Ridge beam hight

[rjwoelk]

Question 1
Where do you take measurements?
I have a 17 ft wide building. 
So  6/12 pitch roof. So 6x 8.5 is a 51 inch rise.were is this in relation to the top of the ridge beam?
If i have a ridge beam and the rafters sit on top of the ridge beam. Now you have a birds mouth top and bottom. 
How does one calculate this out.
Question 2
If you have 5 tiebeams. Say one or 2 have a bow in them. The bow is to the top.
 Do you adjust the length of the king post so the ridge beam stays level ?

[Don P]

It "would" be 51" above the point where it crosses the 17' width. But the ridge has some unknown width that needs to be accounted for so the 8.5' of width is probably incorrect.

You are using the terms tie beam and kingpost so I assume we aren't talking a common stick frame rafter but rather the top chord of a truss? Are we talking about 5 sets of trusses with no common rafters? Sorry kinda lost here, a sketch would help... drawing it in sketchup would also answer all those questions. As far as the bowed tie, you can snap a line and either remove material from the tie down to the line which would make all your kingposts identical, or you could use the difference between line and edge of timber as a distance to shorten the king, square rule or scribe.

I would rip the top of the ridgebeam to a 6/12 bevel and rest the rafters on it, hmm we're now talking about a crownposted roof rather than a kingpost the ties there would be in bending and sized to carry load not just tension ???... need a pic.

This is a ridge height calc I made for stick framing, not sure if it'll help with thinking through it;
http://forestryforum.com/members/donp/ridgehgtclc.htm

[Roger Nair]

RJW, I don't know where to begin because there can be any number of solutions depending on your particular details.  However, the concept applied involves the specification of offset roof planes relative to the outside corners of the top wall plates because in many cases the roof surface doesn't intersect with the plates.  You can use the outside corner as the arris and use the arris as a springpoint for the regulating roof slope.  The finished rough top roof plane will be parallel to the regulating plane by a consistent value.  The size of material, joinery decisions and other factors determine the offset.  Careful drafting, striking full scale lines on a deck or calculation could be the manner of solution.

You will need to, as Don suggests, to supply drawings to clarify your particular problem.

But if I can make an assumption or two:  The ridge is centered, the ridge is flat top, the outside upper corners  of ridge are on the reference for roof slope.  Then the rafter's run is (.5 times building width) minus ( .5 times ridge width).  Ridge height is .5 times rafter's run above the wall plate.  Pythagoras will calculate rafter length from corner of plate to corner of ridge.

[rjwoelk]

Sorry Don p should have been more informitive.
Timberframed building. 5 bents 4 bays
The king posts are on the tie beams .51 inches plus another 8 inches because they are below the top plate by 8. Then another 10 for 2, 5 inch tenons. But does that put my ridge beam . Would this give me a 6/12 at the top of the ridge beam or bottom. I need the 6/12 pitch to be at the top. Then i set rafters on 24 inch spacing. 

[rjwoelk]

Don i was thinking of doing a 6/12 angle cut on the ridge beam or just birdmouth the rafters. 4x6 rafters.
Also was talking with Brad_B and we came to the same idea of chalk line and go from there. 
Some of the questions that pop into my head while driving long haul.  :D

[Don P]

Talking about thinking while the miles go by... so we're riding the percussion hammer trying to make it through the wall, the concrete is 10" thick poured in '48, perfectly on dimension even though rough board formed, vibrated, dense, no voids at all, fully cured but unweathered, not a crack in the foundation that I've seen, flat out laughed at my Milwaukee hammer drill. In the back of beyond in the Blue Ridge. Beautiful stuff in a cussed way. I'm thinking gotta be the Seabees come home. Anyway, with concrete slowly eating steel away the mind has time to wander.

This would act as a truss supporting the ridge and then the rafters, thoughts?




Doh, Michelle looked at it and said "that's kind of like Beyer's roof"

[rjwoelk]

  Don here is a skitch of how the roof looks.

[rjwoelk]

I am coming up with 57 inches.
51 for the rise . Minus the 8 for the ridge beam. Minus the 2 inches from top of ridge to bottom of rafter. Then add the 16 for sill plus 8 inch drop to the top of the tie beam.
So 57 plus 5 inch tenon top and bottom so 67 in total length. Sound right?  What do you get?

[Don P]

 


The carpenter part;
I did it the lazy way with sketchup rather than using trig, there are several ways to climb the mountain here.
I started with the 17' building width, the dropped tie and plates. Then the birdsmouthed 6" rafters. I notched the birdsmouth the max allowed by code, 1/4 depth or 1-1/2" this left a vertical height above plate (HAP in building lingo) of 5" above the outer edge of the plate at the building line. Understand what I just did there or keep asking, that is a critical reference point. Then I drew a 6/12 line up to the centerline of the building and another parallel line 6" below it establishing the rafter edges. At the peak I went out 4" each way, hit the bottom edge of the rafters and dropped 8" to box out the ridgebeam. Then I dropped down to the tie 55 3/8", our difference is the birdsmouth.


The engineer part;
This is not a kingpost truss, I would call this a crownposted roof. The "king" is a post bearing on the tie beam and supporting the ridgebeam. I wouldn't hurt it with much of a mortise at midspan if at all. It needs to be sized to support half of the roof load, 4-1/4' of rafter to the left and 4-1/4' of rafter to the right so 8.5' wide and halfway to the next kingpost fore and aft. Just guessing the bays are 8'. If so the tributary area loading the post which is center point loading the tie is 8.5'x8'... 68sf. If the snow load is 50 pounds per square foot and the roof structure weighs 10 psf, 68 square feet times 60 pounds per square foot=4080 lbs landing in the middle of the tie. Fill in the appropriate variables and then hit the center point calc in the toolbox to check that 8x8 tie.

[rjwoelk]

Yes there is another 8x8 post under the tie beam at the midspan. Just did not think of adding it ad i was just trying to figure out the length of the king post.
Thanks for the info. Bob