Roof Framing Layout 5

[STrctPono]

I was sent a preliminary house roof framing plan by the drafter/designer who is playing architect and it gave me some pause. Can you guys help me wrap my head around this framing plan and the concerns that I have. It seems as if he drew everything as it was convenient to the exterior perimeter layout but didn't really think about the areas that need interior supports. You can see the outline of the interior walls (dashed) and they don't line up with the locations that need posts.

1. I circled some areas in green that are unsupported. Can you confirm that these areas will either need to be supported with a post to the floor below? Any other areas I'm missing?
2. Can you frame a girder truss into a hip/valley truss? Is there any reason why this would not be allowed?
3. Any other suggestions or problems you see with what is shown right now would be greatly appreciated.
4. The very little guidance I was ever given in regards to designing homes was to callout a minimum 2x6 pre-engineered roof truss at 24" o.c. Not sure if this is even correct and seems more like a bare minimum requirement if anything. What's the standard approach to calling out pre-engineered roof trusses on structural roof framing plans? This particular house is using concrete monier roof tiles. Would that change anything other than how I account for the loads to all the supporting elements? This is in a hurricane prone region.

This is still very preliminary and can be changed as needed but I was hoping for some input from you guys who do this a lot. I do not design houses very often, and my experience with rooflines that are anything other than a rectangular box or stick framed with ridgebeams and rafters is nonexistent. I don't really have anyone that I work with that I can turn to for help.

 

[jimstructures]

STrctPono,

Is this roof a truss package roof or a stick built rafter & ridge beam/ridge board?

Jim

 

[TheDW]

I haven't seen houses done with hip/valley trusses before. I'm sure it is done, but they will probably prefer to over-frame smaller trusses onto the main gable trusses, and will probably want to do this at hips:

 

[jayrod12]

Agreed with TheDayWalker. That is how I've seen hip roofs done on all pre-fab truss jobs. The original way is how they framed wood hip roofs back in the rafter and ceiling joist days.

Edit: Just to add to dik's comment below, 6ft or 8ft girder truss offset from the exterior wall are the common ones. It depends on the truss supplier. Take a look at what happens at each offset to see if all of a sudden it ends up over a lintel that wasn't designed for the girder truss loads.

 

[dik]

Girder trusses are, in these locales, located 8' back from the end wall... Hip areas over main roof trusses are stick framed over the truss system to achieve the slopes.


Dik

 

[KootK]

1) I spent a couple of years prior to engineering school and all of my summers during engineering school as a pre-fab truss builder / designer for what that's worth.

2) I agree with the 8' girder set back when there's a choice to be made in that regard. It's usually 7-11 1/4" for sheathing layout but, as the EOR, all you really want is to specify something reasonable enough that the girders will land about where you expected them to.

3) It appears to me that you could indeed support the entire truss system from the perimeter walls alone and that is often preferred when possible.

4) My preferred layout is shown below. Shaded areas are anticipated valley sets. The upper part of the plan is admittedly non-obvious and there are a number of ways that could be handled. This stuff varies a bit regionally and from contractor to contractor as well.

5) I see no reason to specify minimum 2x6 chords unless you're paranoid about long overhangs under heavy drift loads or something. Let the truss designer handle that. All other things being equal, I'd prefer to have smaller chords and more web panel points. It's easier to build them straight that way.

 

[KootK]

This isn't particularly important as an EOR but it's interesting nonetheless. Look closely at how corner sets are typically framed. In plan, we usually show the top chords alonge but the bottom chords don't match in terms of their plan extent. Most truss neophytes don't pick up on that. Graphically, it should probably be shown as below. I sawed and and assembled about a zillion of these one summer before I ever saw one put together in real life. And I was terribly confused. This was back before the internet and, apparently, any serious attempt at isometric drawing.

 

[KootK]

Another trick that I always thought was clever was to drop the top chords of hip trusses a bit so that flat over framing could be set in for sheathing fastening. We used to asseble the flat strapping things in the shop with plates too but, for the life of me, i can't remember what they were called. "Batman Hips" for the trusses. You know, on account of the "ears".

