# Finishing attic. Questionable rafter ties. Will this work?



## homestudio (Mar 22, 2011)

Hello all.

I sure hope I am asking this in the appropriate fashion. If not, please advise.

I'm from Wisconsin, and am in the process of an attic finishing project. I'm concerned with the fashion in which my roof rafters tie into the floor joists, considering the added weight of 5/8" drywall... plus the prospect of snow weight.

First, a shot of the dimensions of the total space:









Note. The plan is to have the existing collar ties exposed. Drywall layer will terminate very near the roof peak, with just enough space for an exhaust vent... so nearly full vault. Knee walls are to be rebuilt (currently hap-hazzardly placed with random spacing) with a full footer.

In this next shot, you can see how the roof and floor tie together, along with my idea for beefing up the junction colored in red. I have also rendered in hurricane straps. Figured they can't hurt.









Essentially, the current construction is 2x6 16"oc floor joists. 2x6 24"oc roof rafters, resting atop a 2x4 top plate. I would like to simply be able to attach an additional "top plate" a foot or two in from the junction, and cross brace to the rafter to prevent shearing. Plausible?

FWIW, roof ventilation is accounted for, eve vents/duravent/ridge vent.


I realize the value of public forums and understand how frustrating it is for noobs to 'barge in demanding answers'.:whistling2: Links to articles or threads appreciated. 

Thanks for your time,

John


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## homestudio (Mar 22, 2011)

I guess it isn't appropriate to call these "rafter ties". They are the floor joists.


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## pyper (Jul 1, 2009)

Are your proposed floor joists up to the task? 2x6 is pretty small for a floor joist.

Your rafters appear to be under sized. You need a knee wall or something to reduce the span. But then that aggravates the problem with the joists.


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## homestudio (Mar 22, 2011)

Hey pyper, thanks for the response.

I'm including a new rendering that has the knee walls drawn in. Sorry, I missed that they weren't there on the first drawing.









Yeah. 2x6 is probably a bit light. However, the live load in the space will be reasonably light. It's the mixing room of a home studio, so very minimal furnishings. Here's a rendering of the finished space:









While all of the maroon colored stuff may look big and heavy, it's essentially fabric covered insulation... or "bass traps". Similar treatment behind the knee walls where you see wood, behind that is bass traps. NO aquariums, water beds, or pool tables... this space was considered 'finished' when we bought the house. It was uninsulated and walls were comprised of little more than cardboard. But still...finished. We've previously used the attic as storage and have had a lot more weight up here than what will be in the new design.


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## homestudio (Mar 22, 2011)

Also,

Home is roughly 100 years old. SO when I say 2x6... the dimension is a LOT closer to 2x6 than it is 1.5x5.5. And dense old growth lumber at that.


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## Ron6519 (Mar 28, 2007)

The issue is that you don't just build for you. You build for the house and therefore future owners. The structure needs to be engineered for a living space, not a lightly used living space.
You may be correct and the flooring system might be to code. And you might be wrong.
I'd have the space engineered to see if it will meet the current code.
Ron


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## Gary in WA (Mar 11, 2009)

Here are some requirements; The rafters depend on local snow load, spacing, species and grade; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_8_sec002_par023.htm

Check with your local B.D. on the collar ties: http://www.finehomebuilding.com/how-to/qa/removing-collar-ties.aspx

You want the struts to be side nailed to the joists, check local B.D.; http://www.smandf.com/25-ConventionallyFramedRoofs-TheGabledRoofProblems&Solutions.pdf

Your floor joists require; http://docs.google.com/viewer?a=v&q=cache:EJENK_uA9RQJ:www.cr-ar.com/pdfs/Building%2520Code%2520Changes.pdf+R602.3(1)+2006&hl=en&gl=us&pid=bl&srcid=ADGEESh311Kosslp7jsjp9WhUXxZsgSiyNaWb-eW0YminVwmPUavLlW75BKLoV9em4LFCAydmTGQlkUFOPm8AqHkRP5wLQGQQGkwyzAGtlN4D2DqCiCLwprOpEbvvjeGk6CEhyHiVdtK&sig=AHIEtbRa4Ah_IICPQ3BR1txlzU01V4JQIg

Gary


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## homestudio (Mar 22, 2011)

Hey guys. Thank you so much for taking the time to reply.

Gary.

