Converting Garage Truss to Cathedral

[wildbug123]

I'm still in the design phase of my Garage Conversion.  One thing I'd like to do is modify the Fink Truss in the Garage to a Cathedral Ceiling with a Structural Ridge Beam.  The attached image shows the Current Truss with the Sunporch on the Left & the Garage on the Right.
This will be my first project involving Truss's so I'm learning as I go.  I read through the KY's 2013 Codes for Roof Loads the other day.

http://dhbc.ky.gov/Documents/2013%20KBC%201st%20printing%20(rev%20June%202013)%20-%2008.02.2013.pdf

and came across this site that I'm using for my step by step:
http://www.totalconstructionhelp.com/beams.html

Questions:
1) I'm stumped by the additional Rafters that extend over to the SunPorch. 
How do they factor in with everything?  I'm assuming that I have to factor this in for my Load Calculations as well as the Garage. 
You can see that slope ratio changes about 1/2 to 3/4 of the way down the Top Chord on the left of the Garage.
Below are my pitch calculations for the two individual pitches. 
And as a side note, not sure if this factors in, but apparently the Sunporch's current Roof is not even steep enough to support proper Snow Loads in KY.
I'll hoping to deal with that externally.

ROOF                         RISE       RATIO per ft.   ANGLE
Garage                       3-7/16"  ~1:4               15.98480152°
Garage to Sunporch      1-1/4"    ~1:12              5.9468630539735°

2) The way I understand it, the outer Ridge Beam will be supporting half of the load of the entire roof (not just the Garage section).
I know that I need to calculate everything for the Beam itself.  How about the support Columns?
Do they need to be calculated and is there a particular way they need to be designed?

3) How much of a load will the Concrete Slab Floor be able to hold?
I will be doing a Room within a Room design for the Garage Portion which'll be made up of 2x6 Construction with 3 layers of 5/8 Gypsum and a second Ridge Beam.  This is for a Music Studio and needed in order to decouple the structure from the outer walls.

Thanks

[Don_P]

Is this a detatched structure... or more importantly, can you remove and replace the roof with something with better pitches?

[Patrick]

I modified existing trusses before and the city needed plans from the truss manufacturer to approve the modification it was around 1,000$ to have that done, it may be better to start with a new roof we paid the fee because it was in a complex with a massive roof. And the amount of weight a concrerte slab can withstand depends on many factors generally a larger footing is poured where there will be load. Also frost depth should be considered especially if attatching to an existing building you do not want the seperate foundations to shift.

[wildbug123]

Hey Don, yes this is a detached structure.  It's possible to replace the roof...but I'd rather avoid it unless everyone agrees it's necessary.
The home was built in the 50s, and although I've been coming across a lot of failed and unorthodox solutions, this one doesn't appear to be a complete failure as of yet.  Would you know the cost offhand for materials on rebuilding the Roof..?

Wow Patrick, I've never heard of that before.  Sounds expensive.  Usually I read that they'll need the designs drawn or approved by a qualified Structural Engineer.
But if it was complex like you said, I'm guessing maybe that solution was more practical.  Hopefully I can steer clear of that.

[wildbug123]

Also, is there a name for this type of modified Truss?  Garage by itself appears to be a Fink Truss but I'm not sure how it's labeled as a whole.
Knowing this could help me narrow down my research.

[rick91351]

Around here thou shalt not modify a truss without the truss engineers approval. 

Side bar if a truss becomes damaged or falls to the ground it needs to be reinspected and certified.  The was from the truss plant and the building inspector.  Both know fat chance of that.  Both being recertified and or even inspected.

I would dig down and get a better idea of what the footings or footer is under your existing structure.  Heck they might not even be there if the structure goes very far back in the Way Back Machine.....  Hard telling what was required if anything. 

[umtallguy]

yeah without a truss engineer that would be a nogo as you are placing an unbalanced load on a point in it that was not designed for.

