Roof Slope Question

[tallyho8]

Curly
If the reason you are going from 2x4s to 2x10s is just for the insulation and not for added strength, then I see no reason not to replace the 2x4s with 2x10s as I have done in the illustration.

This does not change the roof height. It does not change the ceiling height. It does not change the roof pitch. It does not change the wall height and it does not change how the sheetrock will fit.

In the illustration I use 2x8s but the same technique is used for 2x10s.

 

[CurlyDave]

tallyho8:

Back at post 16 I was trying to convey that we were using vaulted ceilings.

The height limit we have means that the stud wall in the upstairs can only be 7'6" and one of the only ways to minimize the "squashed room" visual effect of low ceilings is to vault them.

When someone asked if I was intending on cathedral ceilings, I said "maybe chapel" in an attempt at humor, since the house will only be on the order of 2050 square feet. Not quite large enough for a cathedral.

 

[yooperdave]

Curly
If the reason you are going from 2x4s to 2x10s is just for the insulation and not for added strength, then I see no reason not to replace the 2x4s with 2x10s as I have done in the illustration.

This does not change the roof height. It does not change the ceiling height. It does not change the roof pitch. It does not change the wall height and it does not change how the sheetrock will fit.

In the illustration I use 2x8s but the same technique is used for 2x10s.

The sketch of the deeper rafter is not structurally acceptable. As drawn, the rafter may split, and exponentially reduce the strength of the rafter.

The notch of the birds mouth needs to terminate at the face of wall, which will increase the height of the roof.

Yooper Dave

 

[tallyho8]

The sketch of the deeper rafter is not structurally acceptable. As drawn, the rafter may split, and exponentially reduce the strength of the rafter.

The notch of the birds mouth needs to terminate at the face of wall, which will increase the height of the roof.

Yooper Dave

Well, the point is moot since he is building a cathedral ceiling, but as I said, if the purpose of using 2x10s instead of 2x4s is not for added strength, that setup should work.

Since the rafter has such a large area to connect to the joist, I do not agree with you that the 2x8 rafters and joists as shown in the diagram would be weaker than 2x4s.

 

[Pops15]

Well, the point is moot since he is building a cathedral ceiling, but as I said, if the purpose of using 2x10s instead of 2x4s is not for added strength, that setup should work.

Since the rafter has such a large area to connect to the joist, I do not agree with you that the 2x8 rafters and joists as shown in the diagram would be weaker than 2x4s.

The problem is the likelihood of the rafter splitting, thus weakening it.
The rafters will flex under load if a roof was built with 2X4 rafters as in the first diagram.
The rafters in the second diagram won't flex as much (if any) under load. Since they won't flex the stress has to go somewhere. They'll tend to split at the inside edge of the plate at which point part of the rafter is carrying the load and the other part is the load (due to the oversize bird mouth). Using joists would help reduce the likelihood of the bigger rafters splitting, but I doubt they'd eliminate it.
The OP is going to use 2X10 rafters with the proper bird mouth so he shouldn't have any problems with the splitting issue. Since he's not using joists I am kind of curious what his plans are for keeping the walls from kicking out.
Pops

 

[CurlyDave]

I think Pops is right on this--if I wanted to cut a rafter as shown in post 51, I would nail a metal strap across its full width very close to the birdmouth to prevent splitting.


...I am kind of curious what his plans are for keeping the walls from kicking out.

That is an excellent question. I have wondered about it myself.

About the only answer I can give is that I have a licensed architect and a licensed professional engineer drawing the plans. I have asked them directly about this issue and they assure me that it is going to be OK.

When You get right down to it, I suppose I am relying on their Errors and Omissions insurance to keep the walls from kicking out.

In a less facetious vein, I may have the terminology wrong here, but the roof itself is just like an inverted V, and the interior sheet rock follows it up to the peak, which is what I have always considered a cathedral ceiling. But, this is not one large room, there are interior walls and partitions in many places which continue all the way to the ceiling, and which can carry the downforce from the peak. This means that there is no spreading force, just like when there is a ridge beam.

 

[mjncad]

I think the 4/12 pitch would be just fine; but unfortunately CurlyDave has to deal with the PRC (People's Republic of California) and all their additional rules and regulations and fees that add additional headaches and $$$$ to his project.

Good luck Dave, you have my sympathy dealing with the meddling bureaucrats.

 

[yooperdave]

To prevent thrust (kick out) at the top of wall, I hope you have a load bearing ridge beam - this is not the same as a ridge board. The ridge beam will support the top of the rafters, so they will not try to deflect downwards.

Usually, the ceiling joist will tie in the bottom of rafters as located at the top of the wall - thus preventing the kick out. When the rafters can not kick out, the rafters are also prevented from moving downwards, so the roof stays in-place.

Some people like to move the ceiling joist upward to create a tray ceiling. This is ok only if the rafters are significantly upsized to resist the (significant) bending induced from the raised tie.

Yooper Dave

 

[EddieWalker]

What Yooper Dave said.

You either have to have a ridge beam across the span of the room, supported by continuos, lumber from the beam to the floor, or a load bearing wall to the height of the peak. Either way, the footings in the slab should have been poured to support the load.

Eddie