Got a couple of dumb DIY questions about framing in an attic, if anyone is willing to entertain….
Been reading in the archives about collar ties (vs rafter ties). So collar ties high up are BAD because they put stress on the rafters the higher they are, but are GOOD because they keep the roof from opening up like a clam shell (thanks Boss) in high wind. So does this stress level peak and go back down as you near the top? Dumb reasoning suggests that the closer you get to the ridge, the more stress a collar ties puts on things, but code says it can’t be any further than 1/3 of height from top. Is there something about keeping that stress as far from the center of the rafter span as possible? (I’m asking because I’ve got severely undersized rafters, hairs on the nape of my neck went up when I glanced at some rafter load tables….).
Second dumb question: I read about people putting kneewalls in attics. Seems to me that’s a Really Bad Idea if like me you’ve got undersized attic floor joists, unless there’s a load-bearing wall underneath. On a related note, when running some wires in the attic I hung some 2x4s from the rafters to bring wires down to run between the joists, but cut these 2x4s at the bottom so no load would transfer to the joists….
Replies
And the bathroom is ?
Remodeling Contractor just outside the Glass City.
Quittin' Time
Relax, Cal, everything's under control.....Really.
Sorry man, my curiosity got the better of me.
carry on.Remodeling Contractor just outside the Glass City.
Quittin' Time
About collar ties - I never heard they had a down side to them....
RE kneewalls, if attic floor joists (ceiling joists) are undersized, you probably shouldn't be finishing off the area anyway...
Not finishing, was thinking of a kneewall to isolate the AC stuff then thought better of it....Reading in the archives about collar ties and moments of force and other stuff.... Let's see, 28546.8, 28546.50, 44081.70, and 13773.19 has a good engineer's take....
Edited 10/8/2005 10:05 pm ET by Taylor
Sorry - re collar ties, I read those posts, but I am convinced of nothing.
28546.8, is saying that collar ties don't take the place of ceiling joists in preventing the walls from spreading. Further, based on the fact that he starts with "I don't know for fact what they are for" at least insinuates that he is not a residential building professional, and definitely not a PE of the residential discipline. Could be a civil engineer or who knows what?
28546.50, says >> Back in the dark ages, in architecting school, I was taught that a collar tie was used when there were no conventional ceiling joists (coffered, miter, cathedral, etc., ceilings). The collar ties "took the place" of the joists in preventing the spread of the walls below (from any roof loading). This is why they had to be in the bottom third of the rafter (closest to the ceiling). << This guy is talking oranges, we are talking apples. I think his knowledge might be a little rusty - either that, or he talks a different roof framing language than I do. Or maybe he sat in on a class were there was a teacher who could only teach...
44081.70, >> Collar ties (which are required by the Southern Building Code and no other) << An incorrect statement. NC state BC requires them and we use a version of the IRC2000, be it a modified version. Before that we used CABO. Again though, he is saying that collar ties are not a substitute for ceiling joists.
13773.19 I don't think we can assume this guy is an engineer. He says he is "consulting" on a job - which really sounds like a garage - so what does that mean? Is he a fairly intelligent guy with some knowledge but no actual credentials or working experience who is just helping a friend? I don't know. Is he a good carpenter helping a friend with some advice? Maybe. At any rate, in his project he is recommending a structural ridge - a double LVL - and, as he says, collar ties for aesthetics only. Further, at the end of his technical explanation he says >> easiest best answer is to set the ties on the top plates << Which again, he is saying that ceiling joists are necessary to keep the walls from spreading. Collar ties don't do this.
So - I see no reason to think that collar ties are bad, just that you have to realize what their purpose is - to hold the rafters to the ridge - and that they not intended to keep the structure's walls from spreading. The prevention of the spreading of the walls has to be dealt with by methods other than collar ties.
If it's OK with you, I'll just keep using them to hold the ridge together and pass building inspections.
I know that discrediting a writer is not the best form of persuasion (criticize ideas - not people) but what I am saying is that you gotta take everything you read here with a grain of salt. There are a lot of people here who make statements based purely on conjecture or perhaps their experience is limited to what they learned building or working one or 2 of their own houses. Just remember - you always get your money's worth for the information you get here - and some of it is a great deal and some is not.
Classic Matt, we should include it in all our posts.
Just remember - you always get your money's worth for the information you get here - and some of it is a great deal and some is not.Remodeling Contractor just outside the Glass City.
Quittin' Time
I guess I tend to be a little testy first thing in the AM...
No, not testy...........truthful. This forum is like any other information source. You have to temper the answers with the knowledge and experience of the giver. Hard to do reading this type.Remodeling Contractor just outside the Glass City.