Starting with the roof rafters. Using this table from your link (thanks)

TABLE R802.5.1(6)-continued RAFTER SPANS FOR COMMON LUMBER SPECIES (Ground snow load=50 psf, ceiling attached to rafters, L// = 240)

There are 2 dead load ratings... not sure which applies. Also confused as to what the "SS" grade of lumber would be. Again, my 2x6's are more like 1 7/8 x 5 7/8... and dense as heck. Having a hard time deciding how to compare this to modern lumber criteria. 

The knee wall location divides my roof span 9' 2.5" and 7' 4 3/8"... this is linear following the slope. Is that correct? Or am I looking at total horizontal ground covered by the span? If that is true, does this mean that the 13' between knee walls is the span I am looking at as a weak link to reference the tables?









If it is the 13' span that is of concern, I'm having a hard time understanding how pitch isn't a factor in these tables.

One more thing. 50lbs. per square ft of snow?!?! holy moly. I'd be impressed to see 50lbs. accumulate on a 45 degree pitched roof. Better safe than sorry I suppose.


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## homestudio (Mar 22, 2011)

Collar ties are going to remain in place. They add a structural interest to the open space I enjoy. They exist on every rafter spacing. Additionally, I have added a second collar tie with 4-5" of vertical space between it and the peak. This to serve as the drywall layer, and also provide a space for an exhaust vent run.


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## homestudio (Mar 22, 2011)

Concerning the 3rd link, the closest to my situation is what would be rendered as "balloon framing". The only space between the attic floor joists and the roof rafters is the 2x4 top plate on which they rest. It would appear that the requirements would be for no more than 8" of separation... does this suggest that I am in the clear already on this aspect?


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## homestudio (Mar 22, 2011)

Floor joists.

The knee wall locations are supported from below by load bearing walls. (Well, there are some exceptions of arch ways, but I'd say 70% of the length is supported).

Should I then use the live load requirements in your las link, and refer back to the tables of the first link?

If I do find some weaknesses, Would it be wise to "daughter" additional 2x6's to the original joists?


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## homestudio (Mar 22, 2011)

One more thing.

The roughly 25' length of the room...

This is terminated by a "back" wall of the room. New construction. This dividing wall is eve to eve, and to the peak. It's location is roughly half the length of the house, and is directly above bearing walls. Surely this solid support dividing the total roof's length in half will have some positive effects on overall load distribution?


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## pyper (Jul 1, 2009)

If your knee walls are located over load bearing walls, and those are in turn supported by an appropriate foundation, then you measure the spans from the wall location to the next support. So if there's a load bearing wall in the center, then you only have about 6' spans.

An engineer can calculate the values for your arbitrary sized rafters and joists. There are also programs online that will let you do the calculations, but you need to either know the species of the wood, or the design values for it.

The 50 lb/sq ft figure is the total live load. That includes the snow, the wind, the people, the furniture, and everything. When your roof has more pitch you get less snow/ice, but you get more wind.

Rafter span is measured as the clear area between the top plate and the ridge board (or structural knee wall), parallel to the floor. As you increase the pitch, the depth of the joist (measured in the direction of gravity) increases as well, so these factors offset each other.

So if you measure between the top plates and get 24 feet, then if you span it with a single joist/rafter, it's a 24 foot span. But if you build a 4" wall/beam (actual dimension) in the middle, then the two spans are 11'-10, not 12'-0.

So it sounds like your joist spans are really only about 6 feet. Maybe those 2x6's are OK. 

I think we know enough to say it might be reasonable to proceed, but for the safety of your family (and future residents) you really need to pay an engineer to take a look.


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## homestudio (Mar 22, 2011)

Well. Not exactly.

Roof rafters have a span of just over 9' from knee wall to peak. Disreguarding collar ties.

Floor joists are 13' across the middle, but upwards of 18' in a few locations... (again archways on the floor below.)


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## homestudio (Mar 22, 2011)

pyper said:


> Rafter span is measured as the clear area between the top plate and the ridge board (or structural knee wall), parallel to the floor. As you increase the pitch, the depth of the joist (measured in the direction of gravity) increases as well, so these factors offset each other.


nevermind the above. just read this again.

though the concerns regarding the floor joists remain.

would daughtering or sistering... be of good piece of mind?


Also, the original purpose of the thread (much appreciated on the checking of the system as a whole BTW) still remains. Would the cripples that I had originally purposed be necessary?