 

[dik]

KootK... I see you ran the trusses down rather than across... there was a bearing line running them down...

Dik

 

[STrctPono]

KootK, your responses are extremely helpful. Your revised layout using trusses that span only to the exterior walls with valley sets makes perfect sense. This would be the preferred option. I have reached out to a local pre-engineered roof manufacturer to bounce that idea off of him. I guess what I am worried about is showing something that is not in keeping with local regional practice and creating issues after the contractor has come on board. Is using valley sets the only way to maintain the desired hip and valley lines while bearing the trusses on the exterior walls?

jimstructures, currently it is all pre-engineered roof trusses but I get the impression that a portion of it over the living room and kitchen will change to ridgebeam and rafters. That portion I'm not worried about, but may have questions for how to mate the 2 systems together.

Daywalker/jayrod/dik, the plans that I have seen my old coworker prepare always had hip trusses called out. I'm not sure if he was doing it correctly which is why I'm hoping to get some input from my local truss manufacturer. I appreciate the note about the setback limit for the girder truss. I was missing this.

 

[KootK]

KootK... I see you ran the trusses down rather than across... there was a bearing line running them down...

I don't see it. If anything, I think my layout provides the best option for interior support if that is desired. Regardless:

1) To each his own.

2) There are some good reasons to abstain from interior bearing even when it is a natural fit:

a) renovation flexibility for the owner.

b) one less foundation element.

3) Spans and profiles here are such that I don't expect any of the girders, or hangers, will present any difficulty.

 

[KootK]

KootK, your responses are extremely helpful.

I'm glad. You contribute a lot here so I'm happy for the chance to chip in on something of yours in a meaningful way. Were billable hours of no consequence, I would have also drawn quick and dirty truss profiles for you to accompany the framing plan. If you feel there would be value in that exercise, let me know and I'll put that together. And please don't be insulted by the offer. It's the anticipation of the profiles that really informs the layout choices and it's not something that most heavy construction engineers are likely to nail right out of the gate. I do realize that you're probably familiar with triangles.

I guess what I am worried about is showing something that is not in keeping with local regional practice and creating issues after the contractor has come on board.

To some degree you have to accept that this is inevitable in this space. I'd say that there's at least a 50% chance that the truss supplier, left to their own devices, would tweak some aspect of the layout for reasons that are particular to your market or their setup. Sometimes it's a function of nothing more than their current shop situation: some extra of something or other lying around or a particular piece of equipment being out of commission. For your purposes, I think that you want the following:

1) The reasonable expectation that girders will be where be where you plan for them to be to minimize rework on your end.

2) The confidence that, if you choose to force the truss supplier to do it your way (which you can), your way will be reasonably doable.

Without question, the layout that I suggested accomplishes those things.

Is using valley sets the only way to maintain the desired hip and valley lines while bearing the trusses on the exterior walls?

3) No, it's not the only way. It's probably just the only practical way.

4) Modern truss manufactures can fabricate pretty much any shape imaginable and absolutely could get this thing done without valley sets. The consequences of that, however, would be:

a) A bunch of one off trusses that are difficult to jig instead of simple shape that are repeated and create a rhythm to the fabrication effort.

b) A bunch of odd shaped units that are so location specific that they can only go in one, very exact location. This complicates shipping, erection, and sheathing installation.

These were some of the things that informed the layout that I proposed.

After college I spent a year working for the Wood Truss Council of America and got a much better sense for the practices used across the country. While I wasn't involved in a single thing in Hawaii, I can tell you with some certainty that valley sets are commonly used in every one of the 48 contiguous mainland states. If it's different out in the Pacific, I'd be astonished.

