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Jack
Any time you have a hip or valley where one side is laid out from the opposite end of the wall,and the other starting at the corner where the hip/valley begins you have the potential/likelihood for two sets of jack rafter layout lines. Structurally it doesn’t matter if the opposing jacks line up perfectly, but if your concerned, you could fudge the layout on one to correspond to the other keeping in mind the potential waste and hastle with the 4’&8′ centers of your sheathing, and if your concern is how to deal with an odd sized rafter cavity at the end of the layout and the last one or two aren’t critical centres, they also could be fudged (split the difference on the spacing( in order not to waste material & time.
Architectural drawings make everything look so nice . . . on paper.
Patrick
Replies
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Jack
Any time you have a hip or valley where one side is laid out from the opposite end of the wall,and the other starting at the corner where the hip/valley begins you have the potential/likelihood for two sets of jack rafter layout lines. Structurally it doesn't matter if the opposing jacks line up perfectly, but if your concerned, you could fudge the layout on one to correspond to the other keeping in mind the potential waste and hastle with the 4'&8' centers of your sheathing, and if your concern is how to deal with an odd sized rafter cavity at the end of the layout and the last one or two aren't critical centres, they also could be fudged (split the difference on the spacing( in order not to waste material & time.
Architectural drawings make everything look so nice . . . on paper.
Patrick
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Adirondackjack,
If you subtract the width of the building from its length, you have the "line length" of the ridge. Divide this "line length" by the O.C. measurement. If there is no remainder, every rafter in the roof can be put exactly on the O.C. measurement.
If there is a remainder, and at the same time, you want the jack rafters on opposite sides of the hips to match up, then somewhere in the roof you are going to have at least 2 rafters that are not on the O.C. measurement with each other.
The best way to handle this in my opinion is to put a pair of common rafters at each end of the ridge board face, and a "king rafter" at each end, then layout the ridge board from one end. The last two rafters at the end you finish the layout at will not be O.C. with each other, but now you can cut the 4 sets of "lefthand long" jacks and the 4 sets of "righthand long" jacks, using the same rafter lengths for both sides.
If you are sheathing the roof with some type of 4x8 sheets of plyboard, you will have to make some additional cuts in the areas of the roof where there two rafters that are not O.C. occur.
If you use single cheek cuts at the tops of the hip rafters on one end of the ridge board, (eliminating a set of common rafters at one end of the ridge board) it would be possible to keep all of the rafters on the two long sides of the house O.C., but now you will find that if once again you wish for the jack rafters to match up on the hips, then at least 2 of the rafters along the width of the house will not be O.C. with each other, which will result in additional cuts in the sheathing in these areas.
Only if you drop the requirement that the jack rafters match up on the hips would it be possible to put all of the rafters exactly on O.C. when there is a remainder.
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Dear Adirondack.....
Here goes..some theory and history regarding hip / jack layouts.....
The way I was taught..in old school...was the following...
For the layout of the hip and common rafters at the end of the building...do the following....
The Hip Rafter...splits either corner of the building.
One common rafter...Splits the span and is centered on the span..at the end of the building.
On either wall perpindicular to the end of the building...at a distance of 1/2 the span of the building..is another common rafter set to split on half the span...so that all three commons are the same dimension off the corner. [IMPORTANT]
The ridge is set back from the end wall one half the span of the commons + 3/4 "towards the end wall. The 3/4 inch is for 1/2 of a 2x ridge board...So...theoretically...all three common rafters are set exactly the same distance from the corner of the building...and they all hit and assemble the ridge at a point 3/4 away from the center point of the rafter intersection. Whew!!!
Now...comes the jack layout. You should be able to lay jacks out on center from any common in any direction. Because all commons are the same distance from the corner...you may lay out from that set common, and the centers and matching jacks will be correct.
Then you pull out your good old framing square...It will give you the difference in length between the jacks on the pitch desired and either 16" layout or 24" layout. You then...take a rafter...and step off that length along the hypotenuse in the increment given by the framing square. [If the length difference is 24 5/8 then you mark from the head of a common rafter on 24 5/8 increments until you run to the birds mouth. The number of marks you have on the common will correspond to the number of jacks necessary. You then have the jack dimensions [from the layouts on the common] and you will cut one right handed and one left handed jack for each measurement.
Note: All rafter layouts of a hip roof are laid out from the three end common rafters...Jacks and commons both...so the center of the span of the roof sets the layout of the rafters...
If you do all of this correctly...your jack rafters will fit like a glove...Pythagoras will be congratulated by all comers...and life....will be again beautiful....
It really does work.
David Nickelson
*DaveBut how often does this type of layout result in waste of material and time when sheathing with 4'x8' material. All the time, or just almost all the time? Methinks that that layout style dates from the days when roofs were sheathed with 1x boards and continuous runs of centres was irrelevant, and carps. got paid in sawdust. The only thing I would change would be the centres for the jacks to jive with the run of centres down the length of the roof. Your way does look really slick though!patrick
*PatrickThere is very little waste on either the rafter side or the sheathing side. You are laying the jacks / rafters out on either 2' centers or 16 " centers. If your layout is correct, there is no waste on the plywood side. The jack rafters...are also cut from the same length material as the common...the shortest and longest jack can be pulled from one common length...and on down the line. The centers of the jacks....are the same centers as the commons...no difference...If you look in your Full Length Roof Framer on pages 9 and 10...you see the plan view of a hip roof..and...if you go to page 13...Figure 15...you will see a drawing of the ridge / common / hip detail...and the geometry of the intersections...This method...works extremely well..I have cut and stacked more roofs than I care to number....David Nickelson
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Thanks all for your input. This forum is the best tool I have added to my toolbox in a long time. The written answers seem great however if anyone has drawings of what is being discussed here or can refer me to articles of the "readers-dijest-pictorial-style" I would apreciate it. As for my two cents worth, I would think that there are a couple of best production methods(California boom times, tract, new school, time is money) and maybe most pleasing to look at (exposed timber) methods(older school.) Any one have any thoughts?
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Adirondack
Try....
A.J. Reichers Full Length Roof Framer...it is the bible...
Also...I am sure Taunton must have some L. Haun Roof framing techniques.....
DN
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In all the articles I have seen on roof hip and valley construction the drawings that accompany the article show all lining up perfectly. I would like to see what's the best ways and reasons for dealing with the on center remainder. Most of what I have built has been from plans that I provided so in the past I have just changed dimensions to eliminate remainders. If anyone knows what I am talking about, let me know.