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## Underdog (Sep 4, 2009)

homestudio said:


> Hello all.
> 
> Essentially, the current construction is 2x6 16"oc floor joists. 2x6 24"oc roof rafters, resting atop a 2x4 top plate. I would like to simply be able to attach an additional "top plate" a foot or two in from the junction, and cross brace to the rafter *to prevent shearing*. Plausible?
> 
> John


What exactly do you think will fail in shear here? I'm not following the your reasoning for adding the cross braces. Are you worried that the connection between your joists and rafters is not adequate and the added weight of the drywall will start to cause your walls to push out?


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## homestudio (Mar 22, 2011)

No, I'm worried that the joint itself is weak, and if the added weight may cause the roof to kick out off the wall top plate. 

it's more than possible that "shearing" isn't the proper term.


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## Underdog (Sep 4, 2009)

homestudio said:


> No, I'm worried that the joint itself is weak, and if the added weight may cause the roof to kick out off the wall top plate.
> 
> it's more than possible that "shearing" isn't the proper term.


I had to look at your picture again to understand what you are worried about. It looks like your joists are not physically connected to your rafters, is this true? Typically, the joists sit on top of the top plate and then the rafters are physically connected to the joists.


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## homestudio (Mar 22, 2011)

Correct. Rafters are notched and rest atop the top plate. One would assume them to be nailed down, but given that would have to happen from the top side, and the roof is sheathed... I have no way to guess what gauge or quantity of fasteners that were used.

Thus the concern.


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## Underdog (Sep 4, 2009)

I understand why you are concerned but are you even sure that you have a problem? If the house is 100 years old, it surely has experienced snow loading before. So really, you are only concerned about the additional load on the rafters due to drywall? I would look up the unit weight of drywall versus the unit weight of snow (and wind loads) to see if you should be worried.

Also, if you wanted to strengthen the connection, I would look at the Simpson StrongTie website to see if they have a connector (besides the hurricane ties because they only resist uplift) that can strap the joists to the rafters:

http://www.strongtie.com/products/category_list.html?source=topnav

What you are proposing won't help much if there truly is a problem with lateral spreading. I can see that you are trying to create a triangle to resist the spreading, but triangles are only strong when all corners are strong connections. If you have a lateral spreading problem, that corner of the triangle will simply open up and you will lose all of your strength. 

With that said, what you are proposing also won't hurt your situation. I would just lose the second "top plate" (that's what I think you called it) and connect the 2x4 to the side of the joist and the side of the rafter with multiple nails. Try to keep your nail forces in shear, not withdrawl.


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## homestudio (Mar 22, 2011)

Hey Loftezy,

Thanks for your reply.

I'm certainly not one for doing more work than is necessary... particularly if it will be ineffective. Having thought about a triangle losing it's corner, I can see where that system would fail, but if the "eve" corner fails, that means one of the other two would of had to fail first; given the notched rafter, no?

Understood that drywall weight is a drop in the bucket compared to that of snow. Just trying to dot all my "i"s and such.

While the application may be useless (or at least wouldn't help much), I wonder if you can shed some light on your suggested implementation. You suggest removal of what I had referred to as a "top plate" (sorry... I'm new here). I understand this would clearly create a stronger connection. Being the floor is 16oc and the roof 24oc... am I missing something... how would I carry this out? Realizing it's possible you missed the reference to the spacing discrepancy earlier, but if there is an accepted method of achieving this joint given the difference, I'd like to know... for future reference if nothing else.

Again, thank you

John


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## Underdog (Sep 4, 2009)

homestudio said:


> but if the "eve" corner fails, that means one of the other two would of had to fail first; given the notched rafter, no?


It is hard to explain online but I can give it a quick shot. The triangles you want to build are in essence very simplistic trusses. Truss joints are only designed to resist tension and compression forces - they are not supposed to resist moment forces. So, if you truly have a week joint at the eve(which I'm not even sure you do), that joint will fail to resist tension and compression forces, making the other two joints resist moment forces. And the other two joints that you are creating aren't very strong when it comes to resisting moment forces...hence they will fail too. 



homestudio said:


> Realizing it's possible you missed the reference to the spacing discrepancy earlier, but if there is an accepted method of achieving this joint given the difference, I'd like to know... for future reference if nothing else.


You are right, I overlooked the spacing issue. That is something that you will have to be inventive to overcome. 

