Author Topic: KS Flint Hills 20x30 1.5 story  (Read 4759 times)

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Offline Nate R

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Re: KS Flint Hills 20x30 1.5 story
« Reply #25 on: February 25, 2021, 10:13:57 AM »
Thanks for posting the spec sheet.
In truss manufacture there are some got-ya’s that some designers use as to make the bid “more competitive” as Don_P was referring to. In your case no worries but for the rest of the readers I’ll mention the things that one should look for or might be of interest.


Thanks for that education! I read up a bit on these spec sheets previously to understand what I was looking at, but that was from Mitek's own publications, and I hadn't run into any truss designs that had loading numbers that scared me, etc. Good to know that's out there!

Just for others, and to explain a bit in my case: I ended up with a slab foundation that was 3 inches longer than I had spec'd to my concrete guy. (~3 inches wider, too, and thus the odd span.)  Everything HAD been laid out for 24"oc. So I had to deal with this at the truss level with either an extra truss, or with a truss that could span 27", or something like that. It was cheaper in this case go with a truss design (And thus the 1.8E members) that could handle 27" OC. My truss designer told me that the software automatically added purlins over 24 OC, and to just ignore them in this case, since I was still sheeting the roof.  So we didn't care about the purlin layout or fastening schedule for them.
I ended up building 24 OC except for one truss bay that's 27". This one will contain my wood stove chimney, so gives me a bit more clearance between the chimney and the trusses. Roof sheathing was already spec'd at 5/8, so it could handle the extra span, too. It did make for some oddball sheathing lengths, and I probably ended up using more sheets than I would have.

This is the value of being on site between forming and pouring.......a similar thing happened with my driveway..... If I could have been there, I could've stopped mistakes before it was too late. Would've saved me some money if I got the smaller slab size I asked for....instead I spent more on lumber/trusses, and head scratching time, but gained about 9-10 square feet, I guess.

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #26 on: February 27, 2021, 04:05:54 AM »
OK here is my truss design... It looks like I'm ok, but I'm worried about the loading numbers.

We don't have a lot of snow here, when we do its here and gone as fast. Winds are my biggest concern, we will have sustained winds for days. Not like Western Kansas that has 40mph winds and 70mph semi flipping gusts, but we will have multiple days where we have 20mph winds with 40mph gusts.



Offline Don_P

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Re: KS Flint Hills 20x30 1.5 story
« Reply #27 on: February 27, 2021, 06:19:43 PM »
Reninco, thank you, good explanations.
KJones, It looks like .79" horizontal deflection. That means it is possible for the truss to push a long, unreinforced wall out of plumb that amount. To me that is excessive but it is a judgement call. If the truss is resting on a floor then that would be tying the walls together and no worries, if it is over something like a open greatroom I'd try to reduce that horizontal deflection.

Offline Reninco

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Re: KS Flint Hills 20x30 1.5 story
« Reply #28 on: February 28, 2021, 05:06:23 AM »
I agree with Don_P, I would talk to the designer and mention this concern if you have long walls without any partitions. Also the design is based on peak values which may not be a concern in your location but its best to talk to the designer with this concern.
KJones I like the finished look of this style of truss as it always gives more volume to a cabin or house. Your plan specs are about the same as the comments I made for Nate. All wind conditions for your region are considered as is Nates comments. Mechanical fastener is requested etc.
If wind force is a concern lets follow the wind load…
The wind hits the truss…it will not blow off the top plate (addressed as a mechanical fastener needed) and if the top plate is fastened to the plywood sheeting with the correct schedule…and if the plywood sheeting is fastened to the bottom plate and if the bottom plate is bolted to concrete the load path is complete and not a problem. Also they are considering the dead weight of the completed roof assembly.
If wind intrusion is a concern…
Detailing for wind “intrusion” can be addressed with peel-n-stick underlayment on the roof sheeting and caulking at blocking. This should also include the gable end truss’s. Caution should be used with standard venting procedures detailed in many code books. These become problematic with openings on the “windward” side. In my own house (featured in the build section) is built in a “constant” wind region where it is not uncommon to see speeds of +20 mph for 72 straight hours. I have sealed the windward side of my ridge vent with caulk because it was very obvious this was where any rain was going to go! 
Erection Considerations for this style of truss…
These will be top heavy, you will want to setup temporary bracing on the gable end as a starting point. Trusses of this design can not be set in one pile standing straight up like a flat cord 4/12 truss. If this were me I would breakdown the load into two separate piles on the top plate, with the tops facing each other – if you don’t have room just have one top lay over the other. Then roll the truss into position. At the middle you will run out of room so the last 3 will have to be stood at the same time and then shuffled into place. If the driver has extra time I would set the gable truss with a boom truck. Or pay the time to set each one by truck. Extra bracing is better than less bracing setting this style of truss.


