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Thread: Rafter size

  1. #1
    dan orourke's Avatar
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    Default Rafter size

    Last edited by dan orourke; 12-31-2007 at 10:05 AM.
    Elite MGA Home Inspector E&O Insurance

  2. #2
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    Default Re: Rafter size

    That is not a problem.

    If that is an older house and/or the rafters are overspanned, I would comment on that. Based on the snow region, the comment would vary of course.


  3. #3
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    Default Re: Rafter size

    That *IS* a problem.

    The ridge should always be as large as, or larger than, the cut rafter ends (typically that means the ridge is one size larger than the rafters).

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  4. #4
    Jon Randolph's Avatar
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    Default Re: Rafter size

    Quote Originally Posted by Jerry Peck View Post
    That *IS* a problem.

    The ridge should always be as large as, or larger than, the cut rafter ends (typically that means the ridge is one size larger than the rafters).
    With the rafters/ridge board installed like that, and many of them are, there is no support for the rafter wood that hangs down past the ridge board. Heavy roof loads or age could cause them to crack and fail.


  5. #5
    Kevin Barre's Avatar
    Kevin Barre Guest

    Default Re: Rafter size

    While Jerry is correct in that best practices would dictate a full depth ridge, I would not necessarily get too worked up about it. It is quite common in older homes. In fact, I've seen numerous homes over 70 years old or so with NO ridge board at all. And ya know what? they are still standing. Admittedly, most show noticeable deflection or spread, but that's from inadequate bracing, not the lack of a proper ridge board. Judging from the 1X decking and fully dressed lumber, I would guess this home was built in the 1950's. Early 60's at the latest. If there were any significant ill effects from the ridge size you would probably have seen them. So, if it's 50 years old and hasn't shown signs of real problems by now my money says it won't in the near future.
    IF (and this is a big "if") the bracing is correct -- especially the collar ties with a low pitch roof like this -- I would not make an issue of it. I do usually insert a comment to the effect that while the roof structure does not meet modern requirements no problems were noted at the time of the inspection due to it.


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    Default Re: Rafter size

    Quote Originally Posted by Kevin Barre View Post
    While Jerry is correct in that best practices would dictate a full depth ridge, I would not necessarily get too worked up about it. It is quite common in older homes.
    Kevin,

    Did you notice the rafter on the right in that photo?

    I forgot who, but some here has a great photo of *why* that is not only 'good practice', but *required too*.

    In fact, I've seen numerous homes over 70 years old or so with NO ridge board at all. And ya know what? they are still standing.
    Totally separate framing design and issues, with no ridge board, the two rafters meet fully, the problem shown in the photo does not exist.

    Admittedly, most show noticeable deflection or spread, but that's from inadequate bracing, not the lack of a proper ridge board.
    Yep, like the 'no ridge board' that is a separate issue and has no relationship to the issue caused by the rafters having been cut larger than the ridge board.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
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    Default Re: Rafter size

    From the tongue in groove underlayment I would think this is a 50's home. It's been this way for a while. That dosen't make it right. I understand the problem. What I would like to know would be, what do you all think a practical solution is.


  8. #8
    Michael Greenwalt's Avatar
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    Default Re: Rafter size

    Ridge boards were not required until late 50's, minimum depth of cut until 1978. (varies depending on your region of course as it could be much later). Ridge boards are *required* now of course, minimum depth...of course. Depending on the date of construction well........... So, delima, "What" do you report.

    Mileage may vary depending on driving habits.


  9. #9
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    Default Re: Rafter size

    With a house that age that has lets say "stood the test of time", it would not even make my report.

    Scott Patterson, ACI
    Spring Hill, TN
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  10. #10
    Jim Hime's Avatar
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    Default Re: Rafter size

    This is a problem. The bearing forces of a rafter are at the bottom of the rafter against the ridge according to the engineers at the Texas Dept. of Insurance. Besides, it's also codified at IRC R802.3. The rafter could crack laterally down its length. A common repair I have seen is to nail a 2x2 on bottom of the ridge that appears to be acceptable to our Gulf Coast windstorm engineers.


