Insulating a Cathedral Ceiling.

[rwalter]

My current design and lumber order has 2x10 rafters for the roof which is a 16/12 pitch. My question is should I used blown in foam insulation which should allow me to reach R38 or should I scab 2x4 materials with gussets to the underside of the rafters or just change my rafters to 2x12's. I could also stick build scissor trusses under the rafterboards but that might lower the ceiling to much for the second floor overlook at the stair landing heading up to the second floor of the 1 1/2 story design. For scissor trusses to be effective at what point is the maximum point or the closest poing to the ridge board that they can be attached. I know typically they are attached at about 1/2 the pitch of the existing roof line but that would drop the ceiling to low in my applicatoin. If I go with a straight cathederal ceiling I will use collar beams to stablize the outward forces on the walls. I gues the main thing I am looking for is the most cost effective way of doing this. Thanks for any input.

[Amanda_931]

Our friendly local auto mechanic's new shop has a cathedral ceiling.  (OK, he wanted the room in the middle for a lift--raising cars to heaven?)  Rafters are ugly but warrenteed strong truss-joist types.

The video Building With Awareness showed some all-2x4 trusses--already shaped, just had to put them up!) on the center parts of that house.  They would get heavy fast, the wider you got.  Worse than your standard trusses, I'd think.

Your design (maybe even location) may make it reasonable to price either of those types of deep--room for lots of insulation--prefabricated rafters or trusses.

[Shelley]

I would think that in most places the most cost effective way is to use a mfg truss.  I know that here we can buy them cheaper than we can build them.

If you have local truss companies go and talk to a few.  Prices are generally the same.  Pick one that treats you right.  They can glance at the plans and pretty much tell you what you need.  Offer options if there are any.  And, wag you a price/truss.  That way you can determine which method is cheaper.
Amount of help you have comes into play too.  Mfg truss can be easily handled by two people if necessary.

You'll need either batts or foam with that pitch.  If you can afford foam it'll be better.

[rwalter]

I not sure if I made my self clear on the scissor truss concept. I would still be using the 2x10's for the rafters. My question is what is the closest or highest point  to the ridge board that the actual "scissor portion"  of the truss can be attached. I know typically the are installed at 1/2 pitch of the main rafter and in this case that would be 8/12. However in this case that would not even meet the 7' 6" ceiling in the seond floor. I would perfer not to have to have a crane in to install pre-manufactured trusses. You can very easily install the lower scissor once the main rafters are in place. This is what I did for an addition on my house and it worked well and I had no problem with the building inspectors.

[Bart_Cubbins]

It sounds like the lower cords of the scissors "truss" aren't going to be structural, so you can install them at any angle that makes sense.

If you decide to build down with gussets, then you might be able to downsize your rafters to 2x8 if that is all you need for structural strength.

[John Raabe]

If I understand your concept correctly, you are doing a standard raftered ceiling with a non-structural ridge board and then proposing on adding, struts, let's call them, to form a site built scissors truss. These struts configure the truss so that it resists outward forces and does not therefore require collar ties. Correct?

Very clever! It is however a fairly sophisticated bit of engineering and is well outside any "rules of thumb" I know about.

What I show in the diagram are 2x4s on either side of the rafter forming the bottom of the truss. These should be nailed off well as they provide the shear force to counteract outward forces. The steeper the angle of the struts, the more of a lever arm on the nails and the greater the shear force.

Of course you could do this as a flat truss with glued up plywood gussets but I like the framed in place version better as a site-built flat truss would be even heavier than a manufactured truss with metal plates.

[JRR]

Kind'a looks like a regular scissors truss with vertical elements missing.   There are some span tables published by the Wood Truss Council of America that may make good reading ... they include scissors trusses.   Also some interactive website links:
.
http://www.woodtruss.com/projects/woodtruss/technical.php?PHPSESSID=7da003407e77474f4c4ce5977f472306

[rwalter]

John,

You hit the nail right on the head! That is exactly right. I designed a two story addition to my house a couple of years ago and used a local contractor to do the framing for the addition. We used this exact same set up for the cathederal ceiling in the second floor master bedroom. The idea actually came from the building inspector when I met him to show him my plans. I then ran the idea buy my contractor and he liked it also and ran with it. The problem is I don't know how they figured the insided pitch of the ceiling scissor. Unfortunately he doesn't do work in the area where my cabin is going up other wise I consider calling him, which I might just to do to pick his brain on this anyways. To help stabilize the truss they did run a verical strut from the ridge board down to the center of the scissor. The building inspector had no problem approving the framing.

As for how they framed my addition your idea is exacly how they did it. They framed it all right in place. I am having trouble remembering but they might of used 2x6 lumber on the struts to create the scissor.

"What I show in the diagram are 2x4s on either side of the rafter forming the bottom of the truss. These should be nailed off well as they provide the shear force to counteract outward forces. The steeper the angle of the struts, the more of a lever arm on the nails and the greater the shear force.
 
Of course you could do this as a flat truss with glued up plywood gussets but I like the framed in place version better "