These guys are rolling smaller scissor’s. I would not put the vent block in before…the truss always has some type of discontinuity at the plate – this often makes those precut vent blocks fit a problem. Also I would use way more temp braces for larger trusses. These style of vent blocks are not my favorite either.

« Last Edit: February 28, 2021, 12:20:07 PM by Reninco »

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #29 on: February 28, 2021, 04:02:16 PM »
Reninco, thank you, good explanations.
KJones, It looks like .79" horizontal deflection. That means it is possible for the truss to push a long, unreinforced wall out of plumb that amount. To me that is excessive but it is a judgement call. If the truss is resting on a floor then that would be tying the walls together and no worries, if it is over something like a open greatroom I'd try to reduce that horizontal deflection.

Don, thanks for the input, I went back and looked at some loading charts and talked to a contractor friend and he thoughts the loading was fine. I didn't talk to him about horizontal defection. I'm following the 1.5 story plan, with a loft and then open area (open for about 16'). Along on long wall there is a deck and there will be a deck roof merged into that wall (not sure it matters for this discussion just sharing).

I will call them tomorrow and have it reworked, but sounds like 0.29" is ok, is there a target (other than zero) I can shoot for? We will not be putting sheetrock in the greatroom, if we use sheetrock it will be in the bath and bedroom area under the loft. Again not sure it matters just letting you know.


Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #30 on: February 28, 2021, 04:51:42 PM »
I agree with Don_P, I would talk to the designer and mention this concern if you have long walls without any partitions. Also the design is based on peak values which may not be a concern in your location but its best to talk to the designer with this concern.
KJones I like the finished look of this style of truss as it always gives more volume to a cabin or house. Your plan specs are about the same as the comments I made for Nate. All wind conditions for your region are considered as is Nates comments. Mechanical fastener is requested etc.

Reninco, sounds good. I appreaciate your comments and Don's as well. I'm going to have them look at the design next week, hopefully I can get it ironed out with out breaking the bank.

I ordered all my floor joist, floor sheathing etc this weekend. My wall framing order is queued but I won't pull the trigger on it quite yet. I have to admit I'm pretty excited to get this thing rolling...

Thanks!

Offline Don_P

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Re: KS Flint Hills 20x30 1.5 story
« Reply #31 on: March 01, 2021, 05:07:42 AM »
That horizontal deflection is a judgement call. If both walls are equally "floppy" I try to keep it under around 1/2". This is not a strength issue it is a serviceability problem with no clear cut answers.

Framing of the gables with scissors is done wrong more than right. Studs must run unbroken from points of lateral support. That is, from the nearest floor to the underside of the scissor trussed ceiling. The truss shops often create a flat bottomed gable truss to go with the set. That is what you are seeing in the video. This is wrong. That creates a weak hinge in the gable wall. What you will see in post wind event pictures is that wall breached, the gable truss on the ground and the roof somewhere across the neighborhood. Balloon frame that wall to the underside of a scissor pitched gable truss. Less fun but much stronger and to good code.

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #32 on: March 01, 2021, 07:58:51 AM »
That horizontal deflection is a judgement call. If both walls are equally "floppy" I try to keep it under around 1/2". This is not a strength issue it is a serviceability problem with no clear cut answers.

Don,
I'm not sure I understand what you mean by serviceability issue?

The designer came back and said to hit 0.25" H defl that the bottom chord would be a 2x10 and the resulting heel would be about 24", that is to much height for me.

So question, since I'm just seeing the output of numbers and not the equations... The loading generates the deflection taking into consideration the lumber size/angles etc. That is my assumption here so could one of the following be a solution?
  • What if I doubled up rafters every 4 ft?
  • What if I changed the rafter spacing from 24 inches to 16 inches
  • What if I added a collar in the lower third of the scissor every 4 fit?
  • What if I used a tension cable every 4ft reduce the splay?
  • Sheathing the truss
I did confirm that is total deflection, so 0.395 per wall.
Whew, I doubt that this will blow away but regardless I want it to be right...
Thanks!
« Last Edit: March 01, 2021, 11:10:19 AM by KJones »

Offline Don_P

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Re: KS Flint Hills 20x30 1.5 story
« Reply #33 on: March 01, 2021, 03:18:17 PM »
Strength is a life safety issue, a "must"
Deflection is a serviceability issue, it is a "may". Things like cracked drywall, opening trim, planes out of plumb, excess vibration. These are qualitative things rather than safety issues.
We'll be hitting 25mph sustained with 50mph gusts tonite, 70 is not uncommon. The vibration caused by that in a flexible truss is annoying/disconcerting even if it is safe so I err towards stiffer.