  11. #11
    Kevin Barre's Avatar
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    Default Re: Rafter size

    Quote Originally Posted by Jim Hime View Post
    This is a problem. The bearing forces of a rafter are at the bottom of the rafter against the ridge according to the engineers at the Texas Dept. of Insurance. Besides, it's also codified at IRC R802.3. The rafter could crack laterally down its length. A common repair I have seen is to nail a 2x2 on bottom of the ridge that appears to be acceptable to our Gulf Coast windstorm engineers.
    Maybe someone monitoring this who is an engineer can chime in. I'm having a hard time wrapping my head around the concept that all the forces are concentrated at the bottom of the rafter. Whether it's a snow load or wind load, I just can't seem to visualize it. Certainly not a compression load. Generally, if I see a roof failure manifesting any signs at the ridge, its rafter spread pulling the rafter away at the bottom. That's going to be real hard to do with proper collar ties.That's why I said in my first post: "IF (and this is a big "if") the bracing is correct -- especially the collar ties with a low pitch roof like this -- I would not make an issue of it."

    In looking at the one photo provided and the statement in the original post: "Did not observe any rafter cracking or splitting" I fail to see why it's a big issue. I see one hairline crack about 1.5" long in the right rafter. For all I can tell, that may be caused by a gang nailing pattern on the other side, not rafter stress. What seems most important to me is that there are no gaps along the rafter/ridge joint, so it appears not to have moved. The exposed heel of the rafter appears to be about 1.25" so the portion making contact is about 80%.

    Obviously some of you disagree, but I take a pragmatic approach to these kinds of things. As Mr. Patterson pointed out, it has stood the test of time. After 50 years or so and no visible signs of failure I doubt anything catastrophic is going to happen. And if I see no problems with the bracing -- and most 50's homes around here that I see are generally OK -- then I don't write it up as a defect. My standard report language is that it doesn't meet current requirements, but if there are not any signs of problems I don't recommend any "corrective" action.

    I am not at all interested in turning this into any sort of argumentative issue; I'm just trying to learn. Any PE type want to educate me on this?


  12. #12
    Kevin Barre's Avatar
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    Default Re: Rafter size

    Quote Originally Posted by dan orourke View Post
    Good discussion!

    Further info not in my origional post:

    1968 home. This framing is of the garage attic. Rafters have only one collar tie in the middle of the roof. Did not see any rafter cracking/splitting. 2x4 purlin brace with 2x6 rafters. Roof sag at the front of the garage roof.

    Slab foundation with recent repairs (approximately 20 piers installed one year ago). Foundation still not level to the point you can't even open the front door more than 35 degrees as it binds on the floor. Significant structural movement indicators interior / exterior of home. I went ahead and recommended SE look at the garage framing since he was going to be onsite anyways!

    But....if you assumed no other structural movement and you just had the rafters being 1/4 " to 1/2" larger than the ridge board and its an older home, would it make it to your report? That was what I wanted to get opinions on and why I left the other info out on my origonal post.
    OK, I stand corrected. The home is closer to 40 years old. I guessed 50 years due to the use of what looks like 1X roof decking. I don't see that around here on much built after the real early 1960's.
    Roofs over garages of this vintage commonly show some deflection of the rafters. When you start off with undersized ceiling joists and you just add a stiffback and brace the rafters off of that they are often going to head south. In that case, I can see some force being applied to the bottom of the rafter at the ridge. But I still think there's enough bearing that it is not a big issue. But if this is what is happening, I think the bigger issue is that teh garage ceiling may be undersized to the point that it needs some beefing up. Especially when some owner's store way too much stuff up there.
    I
    f you can, please let us know what the SE says.