Have you looked at the strap ties yet? There are quite a few to choose from and some are even twisted to allow connections of wood members that are on different planes. Look at the website, hover over the name of the ties to get a picture and description. I cannot recommend a certain tie since I have never used them for your application but you might be able to find something that you are comfortable with. Your goal should be to connect the top (or side) of your joists to the side of your rafters. And yes, I realize that will be an issue since they don't line up exactly. 

http://www.strongtie.com/products/categories/straps_ww.html


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## pyper (Jul 1, 2009)

homestudio said:


> Correct. Rafters are notched and rest atop the top plate. One would assume them to be nailed down, but given that would have to happen from the top side, and the roof is sheathed... I have no way to guess what gauge or quantity of fasteners that were used.
> 
> Thus the concern.


If you examine the sides of the rafters where they rest on the top plate, you will probably see some big nails toe-nailed to the top plate. They're probably a quarter inch around. The joists are probably nailed similarly. 

I don't think the wood you proposed in the first post would help anything. It could cause harm. Roofs are complicated and designed to work as a system, and adding random bits can have unintended consequences. 

For a small fraction of the cost of your project, you can get an engineer to look at it, and then you'll know for sure what's safe and what's not.


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## Gary in WA (Mar 11, 2009)

We have touched on this before….. http://www.diychatroom.com/f14/cutting-joists-attic-ladder-90944/index2/

Read all three pages for insulation; http://www.diychatroom.com/f19/rafter-framing-code-question-93001/




Gary


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## homestudio (Mar 22, 2011)

Thanks for the links again GBR... though much of the linked reference remains above my head.

I apologize for my lack of verbage knowledge... but I am looking up words here as we go.

In terms of the rafter/joist junction. I can join them with a "plate" where their spacing meets every 4'. To me, this seems a reasonable strength improvement considering the existing construction has been enduring the wind and snow... this extra fastening is merely to make sure the drywall doesn't break the camel's back.

For insulation, refer to this drawing:









color code. 

pink lumber refers to insulated cavities (rafters are duravented)
green triangle was space I assumed I would need for a shared air space to utilize eve vents
blue triangle is an area that escaped me during the initial "design"... this is a shared air space.

Originally, I was planning to construct the second smaller knee wall (which would also be insulated...forgot to color it pink) to represent the inner drywall layer of the space. Again, to create a cavity to ensure that all rafter spaces would benefit from the intake at the eve vents. It seems reasonable that the smaller blue triangle area should be sufficient for the task though? Should I then fore go the second knee wall, and instead continue duravent and insulation in the rafters, as well as sub floor and drywall, all the way into the eve?

Understood this doesn't meet criteria referenced in GBR's link. While meeting code would be great, and I want to attempt to do so at every opportunity, this project does not have permits. I know. The house is owned by a band member, and was bought from a band before that. Over the last 20 years or so, there have been many projects undertaken with no documentation. (electrical, plumbing, finished basement) It would be a nightmare to have an inspector through here. Having said that, I want to execute this project correctly, though I may not follow to a tee details which, like the insulation link, seem to be open for interpretation.


I'm still looking for more info on the floor joists. Is there a table defining load calculations for sistered spans? Additionally, is sistering a 2x4 to a 2x6 adding anything, or does the sister need to be of equal girth?



I really appreciate all of your help.


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## homestudio (Mar 22, 2011)

OH yeah.

The existing flooring in the attic is 5" planks. We are removing that and installing 5/8 osb. Before insulating the cavities, we are having an electrician friend come in and remove all of the old knob and tube electrical and running new service. I mention this to explain that sistering should be possible.


I realize that engineer suggestions will continue to pop up. I appreciate the need, but am concerned that anyone I have sign off on the project would either require permits, or "rat" me out. I'm nervous about this.


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## pyper (Jul 1, 2009)

homestudio said:


> I appreciate the need, but am concerned that anyone I have sign off on the project would either require permits, or "rat" me out. I'm nervous about this.


I won't comment on whether you should get a permit or not, but the engineer would be consulted prior to obtaining a permit. He tells you what you need to do, and then you get the permit based on his drawing. I can't imagine any engineer bothers to follow up and see if projects like this ever get followed through on.

As far as sistered joists, you just just calculate the design load (pounds per square feet times square feet) and look up how much a beam of a given length can carry. If you use two you add them together. In reality, if you glue and nail them together they're stronger than the simple sum, but not by much.

tables: http://www.awc.org/pdf/wsdd/c2b.pdf

So according to the table, a 13 foot span with a 2x6 beam can carry 349 lbs. Two side by side (sistered )can carry twice that. on 16" centers, you have 17 SF, so 20 lb/sf; 40 for two; 60 for three, and so on.


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