If you are willing to put a tie in, or cable it then in my mind you are willing to lower the inside pitch. Lowering the inside pitch will do the most, the cheapest... it creates more "tie". The other solutions may certainly work or help to varying degrees but I think you'll get the best look and function by lowering that inside pitch a little if possible. (Do coordinate that with the truss designer if doing that, pinning the wrong point can damage a truss.) Depth creates strength and stiffness fastest/cheapest. Have him maintain member sizes and grades but drop it 6"-12" lower at the peak and watch what it does to the deflection.


Offline Reninco

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Re: KS Flint Hills 20x30 1.5 story
« Reply #34 on: March 01, 2021, 04:13:44 PM »
I agree with Don P on lateral support (lack of), serviceability and his suggestion on a slight truss redesign with slightly flatter inside pitch. I bet after you set them you’ll have forgotten the change.
Realize they are using max wind for input and these are somewhat generic in code applications. Considering terrain variation: If you’re building in a hollow you would never see peak wind…so you will never see peak deflection…but you might see peak snow loads.  If you’re on a knoll of some type and see peak wind 3 times a year…well as Don P says a lot of the qualitative stuff with big deflections just gets plain annoying to live with.
I love not building or living in creaking houses.
It costs very little to change on paper and very much to fix after being built.
The video was to show setting this type of top heavy-truss and the importance of bracing the gable end – everything else should be ignored.

Offline Nate R

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Re: KS Flint Hills 20x30 1.5 story
« Reply #35 on: March 01, 2021, 05:46:02 PM »
I assume you're doing a 10/12 interior slope to accommodate head height in a loft?  Maybe drop it to 8/12 or 9/12 as suggested above, but increase the heel height to compensate?  This may be a simple design change for the truss designer, and might not be expensive. If you look at my truss design (posted previously/above), they just add a "spreader" wedge between the bottom and top chords to increase the heel height another 3 1/2 inches.

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #36 on: March 02, 2021, 10:26:43 AM »
I assume you're doing a 10/12 interior slope to accommodate head height in a loft?  Maybe drop it to 8/12 or 9/12 as suggested above, but increase the heel height to compensate?  This may be a simple design change for the truss designer, and might not be expensive. If you look at my truss design (posted previously/above), they just add a "spreader" wedge between the bottom and top chords to increase the heel height another 3 1/2 inches.

Yes I was trying to keep the headroom in the loft and not have to much wasted space in the corners where the roof meets the walls. It will be functional and we will use it.

Lowering the inside pitch will do the most, the cheapest...

Don, I have a request for a new design... 8:12 on the interior, I gave it a little extra heel height too, not sure if they will need it.

It costs very little to change on paper and very much to fix after being built.

Ands that is why I'm here asking questions!  :)

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #37 on: March 03, 2021, 07:32:53 AM »
Well redesign has the horizontal deflection at 0.49" total.

They did revert back to 2x4 Top and Bottom Chords, so I've asked if there was room to gain a bit more based on Don's comment
That horizontal deflection is a judgement call. If both walls are equally "floppy" I try to keep it under around 1/2". This is not a strength issue it is a serviceability problem with no clear cut answers.
Is 0.49" just to close to 0.5"? I'm thinking if I can squeeze some more why not at this point.

They were also about $450 cheaper too and 62% of the weight, which made me happy. I'm sure if they change something it will go back up.

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #38 on: March 03, 2021, 01:42:36 PM »
Well here we go, what do you all think?



Offline Don_P

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Re: KS Flint Hills 20x30 1.5 story
« Reply #39 on: March 03, 2021, 03:18:15 PM »
If the geometry is acceptable it is a better truss. If you need more headroom can you increase wall height a bit?

Take a look at both printouts...
Reactions are the same, both are bearing the same loads
Now look at the difference in the forces. By making the truss deeper the internal forces dropped dramatically. Enough that they are now able to use visually graded #2 SPF lumber vs high end machine stress graded much stronger wood in southern pine, and as you noted they dropped a size to boot. I'm seeing 3/8" horizontal deflection, looks fine to me.

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #40 on: March 03, 2021, 03:37:05 PM »
If the geometry is acceptable it is a better truss. If you need more headroom can you increase wall height a bit?