    Thread drift: Unbelievably, I once found an engine block in an attic. OK, so it was just a 4-banger, but it still would have been a job getting it up the stairs. There was no hoist or any other device to help get it up there. I can see it now...Earl and Bubba sittin' around with a cooler full. After about a dozen, Earl says "Hey Bubba. How much you wanna bet I can hoist that thing up in to the attic?"


  13. #13
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    Default Re: Rafter size

    Quote Originally Posted by Scott Patterson View Post
    With a house that age that has lets say "stood the test of time", it would not even make my report.
    Scott,

    Quote Originally Posted by Jerry Peck View Post
    Did you notice the rafter on the right in that photo?
    I wish I could remember who posted those excellent photos of what happens to rafters when the ridge is undersize (smaller than the cut end of the rafter) - that would explain the reason to Kevin.

    Nonetheless, if anyone looks at the right rafter in that photo, they will see the beginnings of what will be shown in those photos, if whoever has them will post them again.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
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    Default Re: Rafter size

    Quote Originally Posted by Kevin Barre View Post
    Maybe someone monitoring this who is an engineer can chime in. I'm having a hard time wrapping my head around the concept that all the forces are concentrated at the bottom of the rafter. Whether it's a snow load or wind load, I just can't seem to visualize it. Certainly not a compression load.
    I'll bite.

    Let's assume the roof is 9:12 pitch. (That makes it easier to do these calculations in my head.) Also assume we have dead loads and snow loads only (vertical loads) of 300 pounds on a rafter. That loads is at an angle to the rafter, not along it. The loads is transformed from a vertical load to a load along the rafter as follows:

    Draw vertical arrow pointing down that is 3 units long (3 units representing 300 pounds). Next draw another arrow from the tip of that arrow 4 units long. (The roof is on a 9:12 pitch which is the same as a 3:4 pitch.) Now draw a third arrow from the start of the first arrow to the top of the second arrow. It will measure 5 units long. That means the loan ALONG the rafter is 500 pounds. That 500 pound load along the rafter has two components - vertical and horizontal. The vertical load is 300 pounds and the horizontal load is 400 pounds.

    So, when a vertical load is applied to the rafter it induces a horizontal load.

    Would you want to put your finger between the rafter end and the rafter board and then apply a load to the roof? I didn't think so.

    "Baseball is like church. Many attend but few understand." Leo Durocher
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  15. #15
    Kevin Barre's Avatar
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    Default Re: Rafter size

    Bruce--
    Thanks for the reply. But it appears that I did not make myself clear. It's not the idea of the load being transferred to the cut end of the rafter that I did not understand. That's obvious. It's the earlier post stating that the load was transferred to the bottom end--the area w/o a ridge board behind it. That's what I questioned. I may have misinterpreted this quote: "The bearing forces of a rafter are at the bottom of the rafter against the ridge"
    Since the ridge is obviously not the bottom end of the rafter, I interpreted this as saying that the forces are at the bottom of the cut along the ridge.

    Clear as mud?


  16. #16
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    Default Re: Rafter size

    Kevin,

    What I think was meant by this "The bearing forces of a rafter are at the bottom of the rafter against the ridge" was that the loads imposed on the rafter are all transferred to the cut end at the ridge (all the length of the cut end) and the cut end (bird's mouth) at the top plate.

    While the bird's mouth cut rarely is properly cut and fully seated (bearing) on the top plate, that end seems to cause many fewer failures (possibly because the dead load is vertical and that is bearing vertically on the top plate).

    The ridge cut is problematic in that it (the full length of the cut) is carrying the its full share of the dead load (gravity load) in the vertical position wedging the ridge between the two opposing rafters.

    When only part of the cut rafter end is against the ridge board, then only that portion of the ridge is being supported.