Quittin' Time
Yeah, I don't think you are in disagreement with those posters. Everybody agrees that rafter ties or attic floor joists are to prevent spreading, and collar ties to hold the rafters together at the ridge. I was just trying to understand why the argument for increasing moment as you moved the collar ties up the rafters did not lead to max stress at the apex.But I think I got it now. The ridge board wants to sag, and the rafters want to bend to allow that sagging. Putting any kind of tie say halfway down the rafter is putting a lot of stress at the greatest pressure point, where they want to bend. But putting ties at the bottom (rafter ties) or at the top (collar ties) avoids that scenario.Or something.BTW I have a 2x4 post right in the middle of my attic floor, apparently to hold up the ridge board in the center. Sits on 2 2x6 joists, AFAIK nothing underneath.Other fun details: rafters are 2x6 @ 20"oc. Got nasty cracks in two of them. Since attic floor joists are 16"oc, the rafters are rarely connected to the joists directly. Instead all are toe-nailed into the 4x4 top plate. In some places nails are tearing out. If anyone can suggest a Simpson connector to make that connection stronger....
>> BTW I have a 2x4 post right in the middle of my attic floor, apparently to hold up the ridge board in the center. Sits on 2 2x6 joists, AFAIK nothing underneath. <<
It's likely just a temporary that was placed there during construction. You normally put up the ridge first, and then stack the rafters in against it. You gotta have some temp bracing to hold the ridge in place.
>> Other fun details: rafters are 2x6 @ 20"oc. Got nasty cracks in two of them. Since attic floor joists are 16"oc, the rafters are rarely connected to the joists directly. Instead all are toe-nailed into the 4x4 top plate. In some places nails are tearing out. If anyone can suggest a Simpson connector to make that connection stronger.... <<
regular H 2.5 hurricane ties or similar would have worked fine - I just don't see how you would get them in there since I assume the ceiling is covered, and probably a bunch of insulation and other fun stuff is in there too. That's some pretty odd framing though.... although I do new construction which I like a lot better since I have a lot more control... see first pic below.
Anyway, assuming that some of the rafters that are not connected to ceiling joists are at least close to a ceiling joist, I'd put in some diagonal braces as close to the wall's top plate as I could get them. The braces could be any of 1x6, 5/4x6, 2x4, double 2x4, 4x4, etc, depending on how big the gap is between the rafter and the closest ceiling joist. The braces would be about 1' - 1.5' long. Look at the second pic below, bottom left corner. The ceiling joists/rafters are nicely stacked in the pic, but this same method will work in your instance if you have a ceiling joist and rafter that are, say, 3/4", 1", 1.5", 3", or 3.5", etc, apart. This kind of bracing is not "exactly what the doctor ordered" but it will definitely help and, material wise, will be cheap to install. Don't get too mired down though in over analyzing the forces that could be created though. It's not like the house is ready to fall down either way.
Thanks for the bracing suggestion. I actually do have good access to the point where rafters, ceiling joists and top plate connect, for reasons I don't wanna get into....I hear you about new construction. After crawling around in my belly under an air handler putting in joist hangers, or my arm buried in a wall pulling new wiring through, hats off to anyone who does remodeling for a living, they must live for the daily challenges....
they must live for the daily challenges....
That would be the noble thing to do, but I'm leaning towards (and hoping to attain) the lucrative remuneration.
Now, speak to me on the bath. At what point are you? I'd go back (too lazy to look) to your posts on the thing, but since I have you here............Remodeling Contractor just outside the Glass City.
Quittin' Time
Gosh is that the time gotta go....
Some day, some day, I'll post the Breaktime Bathroom Thread... I'll try to provide links to all the threads I started on various details.... I should have plenty of time, since I'll probably be unemployed.....
Edit: FWIW BI said I did nice work, plenty of work out there if I wanted it.... Told him I worked too slow to be able to make a living at it....
Edited 10/9/2005 10:57 am ET by Taylor
Taylor,
Just so you understand, it's the norm here (at least it happens all too often) to ask questions, get a #### load of answers, and then fall off the board. Your threads on that bath from hell were followed by me (and I would guess others) and I'm just interested in seeing the finished product. Your sweat on that project was immense, your pride should be in the amount of obstacles you overcame.
