Contractor Talk - Professional Construction and Remodeling Forum banner
1 - 18 of 18 Posts

·
Registered
Joined
·
30 Posts
Discussion Starter · #1 ·
This is my ceiling and I'm trying to figure out the BEST way to furr out the 2x4's to prepare for spray foaming the entire envelope in 7" of closed cell spray foam. I want to leave the bottom of the trusses/ceiling open so I'm not looking to drywall the bottom of the trusses. Therefore furring out the joists is not something I intend to do. I want to build up the top chord of the rafters.

My first idea was to cut 2x4's and place them inside the webbing of the trusses. That turned out to be nearly impossible to screw them because you have space issues and have to use really long screws to go through the 2x4's. Also hugely time consuming so I abandon that idea.

Hopefully you all can give me some input on my new idea. What if I take 3/8 osb and rip it lengthwise into 8" inch wide by 8' foot long. Then take those pieces and glue (PL premium) and screw them to the face of each rafter. Since it's not fiberglass batts, and it's spray foam it seems like it will be fine if there is a truss in between both of these furred out osb strips. Then after I've got all those strips out there and the rafters are now ~8" deep/tall I could go and add scabs/gusets over the places where the osb meet. I know I'd be adding some weight but since it's in the top chord wouldn't that be acceptable?
 

·
Registered
Joined
·
3,588 Posts
Here are the pictures of the trusses
7" plywood strips 3" wide every 2' to scab the top chord and the furred out part together

or 6" screws
 

·
Registered
Joined
·
30 Posts
Discussion Starter · #7 ·
It would have to be 8" as I don't want them to spray right to the edge. I may want a little extra room there for many reasons. I just don't know if osb or ply is the thing to use. I thought about using pine to keep it very light but it's atrociously expensive. None of this is structural. It's just to create a cavity to fill.

so the red area is 8" wide x 8' foot long. Where the two pieces butt i'd be using osb scabs (the blue areas). I'd be putting these over the faces on each rafter since I don't believe it matters if there is a truss in between the two furred out sections. The truss will just be burried in foam up in there.
 

Attachments

·
Registered
Joined
·
30 Posts
Discussion Starter · #8 ·
my first question is when adding to rafters is osb better than plywood? and also am I in danger of overloading the truss in any way? I know the weight would want to make the trusses bow out, pulling the bottom chord lengthwise or open. Most of the added weight would bare down on the walls which are 2x4 staggered stud (a stud every 8") with 2x6 bottom and top plate. So I know the walls can take it. All the 2x4's have hurricane clips too. It's built like a safe.

I know there is a lot of do's and don't when adding anything to the bottom chord.. like a lot of guys want to lift an engine out buy throwing a hoist on the bottom chord.. and I know that's a bad idea. I just can't think of any other way than using OSB and screws to fur out 8" of height and lamenating these to the face of each top chord.
 

·
Registered
Joined
·
30 Posts
Discussion Starter · #9 ·
Also what thickness of OSB.. 3/8's? 1/2"? and are scabs normally 3/8 or 1/2? I already have the metal plates holding the trusses together so this is just added protection ontop of everything.
 

·
Been around
Joined
·
1,165 Posts
Why do you need furring strips at all?

If you are spraying the underside of the roof deck why wouldn't you just enclose the chord?

But I guess you are trying to leave the bottom of the top chord exposed?

The SPF will do a pretty good job of bonding all your parts together. Glue and screw is overkill, you could pop each strip on with a few pneumatic nails or staples.
 

·
Registered
Remodel
Joined
·
31,138 Posts
The only way to tell if you're overloading the truss or not is to know what it was designed to, then add up what you have. There's no way for anyone here to answer that.
 

·
Registered
Joined
·
30 Posts
Discussion Starter · #15 ·
Why do you need furring strips at all?

If you are spraying the underside of the roof deck why wouldn't you just enclose the chord?

But I guess you are trying to leave the bottom of the top chord exposed?

The SPF will do a pretty good job of bonding all your parts together. Glue and screw is overkill, you could pop each strip on with a few pneumatic nails or staples.
Making the 2x4's of the chord taller was my first idea, but that was so difficult. Now I'm thinking of just using the osb against the face of the trusses rather than try to modify the 2x4's.
 

·
Registered
Joined
·
31 Posts
Is there a reason you need the bottoms of the top chord exposed? Your roof is going to frost on every truss if you don't just spray the whole underside of the roof to 7 in.
 

·
Box Builder
Joined
·
6,320 Posts
So, you are doing a hot roof? Just curious, why 7" of closed cell? Just did something similar in my own house. Still framing stage. Nailed strips of 3/4" ply to side of rafters then added ripped 2x's parallel to rafters fastened into the 3/4" ply. Then we strapped that for blueboard. I'm using open cell though. My bays are vented.
 

·
Registered
Joined
·
15 Posts
7 inches of closed cell exceeds what would most folks would use in a walk inn cooler, I assume you are in south La from. why so much foam?
 
1 - 18 of 18 Posts
Top