This is not a how do you do it type of question, I have over 25 years of residential construction experience and I am sometimes confident and proficient at most aspects of residential framing including laying out and cutting rafters.
I am framing a 3 car garage 24′ x 36′. 2×12 rafters, (two piece) 4/12 pitch, spaning the long distance, supported by triple 18″ LVL at 13′ (measured horizontally). This leaves me with a 6′ piece of 2×12 rafter to the double 2×12 ridge. Whew! it’s for an architect!
My question is this; the plumb cut on the top end of the rafter is open ever so slightly at the top of the cut. This is a condition I have noticed many times before especially with short spans, big rafters and doubling the ridge seems to make the problem worse.
What I have here is basically 2×12 rafters cut for a total span of 10-4; the outside to outside dimension of the LVL girders meeting a double 2×12 ridge.
I hope that this is comprehensible. What I want to know is why do I often come up with plumb cuts that are open at the top. As I stated, it seems to be only when I have the combination of short spans and large rafters and more so if the ridge is doubled or wide.
I checked the layout many times and I am capable of cutting well fit rafters in a more conventional method of construction. Am I missing something?
Thanks,
Eric
Replies
Eric, it seems like the ridge is too high. The thicker the ridge is the worse the cut would be.You are effectively pushing the plum cut up when the ridge gets thicker.
I am assuming the rafter is cut right on the money.Cut two rafters and place them on the floor with a small piece of the ridge between them. Snap a line that represents the top of the wall plates, line birdsmouth on line with the correct span between them.Measure the ridge height, you may find that you are off with original height.
Usually the problem is the rafter is open at the bottom. I've done this myself when I forgot to include the thickness of the sheathing in the birdsmouth horizontal cut.
mike
You are effectively pushing the plum cut up when the ridge gets thicker.
I am reading you in a general sense and I have a notion that the thickness of the ridge is where the problem lies.
What do you mean by pushing the plumb cut up?
I did not take into consideration checking the height of the ridge after all it falls at the tops of the rafters; right?
Doing some drawings this morning on scrap plywood, I was trying to determine what effect doubling the ridge would have on the plumb cut and I couldn't gather any new intelligence. As long as I have deducted the proper thickness of the ridge I can put it at any hieght I wish it being right or wrong being an aside for now.
I'm pretty sure I am making accurate cuts. I done many single ridges on simple up and overs a with great success and clean well fitting cuts.
Thanks
Did you subtract double the thickness (for a double ridge) plus maybe an 1/8th for fat lumber, from your span before you calcitated your rafter length?(3-1/8" for 2x, 3-5/8" for LVL)Mr T
Do not try this at home!
I am an Experienced Professional!
Of course I did or they wouldn't come close to fitting! That was most definitely included in my 'calcitations'.
In my original post perhaps I should have stated that I was talking about an 1/8" or so.
I suppose I am searching for some anomaly or something that I am missing.
Eric
You might try checking your square for trueness.
If you see a little tiny hairline crack right at the crotch, your square might be off.
When you lay out a rafter with a square that is a 1/16" or even less out of true, and then use that first rafter as a pattern to lay out the rest .
Then any error in the first rafter is multiplied by 2 , a 1/32 error becomes 1/16" and a 1/16" error becomes 1/8".
Try double checking your rafter with a "swanson 12" you might notice a tiny little bit of a difference.
Eric, if the ridge is 1" too high for instance, the bottom portion of the top rafter cut will hit first, this will cause a gap at the top.Try the method I described about layout on floor. You probably will come up with a different ridge height.
The plumbcut on the 2x12 is about 12-1/8" with a 4/12 pitch if you measure it. That alone can be cut a little crooked with a circular saw. Now you have 2 - 2x12's for a ridge which is for arguement sake 11-1/2" plumb. If you nail the two together you'll probably have 3-1/16" - 3-1/8"and you might have a a slight cup in the center, these will never be that good so when your 12-1/8" plumbcut butts up against that your going to see some kind of space.
Your 3 micros should be 5-1/4" but when you nail them together they could be 5-3/8" or bigger, so that could effect the birdsmouths and the direction front to back of the micros if you have them set in place already.
I've framed plenty of homes and additions with microlams as a break as you have described but never with such a short span like yours have I split the two rafters like your saying. Couldn't you have used 20' - 22' rafters and notched out where the 3 microlams sit?
When using one rafter it's a better job and alot easier
That could be one of your problems since your splitting the rafters like that the height of your micros and ridge might not be correct. Have you put a line from the top of your plumbcut all the way down to the the plate to see if it's straight?
If the 3 micros are low then the top of your plumbcut will be open.
Is this drawing something like your doing?
Joe Carola
You hit it right on the money. You understood perfectly what I described.
I was a little suprised at the differences in the hieght of the individual LVL's. I am not sure they all came from the same fabricator. I have never worked with a piece of manufacture lumber.
I've framed plenty of homes and additions with microlams as a break as you have described but never with such a short span like yours have I split the two rafters like your saying. Couldn't you have used 20' - 22' rafters and notched out where the 3 microlams sit?
That is exactly what I did. The total span is 36 ft. The rafters overhang the outside wall 3ft. We used twenty's to the LVL and spliced them to '6' chunks' from there.
When using one rafter it's a better job and alot easier.
Until I studied the plans closer I thought this might be the case. They'd have to be alwful long, big$ and Very Heavy. I'm getting tired of lifting heavy things!
That could be one of your problems since your splitting the rafters like that the height of your micros and ridge might not be correct. Have you put a line from the top of your plumbcut all the way down to the the plate to see if it's straight?
The Blue line is straight. I know now one thing that I did wrong was to not copy the upper bidsmouth of the long rafter to the birdsmouth of the extensions. I laidout the extensions and cut them all based on an assumed dimension of the LVL'S while we were waiting for them to arrive. ( that's another story)The extensions were cut out of different stock than long rafters and there was an issue with the hieght. The long rafters fit fine for the most part considering we were dealing with two 600# girders.
One of the points I was trying to make was the short span to the outside of the LVL's and then using 2x12's to a double 2x Ridge.
I was looking for someone to tell me that the combination of short spans, large dimension rafters and multiple tickness ridges can cause problems with plumb cut.
This thread posted twice. I don't know how to post the url for the discussion.
Thanks big time,
Eric
Here's your original post......I think.
http://forums.taunton.com/tp-breaktime/messages?msg=36004.1
Knowledge is power, but only if applied in a timely fashion.
as well as allowing an eighth between your LVLs we may consider the crown of the lumber effecting the fit. I have seen your delema even with sigle ridged roofs with a one piece rafter at that gone down done all the math over and then realized the rafters not fitting aswell as some had a higher crown.