    Here is an example (extreme for example purposes only): Let's say that the rafter is a 2x8 (7-1/4" deep) and that the cut end of the rafter is 10" (I could look it up and match a slope to it, but I'm just pulling numbers out of the air for the example). Now let's say that the rafters are 24" on center (makes it easier to calculate that way). Let's say there is a 30 pound live load and 20 pound dead load on the roof, and that the roof is 10' x 10', or 100 sf. That means the total load on the roof is 100 sf x 50 pounds per sf or 5,000 pounds total load. Let's assume that 5,000 pounds total load is distributed 50/50 along the ridge and the top plate, or 2,500 pounds along the 10 foot ridge. There are 5 + 1 rafters supporting that 2,500 pound load at the ridge, with each rafter supporting 2 feet of the ridge load (except the two end rafters, which only support 1 foot of ridge load). That 2,500 pounds per 10 feet = 250 pound lf x 2 lf = 500 pounds on each rafter end (except the end two).

    Now, getting back to our 10" rafter end cut ... that means that each inch of the rafter end is supporting 50 pounds against a full depth ridge.

    Now, though, let's make that ridge 5" deep, that means the entire 500 pound load is supported only by the top 5" of the rafter cut end, the lower 5" does nothing except hang down in free air (I have not yet seen any "structural air" which has passed a structural loading test) - that means that the top half of the rafter is loaded to 100 pounds per inch of cut end, and the rafter is trying to split itself down the center where the load meets the bearing and where the no load meets free air.

    Given sufficient time and other conditions, this will happen.

    And, in fact, that *IS* happening in that photo.

    Which is why I keep referring to what I asked before: "Did you notice the rafter on the right in that photo?"

    I sure wish whoever has that photo of the split rafter end will post it, this time I will save it.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  17. #17
    Brandon Chew's Avatar
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    Default Re: Rafter size

    Ok, this engineer will take a stab at it ...

    Go to your local big orange box or out in your garage and get a 2x4 about four feet long. Miter cut one end at a 45 degree angle. This mitered end represents the end of your rafter that rests against the ridge board.

    Take the 2x4 and go outside and stand in your driveway (or on another hard surface that you won't damage) with your feet placed about shoulder width apart. Hold the square cut end of the 2x4 in both hands, like you were holding a baseball bat, and point it straight out in front of you. Orient the 2x4 so that the 1.5 inch sides are on the top and bottom, and so that the short side of the miter cut is pointing toward the ground and not up at the sky.

    Now, raise both hands high over your head and point the mitered end of that 2x4 at the sky. Swing it down as hard as you can and strike the pavement with the mitered end. Now, after you stop swearing and the sting leaves your hands .....

    If you hit the pavement with the 2x4 at an angle of less than 45 degrees, you'll find some of the wood crushed at the heel of the miter where it struck the ground, but otherwise the 2x4 should be intact. The forces applied in this case were mostly trying to bend the 2x4, much like loads placed along the span of a rafter or joist are trying to bend them. Within certain limits, the wood can do a good job of resisting those bending forces without breaking. This situation is similar to what you have with a structural ridge and rafters that are attached to it by hangers that have a bearing surface on the bottom edge of the rafter where it meets the hanger.

    If you were lucky and managed to strike the 2x4 on the ground at an angle of exactly 45 degrees, making full contact with the ground and the miter cut, the 2x4 should be undamaged. This is similar to a rafter with full bearing on the ridge board. It is also similar to a wood post or column supporting a load on one end, while bearing flat on the footing end. The wood is strong when the load is placed on it in this manner.

    If you hit the pavement with the 2x4 at an angle of greater than 45 degrees, the point of the miter will hit the ground first, and the end of the 2x4 will shatter, running with the grain. Wood is much weaker when the forces are trying to tear it apart along the grain vs trying to bend it across the grain. This is the situation you have when the ridge board is not wide enough, and a portion of the mitered end of the rafter hangs down below it. You are introducing forces into the end of the rafter that are trying to pull it apart along the grain.