[wildbug123]

I appreciate the warnings, they're more than justified.  And in the end , if I don't feel 100% certain, I'll hand it over to a structural engineer.
Still I'd like to try if anyone is familiar with this territory.  The math part doesn't bother me.  And I've completed a few projects construction & otherwise that were said to be way over my head.  I enjoy a challenge but I won't I won't rush in blindly.  There's programs like Revit which will help with the calculations but I'd rather avoid that too if necessary. 
Is there anyone familiar with this territory?

[Don_P]

The way the Fink works typically is the webs break the spans. The ceiling is broken into 3 spans, the top chord, rafter, in 2. So the first check is to see if the rafters are capable of supporting the roof as single full length members. check the easy side first, the right. With the top chord size, species, and grade, check them against the code span table for your snow load and on center spacing. If we fail there the question is answered before going any deeper.

[wildbug123]

Don I'll check on all this tomorrow.  Although nothing appears out of the ordinary or worn on the garage side.  The sunporch side however does have the far more moderate 1:12 pitch.

[Don_P]

wildbug, Haven't heard back but I'll go out on a limb and say I'd be suprised if the top chords can make the span without the bracing of the internal webs. An engineered truss is normally designed pretty efficiently to save wood and weight. However, one thought. If the roof is removed and the garage trusses are replaced with scissor trusses or a structural ridge/common rafter roof that has a steeper outer pitch, then the rafters from the left side can be steeper and the existing rafters will likely be long enough to be recycled as the new rafters since although they are steeper they will meet the roof earlier... draw it out to scale and see. Scissor trusses would avoid ridge posts and possible foundation modification. I've recyled the wood from trusses by putting an abrasive metal blade in a skillsaw and cutting the plates at the joints. The lumber could then be recycled into your interior walls, just be careful not to saw into a plate with a regular blade.

[wildbug123]

Sorry Don, I got swallowed up with the holidays.  Huge family & a little too much holiday cheer.  I'm hoping to be able to be able to take a look at everything in the next few days.  And will double check current span & loads.  If alright I may also outline the project I'm doing a little better in order understand my options and their relative price tags. Talk soon.

[wildbug123]

Hey Don, I'm able to check this tomorrow once our weather clears up in the area.
I'm going to remeasure the Pitch and Angle from the Outside just to make sure it matches my CAD representation.

"So the first check is to see if the rafters are capable of supporting the roof as single full length members. check the easy side first, the right. With the top chord size, species, and grade, check them against the code span table for your snow load and on center spacing."

How do I go about checking for this?  And not sure how to check each side individually.  My focus has been on Structural Beam Loads only so I'm not familiar with checking existing Trusses and Rafters.
Is it a matter of checking my current builds Truss's and Rafters against results from somewhere like here:
http://design.medeek.com/calculator/calculator.pl
http://www.blocklayer.com/roof/raftereng.aspx
against this info:
http://dhbc.ky.gov/Documents/2013%20KBC%201st%20printing%20(rev%20June%202013)%20-%2008.02.2013.pdf
http://www.fs.fed.us/t-d/snow_load/

[Don_P]

The KBC document is too large for me to download but I suspect it is simply the KY version of the IRC code. If so go to chapter 8 and find table 802.5.1(3)(I'm going out on a limb and assuming a 30 lb snow load more than covers your jurisdiction, check with your building dept as to which table is appropriate). Check the size, species, grade and on center spacing of the top chord of the trusses. Find a gradestamp on a top chord and if you don't understand it either take a pic or write back with exactly everything it says. You don't need to know the pitch, you really just need to know 1/2 the building width, which is the span, measured horizontally. Then consult the table and find the allowable span for the top chord, now rafter, material.

You can also use this calc, but it will go over better downtown being able to point to a span in the codebook, many inspectors don't know this is the people who generated the codebook tables;
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

[wildbug123]

Thanks Don, this helps a lot.  Hoping to get out there soon and check.  We're in below zero temperatures at the moment.

