performer.  Of course, this was in a woodworking application and construction adhesives were not tested.  They might very well be vastly superior for trusses.  I would think planing would increase the strength of any glued joint.

[IMERC]

If yur really gonna do this....

use at least 1/2" ply.. 5/8" preferred...
OSB lacks the inegrity that plwood has..
be liberal with the construction adhesive... (PL is about as good as it gets)...

clean (plane) the lumber at the gusset locations before glue up...

glue and nail (2" nail) the gussets on...
don't use staples...

[IMERC]

I’ve always used the PL stuff...can’t remember which one right now.

PL400 or PL polyurethane construction adhesive...