Sistering Trusses, Purlins, and Skylight
When using “store bought” trusses in a simple one roofline project is it kosher to sister two together on either side of a 4′ opening and then tie them together with purlins? The idea is to accommodate a large opening for a framed in place dormer or skylight.
Also, should purlins run ON the trusses (and thereby run on ALL the trusses, from one gable to the other)
OR
Should the Purlins hang from hangers BETWEEN the trusses only at the large opening?
Not a real project yet, just curious.
Replies
What you describe is perfectly acceptable. The only thing I would add is that the doubled trusses should be nailed together.
I'd run the purlins on edge between the trusses. I can't imagine any reason to run them from gable to gable.
One trick you might want to consider would be to use 2X6 purlins, and notch them out for a 2X4 laid flat where the "missing" truss would otherwise be. That allows you to keep your regular spacing on the plywood, and gives you something to nail the edge to.
If you lay the purlins out from the bottom up, the edges of the plywood should break on them, too. (You have to allow for the subfascia width)
Hope that's not more than you wanted to know.
If the King's English was good enough for Jesus, it's good enough for me! [M. Ferguson - Governor of Texas circa 1920]
Service with a smile - thanks for the reply. I love this BB.
Would/Should I use 2x6 joist hangers for the purlins or just nail?
Should Purlin long face be plumb and the top beveled to the roof pitch, or should the short face be in the same plane as the roof pitch?
Steelkilt Lives!
I've never used joist hangers on them - They just aren't that long. If you're dealing with a building inspector, it might be worth asking him what he expected ahead of time.
I always put them in perpendicular to the roof. Putting them in plumb and bevelling the tops seems like a lot of work for nothing. Everyone thinks of changing the world, but no one thinks of changing himself. [Leo Tolstoy]
Whether you set the purlins plumb or at a right angle, or even some other angles to the roof plane depends on how the skylight opening is to be framed. Most are done as Boss Hog said, but some are designed (usually it's a pretty contemporary look) with the whole opening on other angles.
I haven't seen really large skylights used with trusses, but I've put some real monsters in on stick and timber framed roofs and those really should have joist hangers or at least more than some face-nailed 16s driven through them.
Is it ever acceptable to try going wider than 4' (i.e., three trusses - 6' center to center span - three trusses)? Then use 2x8's for the purlins?
OR
Is this when you call in the structural engineer and truss designer for a pow-wow?Steelkilt Lives!
The rule of thumb is that you can do a 6' opening with a 2 ply truss, and a 10' opening with a 3 ply truss.
That assumes you trusses are designed for 2' O.C., you put purlins ("ladder framing") between them, and your local building department has no problem with it. Ask the place you get the trusses from if you have any doubts.
Actually, this works out great with attic trusses and dormers. Sometimes. If You can move 2 trusses aside by 2' and end up with a 6' dormer WITHOUT buying any additional trusses. *IF* the spacing works out right, or if you can play with the dormer sizes and locations.Why doesn't Tarzan have a beard?
Thanks for the Rule of Thumb reply about accomodating openings. I guess that was what I was really looking for. This sort of goes back to the thread I started about tips and tricks to save dollars and cents.
I want to design/build a structure in an "efficient" manner. In that last thread you informed me that a bunch of the same trusses is a lot less money than a truckload full of trusses with only two or three alike.
As my design continues to develop, I often turn here for opinions and insight to the various things I am considering. My shop/garage/temporary residence when building/guest house has a clerestory roof that I framed conventionally (mainly because of headroom issues related to overall building height limits). I would like to continue that design into the house, but do it with scissor trusses to get it up and covered fast (always good advice, no?).
One of the concerns I had was accomodating the openings for the clerestory windows. Now, as I plan my design I will do so with your ROT in mind. Then, when I take my designs in to an engineer, I hopefully will not have many "revisions" ahead of me
Thanks again for your advice. I often wonder if guys like our naysayer here ever share any gained knowledge with anyone in a positive and/or supportive manner?
Have a safe and happy Memorial Day Weekend. Steelkilt Lives!
In general, you need engineering for trusses.
The engineering will have all of the details you NEED TO FOLLOW.
I'm not building the trusses myself, I was just curious about accomodating openings when using trusses. The my final design will go under the sharp eyes of an engineer.
However, thank you for your concern.Steelkilt Lives!
I understand you are not building the trusses.
Once again ...
Your engineer will specify the details around the dormers. Your builder will assemble the trusses as the engineer specifies.
Your engineer and builder will do well to ignore every bit of information that is provided here or that you might offer.
..........And you would do well to ignore George, who is a moron with no practical experience in trusses.How do you know when you're out of invisible ink?
Boss Hog ---
Regardless of if your information is correct or not (I really don't care which) ...
The local engineer and truss builder are liable for the design. Not you.
The local AHJ is not going to accept any desing because someone found it on the internet.
Finally, I don't do any engineering or give any engineering advice without complete plans in hand. You should not either.
I try to give useful, constructive advice when asked. The advice I give is based on years of experience both in the office and in the field.
The advice I gave was very generic and was correct, based on the limitations I put on it.
.
Your "advice" on the other hand, is useless, smart-a$$ed, and basically designed to stroke your inflated ego. I can't think of a single instance where you've contributed useful info to a thread. Dare to be stupid. [Weird Al Yankovic]
if your building a simple truss roof and want just a 4' hole in it, it's easier, my opinion, to let the truss people supply you with the 2 ply headers, short stubby cut trusses, and hardware to assemble it. Too easy. To start building a spider web of 2x4's and 2x6's to make up the 4' space seems like defeating the purpose of trusses in the first place.
panama,
I know it's just a matter of personal preferance- but for me a simple ladder to fill the opening is way easier than dealing with stub trusses. No spider web to it.
John Svenson, Builder, Remodeler, NE Ohio (Formerly posted as JRS)
Hey dude, I don't know exactly what shape this thing is suppose to be, but you probably need more than just a simple ladder to finnish it. O.k, now the roof is on. But what about the cieling? Now you need to make another simple ladder. Then the skylight well too. None of that work is nessessary if get the components from the truss people.
A stub truss doesn't just hang there in mid air. You've got to order or build the headers. You still need the double trusses, plus the stubs, plus the headers, plus the extra crane time. Not to mention those stubs and headers cost way more than a typical truss.
Like I said, for me, it's easier (and less expensive) than screwing around with stubs.
John Svenson, Builder, Remodeler, NE Ohio (Formerly posted as JRS)
I didn't imply that they hang in mid-air. I said components.They can't cost that much more, compared to the cost of the lumber you need to fill in, and the labor to do so. And if you read back a few posts, some one had this guy notching the blocks and letting in a 2x4. There's some work for you. And if the crane time is running out, you know as well as I do to just install the full trusses and fall back on the short ones later. And you'd rather oommpf a 20' long or so 4' wide 2x6 assembly (that's been sittin in the rain) up there? I'll take the short truss.
Edited 5/22/2003 8:43:33 PM ET by panama red
Panama Red & Svenny -
I think it's a matter of personal preference more than anything. There are advantages to both ways.
Using double standard trusses is cheaper - At lease if you only consider the price of the trusses. It also allows more flexability in the location of the skylight and the shape of the shaft. (Some are straight, some flare out)
Using girder trusses, snubbed trusses, and headers requires less work in the field. But the girer and snubbed trusses add cost to the truss package. You have to supply the exact skylight location ahead of time - It's hard to change later.
I'd say if you're doing something like this, definitely tell the truss company which way you want to go ahead of time. There's no reason they can't do it the way you prefer. An empty purse and a new house make a man wise, but too late. [Portugese proverb]