Attic Trusses Revisited

[phamENG]

We had a good discussion on attic trusses a few years ago: thread507-475689

I'm curious, for those experienced (KootK! RontheRedneck!), what are some good rules of thumb for these? Span vs. depth vs. clear width of room vs. roof slope, etc. I have one client who is always shoving rooms into the attic with funky roof lines, and these are a great way to give him what he wants without forcing the framer into some reciprocal framing mess. But I'm always nervous I'm going to hand the truss manufacturer an impossible problem. I collaborate with the local designers when I can, but there's not always time for that, especially on the initial pass at the framing plans.

 

[KootK]

I'd recommend that you create your own FEM template in order to prelim design these things yourself. I feel that's possible precisely because the hardest working parts of an attic truss are the parts that are, in fact, not trussed. See below. So long as you pick 2x8,10,12 lumber grades typical of trusses in your area, I would have every expectation that a phamattic truss would be supplier workable. Bracing assumptions are critical, as you can imagine. I don't see that tripping you up though.

Pretty much everything comes down to the clear spans of the top and bottom chords between joints. That's part of what makes it difficult to come up with rules of thumb that don't wind up being punitively conservative. You can, of course, use multi ply trusses if your project can absorb the cost of that. Trusses can sometimes also be designed so long that they can't be shipped, at least not in one piece.

 

[phamENG]

Great. Thanks for the guidance, KootK.

 

[KootK]

I'll try to scare up some span table stuff too. No guarantees.

 

[KootK]

I stumbled across this when looking for span tables and thought it curious with respect to the reinforcing of the truss top chord across the non-trussed zone. I feel that it's misguided. It's not like a 2x8 can't span 3' or whatever as a regular beam. And I'd expect the reinforcing to help very little with the Vierendeel action that is the real plague of the attic top chord.

 

[RontheRedneck]

It probably won't surprise you that I have some pretty strong opinions about this.

My first rule of thumb is for the BC in the room. If you saw my post in the recent thread about I-joists, you might recall that I like max a span/depth ratio of 18 to 1. i.e. the room is no wider that 18 times the depth of the B.C.

The BC is no different than any other floor member. I get people who seem to think that because a 2X12 is part of a truss that it can magically span ridiculous distances.

The opportunity for vibration problems is actually increased because the BC is part of a truss. The truss itself is supporting the BC, and it has some "give" in it.

On a practice note - I don't use even room sizes like 16' 0". I use 15' 10". The face of T&G plywood is not a full 48" wide. So the customer would end up with a narrow gap after laying down 4 rows of plywood.

One of my pet peeves is seeing a collar tie that's maybe 5' long and they put a tiny little KP above it. That KP doesn't do a damned thing. But people seem to think it's necessary. I do not put on in unless the collar tie is 10' or longer.

If you're figuring an attic truss by hand, I ask that you avoid using numbers for really high grades of MSR or MEL lumber. You might find that it's not readily available.

i.e. we stock 2X10 MSR 2400. If someone sends us a design that calls for MSR 2800 that's a problem.


We have a supplier that sells PSL lumber that's 1.5" thick, and we can use that in attic trusses. I don't know how widely available the stuff is, but if you ask a local truss place they could tell you what they can get.

I recall that one time Alpine used to recommend larger plates on attic trusses. The reasoning was that since they're non-triangulated trusses the plates make the joints more rigid. They kind of got away from that, but I'm still in the habit of doing it.

That's all I can think of at the moment.

 

[RontheRedneck]

KootK - I just saw the drawing that you posted late last night.

In the past I've been asked to plate lumber under the TC to increase the effective depth of it for insulation purposes. I doubt that member was intended to be any kind of reinforcement.

I also noted that W6 is one of those tiny, useless kingposts that I was ranting about above.

 

[phamENG]

KootK - thank you for that. Looks like Ron has a pretty good idea for the real reason behind those added members. It's actually a pretty good idea for insulation purposes...

It probably won't surprise you that I have some pretty strong opinions about this.

I was banking on it! Thank you for the insights. When I do preliminary truss designs, I don't go higher than No.1 visual grade SP. That way, if something changes later on or I made a dumb mistake, the truss manufacturer has more wiggle room to make it work in the final design with MSR if needed.

What kind of cost differential is there to go from a solid bottom chord to using a space-truss style attic truss with a mini-truss built into the larger truss to support a longer span? (Looking more for an order of magnitude than anything specific, of course.)

 

[HouseBoy]

Great insights from real experts Ron and KK (as usual)!
Not necessarily a rule of thumb but...one other thing I have found is that a greater heel height can be helpful in making some of the wider room sizes work better. Still have the limitations of the bottom chord that Ron described but it does help with the "clipped corners".
Gotta review with the architect though since it affects the gutter line!

 

[SE2607]

phamENG - Looks like interesting and challenging projects! Good luck on your quest to find suitable solutions.

 

[KootK]

In the past I've been asked to plate lumber under the TC to increase the effective depth of it for insulation purposes. I doubt that member was intended to be any kind of reinforcement.

Ahh... that makes great sense, thank you. My attic truss design experience mostly predates the "age of insulation".

 

[lexpatrie]

I tried a while back to see (this is somewhat unrelated, sorry) - are any of these folks still around? The one thread had some dropbox links to a few Excel spreadsheets, and they've gone stale (naturally). The two threads had a lot of helpful back and forth (and some spurious moments), but there were some nice graphics and some RISA models that helped me a bit on a recent project.

https://www.eng-tips.com/viewthread.cfm?qid=457041

https://www.eng-tips.com/viewthread.cfm?qid=470116

Regards,
Brian

ETA - I think some of the "extraneous" pieces you're seeing are a) for insulation depth, b) for shipping/lifting or the size of the toothed plate.

I wouldn't go the step of designing it myself preliminary, usually the truss suppliers will provide something basic you can work from.

 

[phamENG]

lexpatrie - who are you looking for, specifically? r13 hasn't been around in a long time, but most of the others are still here occasionally, I believe. If you click their name, you can see the last time they logged on as well as their posting history.

 

[RontheRedneck]

phamENG said:
"What kind of cost differential is there to go from a solid bottom chord to using a space-truss style attic truss with a mini-truss built into the larger truss to support a longer span?"
I don't know. We do that very infrequently.

A trussed bottom can no doubt give you a bigger room. But it adds to the height of the truss significantly. Pretty soon you run out of height that you can build.

HouseBoy said:
"...a greater heel height can be helpful in making some of the wider room sizes work better."

That's true. If the customer will go for it. Sometimes their expectations are unrealistic.

 

[lexpatrie]

I think I'm looking for 1503-44. And Arpasevan. Both have been active at least somewhat recently (June 28, 2023/today).

 

[phamENG]

I'd suggest starting a new thread and putting their names in the title. Probably the best way to get their attention.

 

[phamENG]

Thanks, Ron. I've had it on a few projects, but it's always hard to get cost information at that level. Half the time I don't even get told when the houses get built...

 

[RontheRedneck]

phamENG said:

"Half the time I don't even get told when the houses get built... "

I often know if projects get built. But I rarely see the projects in person.

Heck, I only see maybe 10% of the trusses I've designed when they're in the shop.