Take a look at both printouts...
Reactions are the same, both are bearing the same loads
Now look at the difference in the forces. By making the truss deeper the internal forces dropped dramatically. Enough that they are now able to use visually graded #2 SPF lumber vs high end machine stress graded much stronger wood in southern pine, and as you noted they dropped a size to boot. I'm seeing 3/8" horizontal deflection, looks fine to me.

Good deal! Lighter and cheaper than the first version too.

I appreciate the input Don, Thank you!

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #41 on: September 13, 2021, 03:09:22 AM »
Roof Venting...

Been awhile since I posted, I meant to get this typed up this weekend but ran out of time. Cabin is going, its been slow and at times painful (literally). Anyway I am dried in for the winter.

When I had my roof (metal) put on, the contractor I hired (who also set my trusses) recommended that I just use gable end vent for roof venting. I have tried to do some reading on here and other places and I have only succeeded in confusing myself. The next order of business is closing the soffits but to vent and how to vent is the question.

Thanks for the help.

Offline Don_P

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Re: KS Flint Hills 20x30 1.5 story
« Reply #42 on: September 18, 2021, 03:13:40 AM »
Things slow down when your feet leave the ground. I prefer soffit and ridge venting. Gable vents will vent and dry the upper "attic" zone but those lower areas will be stagnant.

Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #43 on: September 20, 2021, 07:18:07 AM »
I guess let me restate the question.

Do I need to vent the roof, I'm seeing a lot of people say no but not as to why you shouldn't. If someone recommend a direction and why that would be great. Ridge venting is out of the question at this point.

Thanks,

Offline NathanS

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Re: KS Flint Hills 20x30 1.5 story
« Reply #44 on: September 20, 2021, 07:48:20 AM »
Venting is what prevents ice dams. It's more important the more cold and snowy your climate is.

I actually came to a different conclusion than Don for where I live (lots of snow, very cold). Snow piles so deeply here that the ridge wont properly vent when it's most needed.

The code wants you to balance the area of the soffit vent with the area of the ridge vent. Don's warning about gable vents is that if they aren't big enough not enough air will be drawn through the soffits. Joe Lstiburek, the best building scientist of this generation, suggests that you want less gable/ridge venting than soffit because you do not want to depressurize the attic, which will suck hot moisture laden air into the eaves potentially causing condensation/mold/rot.

I think a continuous soffit vent, and then vent channels in rafter bays that extend to above the collar ties with gable vents would work fine. Air sealing, especially at the eaves, is extremely important, and the more R-value you can get at the location the better as well.


Offline KJones

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Re: KS Flint Hills 20x30 1.5 story
« Reply #45 on: September 22, 2021, 08:05:11 AM »
Nathan and Don P, thanks for the input.

So what ratio do you use for venting. I saw someone mention for the 1-1/2 story (20x30) that 1:150 is fine. So 600 sqft, needs 4 sqft of venting, splitting that you need 2 sqft for intake and 2 for vent. Just checking that I'm doing this correctly.

I should mention I have scissor trusses in my build, 12:12 roof to a 8:12 ceiling, so I have 4ft from ceiling peak to roof peak. Obviously that gets tighter the closer to the walls you get, I think its about 10" at the wall.

Since I have kind of got pushed into gable vents, my thought was to put in a 1+ sqft vent in each end. Then soffit vent the eaves on both sides. Is that to much soffit venting? Can there be to much intake? The gable vents are on the north and south walls of the cabin, not sure if that matters but prevailing winds are north and south. My crawl space vents move some serious air with those winds.

Let me know your thoughts...

Offline NathanS

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Re: KS Flint Hills 20x30 1.5 story
« Reply #46 on: September 23, 2021, 12:42:09 PM »
I think these docs will answer your questions.

It's been a few years so I can't remember all of the ratio stuff off hand. It looks like they recommend 60% at the eave and 40% at the gable, and they give an example calculation in the pdf. They also recommend a 2" air gap all the way, which they would typically build with rigid insulation, I think.

Personally, I used a 2" continuous soffit vent.. installed insect screen, but on our mudroom area I actually had to recently beef it up with hardware cloth because of chipmunks.. ugh. Anyway, I remember doing the calculation and coming to the conclusion to make the gable vents as big as possible... 2.5 or 3 sq feet on each end so ~6 sq ft up top, and closer to 11 sq ft at the eave. We have never had any ice damming yet, just about the only house in the county, I'd reckon.  :D

https://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation
https://www.buildingscience.com/sites/default/files/migrate/pdf/PA_Crash_Course_Roof_Venting_FHB.pdf

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