I would also like to meet the wife. She has achieved sainthood at least from my catholic upbringing. At any rate, hope to hear more from you on the above subject.
thanks
And, as a comic aside. In our work as remodelers, we are constantly challenged. Oftentimes our experience, code, and generally accepted standards dictate how we proceed. However, flying by the seat of your pants plays a large part in our jobs. Quick is the keyword tho in how we deal with the challenge. Thinking in advance of the problem arising is also a mainstay. Put it this way, keep your dayjob. And in no way am I trying to insult you or your work. It's just that you tend to dwell on things. No offense intended, hope none taken.Remodeling Contractor just outside the Glass City.
Quittin' Time
Cal, no offence, I know I couldn't do this for a living. And although I disappear from time to time when I'm busy (or busier than usual), I will definitely come back with a wrap-up when this is all done. I owe this board a lot!I know I over-analyze things. But that comes from (a) doing everything for the first time and (b) spending so much time fixing the lunatic mistakes made by my predecessors. Probably being an engineer's son doesn't help.Speaking of which, I second the sainthood nomination. It has helped a bit that my Dad, retired civil engineer, has convincingly communicated that we were in some scary territory at some points (including some of the work done by the carpenter who was supposed to fix the joists). I haven't even told you about the electrical mess I inherited....
Taylor,I forgot to add the drawing.Joe Carola
On your pic with hurricane ties: I assume that those ties are *not* taking the place of joists or rafter ties? I have the impression that hurricane ties are only good for fighting uplift....
>> On your pic with hurricane ties: I assume that those ties are *not* taking the place of joists or rafter ties? <<
Right, but they do strengthen the rafter - ceiling - wall connection. I think you said that the rafters in your house were not well connected on the bottom end. In the pic, that roof actually had a structural ridge, but I would say that the rafters are quite well connected to the wall.
>> I have the impression that hurricane ties are only good for fighting uplift.... << That is their primary purpose but, again, don't over analyze... common sense tells us that the clips are also gonna keep the rafters from sliding on the level cut out away from the walls - which I think you said your nails were "tearing loose" and asked how to strengthen the connection.
Edited 10/9/2005 12:04 pm ET by Matt
Matt, thanks much for the clarification re hurricane ties, and the suggestions.
My sister lived in an old farm house owned by the university she works for in PA. ~ 1910 would be my guess. The main structure was basically a simple gable house that a story and a half - balloon framed. The front and back wall were maybe 12' - 14' high with a 2nd story floor system attached between these 2 walls at about 8' of height. The roof was ~ 8:12 and and there was a finished room in the upper part of the house. Collar ties held the rafters together forming maybe a 5' wide flat ceiling in the top of the upper room. See attached pic. There was some sag and a lot of floor bounce in the 2nd floor system - I think they were rough sawn 2x6s with about a 14' span - which were all exposed. They had already been sistered twice. The roof ridge was sway backed (sagging) quite a bit. From inside the 2nd story room you could see the non-gable end walls were bowed out about 5" in the center of the walls. I wish I had taken some pics. I'm not sure what kind of shape the foundation was in - it had a "cellar" but I was never brave enough to go down there... An engineer was called in to evaluate the structure and particularly the floor bounce... The house was condemned and her family was moved out by the end of the month. A mobile home was brought in while the old house was torn down and a new one was built (at the expense of the university). They were very happy in the end, but I think they came to realize that all that rustic charm was only good up to a point... To me, the old house was like going camping... although it was dry... the new house is not only dry, it is also cosy, and solid...
So, what I'm saying is that this kind of structure can work, it just has to be engineered correctly. My Sis's house probably should have had a structural ridge, and larger sized framing members and 16" OC, which if memory serves, the members were all spaced further than that.
To further illustrate, in our NC state code book, again a modified IRC2000, chapter 8, on the rafter span tables, a chart is given at the bottom of each rafter table that gives a fractional rafter span adjustment factor for raised ceilings. For example if the total rise of the roof is 8' and the ceiling is raised 1/4 of the way above the wall cap plate; 2' - then the allowable span is multiplied by .76 meaning that a rafter that could have spanned 10' with a normal celing joist arangement can now only span 7.6'. This effectivly upsizes the rafters - the more the ceiling is raised, the more rafters are upsized (smaller the fractional multiplier). So, looking at the most extreme end of the fractional multiplier chart, "2/3 or greater" the multiplier is .5, which effectivly means you only have collar ties, the widest span that you can get out of a 2x12-12' SPF 16" OC, is 11'2" menaing that essentially the house can only be 22'4" wide - inside to inside. That's not a very wide house...