 

[TheDW]

Cool, good info Kootk. I was told when I was first starting that it is preferable to use the setback girder vs the hip/valley girders because it allows for more consistent truss sizes to design/manufacture, simplifying it on the truss guys end...less lines of trusses that are all 1'-0" longer than the last...so thats why I've always gone with it. I'm sure its regional

@StrctPono, we usually get feedback from the truss manufacturer long after we submit our permit drawings, and they're starting their designs. Then, they'll let us know if they want to do something different (or more often, just change it on their own). We just have to make sure to have language on the drawings about the footing locations etc being contingent on a review of their final layouts

 

[KootK]

The subtle genius of the valley set approach is that:

1) It keeps the regular trusses regular more often.

2) While each valley piece may be different, when done right they'll all fit in a common, simple jig.

3) Often, when there is more than one valley set required, you will get some repetition of valley pierce to the tune of two to four of each perhaps.

4) The valley set pieces can usually slide up and down the roof structure below in a way that allows contractors to iron out tolerance issues in the framing. In this respect, the use of valley sets can actually be considered to be a boon to the contractor even if the need for more components overall seems like it might be a shortcoming.

 

[KootK]

I was told when I was first starting that it is preferable to use the setback girder vs the hip/valley girders...

Terminology's always a problem with this stuff. Do you mean the difference between these two framing schemes?

 

[TheDW]

Sorta...moreso the difference between these two. 2nd one is from a set we got from truss guys recently, and is what we usually see. I just looked closer at your corner set post, and your 2nd above isn't as different from mine as I thought. Tomato, tomato...

 

[KootK]

Ah... I see. With regard to piece repeatability, you're trading off:

1) one-off stepped hip trusses with the setback girder versus:

2) one-off (or four off) extending mono jacks with the full ridge girder.

So kind of a wash. The things that make the stepped hip system generally more favorable:

3) Less load on the girder if it's a big roof end.

4) No big ridge girder loads needing to go into spendy, 45 degree skew hangers.

5) Because the top chord of a ridge girder truss will be at a lower pitch than the surrounding trusses, the heel detailing becomes challenging.

6) Because the top chord of a ridge girder truss runs cross slope rather than down slope, it get's more awkward the wider it gets because the top chord has to be lowered. As a result, any need to make the ridge girder trusses multiple ply, which happens, will start to muck up the install. When the truss guys fabricate the single ply ridge beams / boards, they'll usually actually miter the tops of the boards. You don't do that with the trusses although I suppose that you could if it were done with great care.

 

[STrctPono]

Were billable hours of no consequence, I would have also drawn quick and dirty truss profiles for you to accompany the framing plan. If you feel there would be value in that exercise, let me know and I'll put that together. And please don't be insulted by the offer. It's the anticipation of the profiles that really informs the layout choices and it's not something that most heavy construction engineers are likely to nail right out of the gate. I do realize that you're probably familiar with triangles.

I'm not offended at all and my understanding for the proper layout of roof trusses would benefit greatly from the exercise. I did provide some rough profiles for roof trusses on a dutch hip roof job I previously did (which I have attached for reference). If these profiles are what you are talking about then yes, I am somewhat familiar with them but get lost if they get anymore complicated. Please feel free to provide any profiles that you think would be pertinent to show on the contract drawings.

 

[STrctPono]

we usually get feedback from the truss manufacturer long after we submit our permit drawings, and they're starting their designs. Then, they'll let us know if they want to do something different (or more often, just change it on their own). We just have to make sure to have language on the drawings about the footing locations etc being contingent on a review of their final layouts

I will include that language. It sounds like the location of the girder trusses are the big one.

While I wasn't involved in a single thing in Hawaii, I can tell you with some certainty that valley sets are commonly used in every one of the 48 contiguous mainland states. If it's different out in the Pacific, I'd be astonished.

I hope you're right. Hawaii does things very differently than the mainland. It wasn't that long ago that houses were built without stud walls. Walls were nothing more than 3/4" vertical tongue and groove jointed wood panels. Kid you not.

 

[KootK]

Walls were nothing more than 3/4" vertical tongue and groove jointed wood panels. Kid you not.

What? With posts mixed in for support of roof framing though, right? Pic.

This is really for your benefit and I would consider it overkill for your contract documents.