    In all three cases the total force applied to the mitered end of the 2x4 was the same, but how that force is applied to it can greatly effect its ability to withstand that force. Now, this experiment is not exactly the same as what you are seeing in the framing at the ridge, because I'm applying a sudden impact force all at once to the 2x4 while in reality there would be uniform loads placed more gradually upon it. But it should give you an idea of how the rafters would fail with the cut end not fully bearing against the ridge board.

    Back to Dan's original question. I would note it in my report and explain why it's bad. I'd also note whether or not I saw any signs of rafter cracking or splitting and the age of the house. I'd finish up with "while I can tell you that it has been performing ok to date, within the scope of our contracted services I can't tell you whether or not this would be ok "as is" in the future under design snow and wind loads. To answer that question, you'll need to retain the services of a qualified structural engineer to do an engineering analysis and design any needed repairs."

    Last edited by Brandon Chew; 10-16-2007 at 08:14 AM. Reason: Edit to add last paragraph

  18. #18
    Kevin Barre's Avatar
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    Default Re: Rafter size

    This thing has lived way beyond what I intended, so I'll try to drive a stake in it and kill it!

    Brandon and Jerry --
    I understand what you are saying, and I appreciate the effort you put into it. Unfortunately, it's really not necessary. I am not arguing at all that there is no load on the cut ends of the rafters against the ridge and I never did. THE ONLY thing I had a problem with was the statement made in an earlier post: ""The bearing forces of a rafter are at the bottom (emphasis mine) of the rafter against the ridge." This is not correct, and that's all I was questioning. As you stated, and as I already knew, it is along the entire length of the cut.
    I understand the benefits of a full ridge board to evenly distribute the loads off the end of the rafter. But in the case in question -- and my comments apply only to it -- I still say the following points are important. Remember, the original question was how to write up this particular home.
    1. The exposed bottom end of the rafter below the ridge is only an inch or slightly more. There is full contact with the ridge board along roughly 80% or better of the rafter end.
    2. There is one small crack appearing to be about 1.5" long on the rafter to the right, and that split is above the midline of the rafter. Based on the scale in the photo, I would estimate that there is a good 3" of rafter below the split which still is in solid contact with the ridge. Based on the evidence visible in this 1 photo, I am not at all convinced that it is related to the lack of ridge depth and any resultant stress placed on the rafter. In fact, I have seen similar or worse splits from framers nailing 2 or more nails too closely together and angled to the point that the nail nearly (or does) exit out the opposite side of the rafter before entering the ridge.
    3) The original poster subsequently stated he "did not see any rafter cracking/splitting."
    4) It's not visible to us, but depending on how the roof is braced, there could actually be relatively light loading at the ridge end. The original post did not mention a lack of bracing.
    5) The home is 40 years old.
    6) The home is in the Oklahoma City area. While it snows there some, it averages only 9.5" for an entire winter, and only 3.2 " in the worst month. This includes freezing rain and sleet. (Yes, I Googled it. It's a slow day.) The load on the roof remains fairly static throughout the years compared to northern or mountainous climates.

    So in response to the original question, I still would write it up the same way. State that the shorter ridge board doesn't meet current framing requirements, but that 40 years have passed and no ill effects due to it was noted. I don't recommend that they hire an engineer in a case like this, but I give the usual disclaimer along the lines that I'm not an engineer but a home inspector. If you aren't comfortable with this assessment blah, blah, blah... hire a structural engineer.

    I have no beef with those who would state that it's wrong. That's your call. In a newer home I absolutely would write it up since I don't think enough time has passed to accurately assess it's performance in real-world conditions. Not to mention the code issue. But in older homes, with regard to framing issues especially, I like to put emphasis on things that are more conclusively wrong, not potentially a problem. And before anyone jumps me on that, it does not apply to most items in the report, especially those such as electrical or other safety items.

    I would love to see it in another 20 years. Absent any other intervening causes such as further settling of the home or a tornado, my money says it still looks the same.


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