Here is the chart:
http://www.amlegal.com/nxt/gateway.dll/NC%20Residential%20Code/part00382/chapter00982/section%20head00987/section00997.htm/subsection00998.htm?f=templates$fn=main-nf.htm$3.0#JD_TableR802.5.1(1)
look at the second chart: TABLE R802.5.1(2), now look for "Roof live load=20pfs, ceiling attached to rafters, L/delta=240" at top of chart, then look for "Rafter spacing = 16" (left side of chart), then look for "Spruce-Pine-fir #2" (normal framing lumber here) and then slide all the way over to the right to "2x12", "dead load = 20 psf". BTW = a more common size for rafters is 2x8s.
So - the whole point here is that you can move your ceiling joists up, or even way up, but from a practicle standpoint, you better get it engineered, and you may even need to go to a structural ridge. In your instance, it may be a good thing to strengthen the rafter to ceiling joist to wall connection. Also, assuming you have segminted ceiling joists - say a 30' wide house with 16' ceiling joists spliced in the center, you may want to verify that the splice is well nailed.
The short answer to your initial question/statement is: - collar ties are not bad but are best when used in conjunction with ceiling joists unless you have your roof system engineered and/or you upsize your rafters.
You may also want to think about other factors; like how many layers of shingles do you have on your roof... And what is the peak snow load in your area.
Matt, that is great info, thanks.
You may also want to think about other factors; like how many layers of shingles do you have on your roof... And what is the peak snow load in your area.
Funny you should ask. I am considering insulating my roof. Looks like the right way to do this (per Ray Moore and buildingscience.com) is to lay 4" polyisocynurate above the roof deck, then a second layer of sheathing as a nailbase for shingles etc. Can this configuration of rafters take that load? I'm in Northern Joisey, close to NYC.
Right now I have one layer of shingles, until a year ago there were three. One yo roof is, I found out today, toast....
Taylor,You better get an Architect out there before you do all that. I'm from Caldwell and I framed an Addition last summer in Westfield and I used 4x8 sheets of 4" foam board that came with 1/2" plywood on the top of it but these were over 3" x 12" oak rafters.There's work involved with this stuff at the end for the fascia line. I nailed 2x6's across the front and 2x6's up the gable ends to have nailing for the rakes/fascias and gutters.Joe Carola
You better get an Architect out there before you do all that. I'm from Caldwell and I framed an Addition last summer in Westfield and I used 4x8 sheets of 4" foam board that came with 1/2" plywood on the top of it but these were over 3" x 12" oak rafters.
Joe, do you have a reference for that archie? I can only find people who believe attics should breathe etc. Even if I convinced someone of this approach, I'd rather not have them learn on my house...
Your story of your sister's farmhouse made me think of this bathroom project that Calvin has alluded to. This bathroom sits on a 4x10 beam over the kitchen. I believe that beam is sagging. Evidence includes steam pipe that is not (no longer) pitched right, attic joists separating from top plate, top plate splice collapsing (I replaced the window header underneath, which was just 2x4s).... Anyway the weirdest thing is in the outside corner, where the stud at that corner is basically suspended between top plate and beam (no bottom plate), attached to each by some nails. I've sistered another stud to it. It's possible the sagging splice made that end of the top plate seesaw up a bit. There a saddle joint (?) at the corner splicing the top plate to the plate on the adjoining wall, and it's coming out of the joint about 1/4".It's hard to tell from underneath what's happening because of a suspended DW ceiling. What's left of the original plaster ceiling has this godawful textured wallpaper (!) on it. Looks like the POs dealt with plaster ceiling problems (due to the sagging beam) by papering over them.
Taylor,
From where I'm from we call collar ties for example when we frame a clipped ceiling. If we use a 2x8 rafter and a 2x10 ridge and a 2x8 collar tie given the height amount specked from an Architect or Engineer above the plate or from the floor. The bottom of the collar tie will be sheetrocked.
A lot of jobs we do we will put 2 carriage bolts threw each collar tie into the rafter. Every roof is different and I've built many many houses and additions over the years like this without the walls spreading. As far as I'm concerned these collar ties given at a specific height foe EACH job keeps the walls from spreading.
There's also been roofs that I framed where we had a Structural Ridge and Collar Ties with bolts.
So the bottom line here is when you see my drawing there's many houses and additions framed this way using just a 2x ridge and collar ties with hurricane ties on the top plates on every rafter that hold the walls in.
From where I'm from ceiling joist sit on the top plate from one end to the other and nailed to the top plate and the side of the rafter.
Joe, thanks, I think (from reading the archives) Piffin for example will call those rafter ties. A collar tie (in the sense I'm using the word) is just a 2x4 that holds the rafters together at the ridge. I'm not sure about fasteners, I'm just using Simpson 1/4" screws...