Rafter span ?

Started by schiada, September 10, 2018, 07:41:29 PM

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schiada

Working on a 1 1/2 story plan.12' walls. 20' x 40' home. With a 12-12 roof.

Is the Rafter span 1/2 the width of the home ? Saying the width is 20' ?

Thanks, Randy

MountainDon

Yes, basically for a pitched and peaked rafter built roof the rafter span is one half the building width.

More technically the rafter span is the horizontal unsupported span as measured from the inside face of one side supporting wall to the face of the ridge board. So in effect, measure the distance from one interior wall plate face to the other, then subtract the thickness of the ridge material and divide by two.
Just because something has been done and has not failed, doesn't mean it is good design.


schiada

Thanks,trying to read the IRC. This is not my thing. d*

schiada

This is a 2 x 12 ridge board with 2 x 8 Rafters. What do you think ?



Rafter tie is 3' below the bottom of the ridge board. 7'6" above the floor.

MountainDon

Quote from: schiada on September 10, 2018, 08:11:51 PM
Thanks,trying to read the IRC. This is not my thing. d*

The IRC is a rule book, not a how to do it course. ;)
Just because something has been done and has not failed, doesn't mean it is good design.


schiada

But am I close ?  ???

With no AZ. Engineering stamp the "Truss Detail" will not pass .

And something like this was suggested ?

MountainDon

Sorry, I missed the post with the drawing when using my phone. 

Pencil in or imagine a triangle formed with the base stretching across from one side wall to the other and the rafters being the other two equal length sides of the triangle.  Divide the height of that triangle into thirds.

According to code, the rafter tie cannot be any higher than the lower 1/3 mark. If it is higher it doesn't count as a rafter tie.  Note that the rafter dimensions are calculated with the rafter tie being positioned at the lowest point. Across the wall top plates in this case. The rafter tables in the IRC have footnotes that indicate how much the rafters must be increased in size as the rafter ties move up to that limit of the lower 1/3.

FYI what are called collar ties in the code tie the upper ends of the rafters together. Collar ties must be placed no lower than the upper 1/3 mark. Usually, they are installed as high as possible. They are to prevent the peaks from opening up with high winds. Collar ties can be eliminated by using metal straps over the peak on the exterior. I did that over the sheathing.

This is one of the problems encountered when trying to make an older design meet present codes. The short walls the rafters sit on are another code problem.  A truss designed by an engineer can be designed for use in place of a rafter built roof assembly. 

Another way around those problems is to change the wall plan. Platform build the main floor with an 8-foot high wall; higher if desired. Build a regular upper floor on top of those walls; ceiling/floor joists sized to the tables. Then you need to do some drawing and calculating. The idea is to make side walls tall enough to enable building a rafter roof on them, with the rafter ties at the highest level that will keep the rafter ties no higher than the lower 1/3 point.  You would need that distance to be sufficient to give the 7'6" headroom between the finished floor and the finished ceiling fastened to those rafter ties.    There are some other limits regarding floor area that is further out towards the side walls. I have trouble finding that when working on my tablet. Maybe someone else can help or I'll find it when back home.
Just because something has been done and has not failed, doesn't mean it is good design.

schiada

Thanks,Don.

I was playing around with the rafters and was pretty sure it would not go with the wall plan.But needed to ask.
Funny how its ok in some places.

Looks like the way to go is a Engineered Truss. May add some cost but will save the 12' wall. And I don't haft to build them.

schiada

Don, in your experience does this look like it could be built ?



And thanks for the help.


Don_P

No, the tie is above the lower third of roof height. This is not ok prescriptively anywhere, but there is a lack of understanding sometimes which is why it shows up from time to time.  You can also use a ridge beam to hang the rafters from but it needs to be sized for the load and must be supported at its ends and possibly by posts in the middle of the building.  The stairs at the wall, left of your drawing, that needs to be at least 6'8" headroom to the ceiling, at the wall.

schiada

Thought the 6'8" was from center of travel. Will rework that.

The shaded area was the area that could be used for a truss.So moving it up would be fine.That is if it is ok with the engineer ?

Something more like this.


MountainDon

Attic trusses I have seen look sort of like this...

link to some trusses on Medeek's website

That page also has a link to his truss designer. Have not tried it myself.

This creates a problem in that stairs cannot be run parallel to the supporting lower sidewalls.

I just had a thought out of the blue... I have no idea if a truss such as these can be placed on top of knee walls.  An ideal truss has no horizontal outward forces on the support walls. However, I seem to recall seeing some truss design specs (cathedral ceiling designs) that did have some horizontal outward vectors.  That could be a problem, maybe???   Time to ask an engineer as that is out of my arena.
Just because something has been done and has not failed, doesn't mean it is good design.

schiada

Just so I have this right.

The 1 1/2 story plans have no chance of being approved under the 2015 IRC ?

Don_P

As drawn, no.

Some options though. If you bump the kneewall height up to around 5', you'll be able to drop the ties to the lower third and have the necessary headroom. Or a structural ridge, or scissor trusses, or attic trusses. MD was thinking about the horizontal deflection in a scissor. This can be problematic however if you sit down with the truss designer and have him stiffen it by upsizing some of the members he can dramatically reduce that deflection, that will not be the first, cheapest, truss he shows you.