[wildbug123]

Finally able to get out there the other day and am starting to work through everything for the Maximum Span Calculator you linked.
The Stamp on the bottom chord:
Northwood Kiln Dried 1 (with a pine tree looking symbol)   
NLGA Ruled No. 1  S DRY   
RIB 49  S-P-F   

Here's the questions I'm trying to figure out at the moment?:
How can I determine if my wood is S-P-F or S-P-F (South)?
Incised or not Incised?
The Truss's are marked No. 1 Grade whereas some of the Rafters either No. 1 or No. 2.  Does that affect the calculations or is it mainly the appearance of the lumber?
And I'm still trying to figure out my Live and Dead Loads.

And then a few dummy checks:
Spacing: I'm assuming it's the space between both Rafters & Trusses combined.  Mine is 22 1/2" so I'm using the 24" value.  Is that correct?
I'm using these values I found for the Deflection Limit:

Snow Load: I used this site to get my approximate Snow Ground Load of 15 psf-20 psf.  Importance Factor. And used the bottom calculator to get: 12.96 psf
http://www.fs.fed.us/t-d/snow_load/
https://courses.cit.cornell.edu/arch264/calculators/example2.3/index.html
http://publicecodes.cyberregs.com/icod/ibc/2009/icod_ibc_2009_16_par055.htm
Deflection: I'm using the 1/180 value taken from this table for my Deflection Limit:
http://publicecodes.cyberregs.com/st/ca/st/b200v07/st_ca_st_b200v07_16_sec004_par008.htm
CONSTRUCTION            L         S or W   D + L   
Not supporting ceiling:   l/180    l/180    l/120

My Bottom Chord Span is: 17' 11 1/4"

[Don_P]

You have regular SPF, if it's south it is designated on the stamp.
SPF design values are the same for #1 and #2 (the only group I know of that being true for) grading of structural wood is visual but not for appearance, for strength. Above #1 is SS, Select Structural which although normally beautiful clear lumber it is also the strongest visual grade. Some lumber, often used in trusses and engineered products, is machine graded by stressing the wood, recording flexure and correlating that to strength, the gradestamp would have read differently though.
S-Dry = surfaced dry... kiln dried to 19% and then planed
Not incised
24" on center, you measured between and got 22-1/2"
The code table is set up for ground snow load, use 20 psf minimum. There is a snow load map in chapter 3 of the codebook. Local inspector trumps any chart, he gets to call the actual design snow load.
In a cathedral you are supporting the ceiling with the rafter.

And you are saying that inside of truss bearing wall framing to inside of truss bearing wall framing measures 18'? if so the rafter span is ~9'

[MountainDon]

I was typing while Don_P was typing... He beat me.

I edited my reply...  all that's left is that if it was incised you'd know it. It would look like a bicycle sprocket was run all over it in a pattern. Some woods need that if being pressure treated; like DougFir.

[wildbug123]

Thanks (the 2 Dons).  I appreciate the patience on the Dummy Checks too.  I usually refer to more than one source, but you never know.

I just read in a more recent document then the National Snow Load site that I linked before.
KENTUCKY BUILDING CODE 2013 it says that the Ground Snow Load for my area is 15psf
And then Ground snow load determination for such sites shall be based on an extreme value statistical analysis
of data available in the vicinity of the site using a value with a 2-percent annual probability of being exceeded
So does that mean I should factor in a GROUND SNOW LOAD of 17% (15% + 2%) to determine my ROOF SNOW LOAD?
Also is it safe to assume the calculator linked is referring to Roof Snow Load rather than Ground (which for me came to 12.96 psf)

SPAN:
Wall to Wall measures 18'9", but the Bottom Chord doesn't extend all the way.
It starts 4 1/4 from the Left Wall & 5 1/2 from Right Wall
So I measured the SPAN as: 17' 11 1/4" (or roughly 18').
So the Rafter Span I measured as 9'12"
Is that correct?

DEAD LOAD:
I just found this equation:
density = mass/volume (d=m/v)   
So I guess DEAD LOAD is solving for Density.
Any idea on how to figure out the Mass?

I'm still looking for how to Solve for LIVE LOAD..?

