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foz1234 06-22-2011 08:53 PM

Increasing roof rafter depth?
 
I would like to turn a unfinished section of attic into a heated/cooled storage area, with sheetrock, trim, carpet, etc.

My problem is the area that I want to convert has 2x6s for roof trusses (edit - they are rafters, not trusses) . So I can either use R19 insulation in the ceiling, which I rather not do. Or lay 2x4s to the side of the 2x6s and attach both with metal straps or 3/8" plywood on the sides. This method will give me 9" of insulation spacing, so I could use R30C insulation.

Do you see a problem with extending the depth of the 2x6 roof rafters this way? There will be sheetrock attach to the roof rafters. Also, this is in NC, we rarely get snow, but with global warming :whistling2: I could see a foot of snow accumulating on the roof.

Any thoughts?

Here is a rough side cutaway sketch of what I am thinking about .....

http://i1014.photobucket.com/albums/...idecutaway.jpg

project_guy 06-23-2011 12:13 AM

Foz,

First off, is the area designed to withstand the extra live and dead load for materials/storage? Are they storage trusses (the kind with a clear area in the center)? If they are just standard Fink truss or the like (triangle in the center), they likely aren't meant for this application. 5/8" sheetrock alone is 2.2 lb/ft^2, and 3/4" ply is about 2.3 lb/ft^2. Depending on how much you finish, this could add up to thousands of pounds...

Assuming you are OK here, I think I'd be inclined to run the 2x4 perpendicular to the 2x6 and attach using twisted strap ties. This allows you to run batts perpendicular to each other, and would likely be easier to get the 2x4s to match the plane of the truss top chords. They would look like purlins on the underside of the roof. Be careful of converting this space though if it wasn't intended as such...

foz1234 06-23-2011 07:01 AM

Quote:

Originally Posted by project_guy (Post 672424)
Foz,

First off, is the area designed to withstand the extra live and dead load for materials/storage? Are they storage trusses (the kind with a clear area in the center)? If they are just standard Fink truss or the like (triangle in the center), they likely aren't meant for this application. 5/8" sheetrock alone is 2.2 lb/ft^2, and 3/4" ply is about 2.3 lb/ft^2. Depending on how much you finish, this could add up to thousands of pounds...

Assuming you are OK here, I think I'd be inclined to run the 2x4 perpendicular to the 2x6 and attach using twisted strap ties. This allows you to run batts perpendicular to each other, and would likely be easier to get the 2x4s to match the plane of the truss top chords. They would look like purlins on the underside of the roof. Be careful of converting this space though if it wasn't intended as such...

First thank you for your reply.

The attic area is stick built. The roof trusses in this area happen to be 2x6s (to save cost?) supported by a 4x16 (nominal size) beam. The attic floor has 2x10 floor joists with a 3/4 subflooring. I think this area was designed as a unheated area for heavy live loads ..... or originally planned as heated area and changed mid-construction.

I like the idea of running the 2x4s perpendicular. I would fill the 2x6 space with R-19 unfaced and the 2x4 space with R-13 faced. I could forget about using 1x3 purlins, using the 2x4s instead.

I forgot to mention in my previous post the roof in on a 8/12 pitch. Would this have any affect running the 2x4s perpendicular? Would there be added stress on the twisted scrap ties or 2x4s themselves?

Daniel Holzman 06-23-2011 07:22 AM

Are you sure you have roof trusses and not simply roof rafters? From your description it sounds like standard 2x6 rafters.

foz1234 06-23-2011 07:31 AM

Quote:

Originally Posted by Daniel Holzman (Post 672491)
Are you sure you have roof trusses and not simply roof rafters? From your description it sounds like standard 2x6 rafters.

I am sorry, you are correct, I do have 2x6 rafters and not trusses. :wallbash:

joeyr 06-24-2011 10:21 PM

Just to clarify, you have a hand cut roof. Standard engineered trusses so not use a ridge beam to hold up rafters.

What is the thickness of your collar ties? Instead of increasing the depth of your original 2x6 by adding a 2x4 to the edge of it, i'd recommend sistering the rafters with either a 2x8 (7 1'4 inches) or a 2x10 (9 1'4 inches) By sistering the rafter from ridge beam to knee wall you will increase the strength of your roof dramatically and have the proper depth for you insulation. Just NEVER remove a collar tie.. Always sister on the other side of the rafter..

Good luck

Gary in WA 06-24-2011 11:31 PM

Under the new Code you may need a building permit even for "limited storage": http://docs.google.com/viewer?a=v&q=...1txlzU01V4JQIg

Sounds as if the floor/joists are up to code: http://publicecodes.citation.com/ico...002_par017.htm

Gary

Ron6519 07-04-2011 09:05 AM

Instead of going to the trouble and expense of adding structure, why don't you use a better grade of insulation, like closed cell foam?

shazapple 07-04-2011 09:35 AM

To keep your roof from losing life span by overheating the shingles (and warranty), you should install baffles between the rafters, in order to allow for ventilation between the roof deck and insulation (this is assuming you have soffit and ridge/gable vents. If not, you should add these!).

Of course, this decreases your space even more, so I second the recommendation for a higher R value insulation.


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