[Don_P]

Forget roof snow load, use ground snow load. It'll stick up there (even when the PE says it won't)... and I'd use the 20psf table in the codebook. The 2% annual probability of the ground snow load being exceeded means they are counting on an above design load event about every 50 years or so. They aren't saying how far above 15 psf it'll be, clouds are notorious for ignoring weathermen . We had one here a few years back that was about 10psf above design, there were several collapses.

Span;
Span is half the 19' building width, about 9'6". Technically rafter span is half the clear horizontal distance plus half the required bearing length.

Dead load is the self weight of the roof, the weight of the rafters, sheathing, shingles, and ceiling. Generally for low slope 10psf is commonly used, it's rarely above 15psf... maybe if you use that random block ceiling flyingvan just posted on another thread.

Live load is the weight of the snow and the load induced by the wind (another reason to not go below 20psf, for a floor LL would be the occupants and furnishings.

Short answer... if the top chord is 2x4 it's a no go, looks like they blow out at a 15' building width, if it's a 2x6 we can discuss what is going on where those trusses sit on the walls and then go to the next check and look at the left side of the roof.

[wildbug123]

The Top Chord is 2x6 (or 2 x 5 1/4 to be exact)
I calculated my Wind Loads last night.  Which value do I use to calculate my Live Load?
I'm assuming: Minimum Recommended Design Wind-Resistance Loads: Zone 2: Roof Area Perimeter: 49.6psf
Which would make my Live Load: (20psf Snow Load)+ (49.6psf Wind Load) = ~70psf
http://www.roofwinddesigner.com/calc/report.aspx?v=2&p=T1a83muDfec%3d

I really appreciate the help.
There's been some areas that have really been hard to pin down up til now.
Roof Live Load for example is defined as so many different things:
Anything from workers, equipment and materials, and planters which excludes wind and snow.
To just Rain and Snow.  Being able to pin that down alone has shaved off so much unresolved research.

[Don_P]

Design wind loads are in chapter 3 of your code book, when you can find a design value or load in the code it is going to be better (more acceptable to the powers that be) than one from a non code referenced source. For your roof as I see it, positive load is 10psf, highest uplift is -28psf @90 mph... and at 90 mph the snow load is zero, the snow has left the postively windloaded side of the building.

Now that I've been a smarty pants and provided a definition of live load without checking the bible... go look up the definitions of dead load and live load in chapter 2 of your codebook. They are saying snow and wind loads are environmental loads and not live loads. OK, but we'll treat them as loads none the less and I put them under the heading of live loads in my head as they are not permanent loads. I can see their logic because the loads of occupancy are normally considered to be of 10 year duration and with the exception of dead loads the other loads they list are of short duration and design strength values are allowed to be adjusted upwards to account for short duration loads.

Sounds like the right side is ok, we can begin to look at the left side of your original drawing.

You made the comment that the bottom chord of the truss does not extend to the wall. Is there a birdsmouth seat cut in the top chord that gives it a flattened bearing surface on the wall or is it an unnotched rafter bearing on just the outside corner of the top plate of the wall?

For the left side of your picture above, the rafter over the far left room appears to be supported on a post or kneewall and then overhangs that support continuing till it lands on the truss, is this correct?

Is that rafter one piece or is it spliced over the support posts?

What is its' species, grade, dimension and clear horizontal span from inside of the far left wall to the truss bearing wall?

What is the horizontal distance from the support post to where that rafter lands on the truss?

[wildbug123]

Don, I can't locate a design value for Wind Load.
I've been refering to these two code books:
Kentucky Building Code 2013
http://dhbc.ky.gov/Documents/2013%20KBC%201st%20printing%20(rev%20June%202013)%20-%2008.02.2013.pdf                  
Internation Residential Code for One and Two Family Dwellings
http://publicecodes.cyberregs.com/icod/irc/2012/index.htm   

Were you able to take a look at the link I posted earlier by chance?
I used Wind Load designer for my calculations referring to my Local and IRC Code Book for the correct values.
It has all my Wind Load calculations:
http://www.roofwinddesigner.com/calc/report.aspx?v=2&p=T1a83muDfec%3d

MINIMUM RECOMMENDED DESIGN WIND-RESISTANCE LOADS   
DESIGN WIND LOADS           psf
ZONE 1: ROOF AREA FIELD           28.4
ZONE 2: ROOF AREA PERIMETER   49.6
ZONE 3: ROOF AREA CORNERS   73.2

I used this data to reach the Wind Loads which falls into both the Right Side and Left Side of the Roof (1 1/2:12 to 6:12)
ROOF AREA DIMENSIONS (FT)   27x33
MEAN ROOF HEIGHT (FT)   15
ROOF SLOPE   Gable: 1 1/2:12 to 6:12
PARAPET(S) (minimum 36 inches high)   Yes
BUILDING CONFIGURATION   Enclosed
EXPOSURE   B
OCCUPANCY CATEGORY   III
BASIC WIND SPEED (three-second peak gust, mph)   120 from ASCE 7-10, Figure 26.5 Basic Wind SpeedBASIC WIND SPEED (three-second peak gust, mph)
ROOF DECK TYPE   WOOD
ROOF COVERING TYPE   BUILT-UP


Would you know the value I should enter into the Span Calculator based on that?

You made the comment that the bottom chord of the truss does not extend to the wall. Is there a birdsmouth seat cut in the top chord that gives it a flattened bearing surface on the wall or is it an unnotched rafter bearing on just the outside corner of the top plate of the wall?

I'm not sure what a Birdsmouth Seat Cut is.  Here's a shot of the Truss and detail of the Bottom Chord.  Ignore the bottom shelf that may appear to be a bottom chord.

For the left side of your picture above, the rafter over the far left room appears to be supported on a post or kneewall and then overhangs that support continuing till it lands on the truss, is this correct?

It looks to me like the bigger Garage section has it's Rafter supported to the post.  The smaller Sunporch's Rafter is cut at a 45degree angle and is supported by the Top Chord/Rafter of the Garage section.  There is also a Bottom Chord that is part of the Sunporch Assembly.  But I can only view it via the Garage since there's a ceiling in Sunporch area.  Not sure if this answers your question or not.
It appears to have all been built at the same time using the same wood as mentioned for the Garage section.

[Don_P]

I don't have a good enough connection to see the windspeed calc or your KY codebook. The IRC tables and windspeed map I was looking at are here;
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_3_par005.htm?bu2=undefined
I see you as 90 mph with 10 psf down and up to 28.2psf up around the edges, about 12 psf uplift in the field. So, on the lee side, in the field, if you have about 10 psf dead load and 12 psf uplift in a wind the net uplift force is 2 psf. On the windward side you have 10 psf downforce plus the 10psf dead load for a total of 20psf down... there is the control for the rafter in span in wind. There is more uplift on the overhangs, use hurricane ties to hold them down to the wall.

For spans, go to R802.5.1(2),
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec002.htm?bu2=undefined
scroll to the 24" spacing section and #2 SPF, 11'9" allowable span, you're at about 9'. If the sunroom is less tham 11'9" clear horizontal span between walls inside you're good.
Your Ky codebook should read the same, that is the appropriate reference.

A picture is worth a thousand words. I was assuming a commercially built engineered truss, these appear to be carpenter constructed site built trusses. There does appear to be a level cut in the rafters where they bear on the top plate of the walls, so I'm guessing bearing is not a problem. I'd run support posts from the top plate of the wall, alongside the rafter vertically, on up alongside the sunroom rafter and nail to all well. Infilling between the top plate and rafter and between main and sunroom rafters wouldn't hurt... in other words you're building a post to support the sunroom rafter from the wall rather than supporting the sunroom rafter on the garage roof. At that point we don't need to worry about the sunroom rafters bearing on the garage roof in the span.

Next problem is the ridgebeam, what is directly under the ridgebeam at the near end of the photo at the front wall... in other words can you run a post straight up in front of the block wall in the background and a similar post inside the garage wall in the foreground directly under the ridge? Assuming so a double 14" LVL should work for the ridgebeam, the supplier can check that. Then you need to worry about post material and footing.

[wildbug123]

Hey Don, just touching base.  Something's come up as usual, but I'll be back on this asap.