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-   -   Number of rafter ties in an 18' x 18' detached garage (http://www.diychatroom.com/f19/number-rafter-ties-18-x-18-detached-garage-152348/)

cjkb 08-02-2012 01:03 PM

Number of rafter ties in an 18' x 18' detached garage
 
Was considering sprucing up the garage in my starter home by using 1/2" drywall on the ceilings attached right to the rafters. However I am concerned with not only the weight of the drywall causing sag, but of the integrity of the garage even as is.

Garage was built in the 40s, and used rafters/rafter ties .
The roof pitch is 4:12 and the rafters are 2x4s with 24" o.c. spacing. They have only 2 rafter ties (every third rafter, i.e. 6') that span the width of garage and are at attached at the rafters right on top of the walls. The rafter ties are only 2x4s.

Roof is OSB with shingles, and the walls are 8' high. Since this is in Los Angeles county, I tried looking for tables that would let me know if the existing construction is sufficient. However all I found were tables for 2x6 rafters with much meatier ties than the 2x4s currently used. Just based on other houses I've seen, the use of only 2 rafter ties seems really insufficient to me, but I moved here from a very snowy place.

I like going to the inspector armed with as much knowledge before hand, so was hoping to calculate whether this original construction is OK, and if so whether it could handle an additional ~500 lbs to the rafters from drywall. Can someone point me in the right direction?

GBrackins 08-02-2012 02:39 PM

photos would aid in providing useful replies

tony.g 08-02-2012 02:46 PM

Some rough figures:

lets say your board weighs 2lbs/sq ft.
Area of board supported off each tie = 18x6 = 108 sq ft.
board load on each tie = 108 x 2 = 216 lbs.
say each tie weighs approx 20lbs;
also, you will need noggins between the ties to support the board - say a further 30lbs approx.
each tie will then be supporting 266lbs.

the maximum bending moment (M) = W x L/8
= 266 x 18 x 12/8
= 7182lbs in.

the maximum stress in the tie = M/Z, where Z = b x d x d/6
(where b = breadth of tie and d = depth of tie)
so maximum stress = 7182 divided by 1.5 x 3.5 x 3.5/6
= 2,345 lbs/sq in.

Most structural timbers will go up to a maximum of 1000 - 1200 lbs/sq in.
so your ties will be stressed way over their limit. You will need to think of an alternative arrangement before seeing your inspector. Sorry.:(

cjkb 08-02-2012 04:03 PM

3 Attachment(s)
I've attached three photos of the back half of the garage. The more I look into it the more I believe the original construction is inadequate.

tony.g 08-02-2012 04:36 PM

Since I posted those figures, it occurred to me that I might have been barking up the wrong tree.
Did you mean apply the board to the underside of the rafters and not the ceiling ties?
If the boarding was to go on the rafters, the main problem would be the increased tendency for the rafters to spread, because you are adding a greater load. I very much doubt that 2x4 would be acceptable for that span today, and if you add extra weight, I think the inspector will ask for justification, which would be difficult. Two ties down that length doesn't seem very much to provide lateral restraint, and you would only be making the situation worse.

(you said the roof was covered with OSB, yet there are battens, suggesting shingles at one time. If the OSB has been added later, then that might also be adding to the weight. Its an elegant and almost 'minimalist' structure and has stood the test of time, but it might be best not to push it too far!).

cjkb 08-02-2012 06:17 PM

Thanks tony. Yes the roof was redone a year or so before I bought the house. And yes, I was thinking of attaching drywall directly to the rafters (so it would be sloped). Code here would require 2x6s for the rafters and ties, and a friend at work pointed out that modifications to existing work might require that the structure be brought up to code. So probably an augmentation with additional ties would not be sufficient. New roof is out of my budget range right now.

I'll check with the inspectors next week ( they are only available Mon/Tue/Wed), but I'll probably limit any modifications to just painting the ceiling white to brighten it up.

joecaption 08-02-2012 06:47 PM

Without some pictures where all going to be guessing.

Why would you be attaching the the rafters and not the ceiling joist?
Without proper baffels, enough room for insulation, soffit vent and a ridge vent it would drasticly reduce the life of the shingles.

Gary in WA 08-09-2012 01:20 AM

You'll need to look up CBC 2010: http://www.reedconstructiondata.com/...es/california/

I have those book-marked in IBC: http://publicecodes.cyberregs.com/ic...?bu2=undefined

Spans: http://publicecodes.cyberregs.com/ic...?bu2=undefined

Gary

joecaption 08-09-2012 10:18 AM

I think you mean collor ties not rafer ties.
Rafter ties would be what attaches from the rafter to the top plate to prevent roof lift.
Collor tie keep the roof from spreading.

GBrackins 08-09-2012 10:29 AM

technically the 2001 edition of the "Wood Frame Construction Manual" defines them as follows:

Collar Tie- a structural framing member located in the upper third of the attic space that ties rafters together to resist roof suction loads at the ridge (keeps leaward side rafter from pulling away from ridge beam/board)

Rafter Tie - a structural framing member located in the lower third of the attic space that ties rafters together to resist thrust from gravity loads on the roof. (prevents pushing out tops of walls and saddling ridges)



cjkb 08-09-2012 11:00 AM

Thanks for the input. It's clear that the 2 ties (what I call rafter ties, which are the 2x4s attached to the rafters at the top plate that span the width of the garage) are insufficient according to code. I have abandoned the notion of adding weight to the roof in the form of drywalling to the rafters, and instead am concentrating on reinforcing the existing construction. Besides, it was pointed out to me that ventilation is insufficient and drywalling/insulating would decease the life of the shingles.

Here in los angeles I don't have to worry about high live loads, and luckily the garage is protected from wind by surrounding structures (neighbors). The garage has survived for 50 years, but I don't like the idea that only 2 2x4 ties are being used.

tony.g 08-09-2012 12:13 PM

Quote:

Originally Posted by GBrackins (Post 984915)
technically the 2001 edition of the "Wood Frame Construction Manual" defines them as follows:

Collar Tie- a structural framing member located in the upper third of the attic space that ties rafters together to resist roof suction loads at the ridge (keeps leaward side rafter from pulling away from ridge beam/board)

Rafter Tie - a structural framing member located in the lower third of the attic space that ties rafters together to resist thrust from gravity loads on the roof. (prevents pushing out tops of walls and saddling ridges)

Wonder what they would call them if they were half-way up?
Colafter ties, perhaps? :laughing:

GBrackins 08-09-2012 01:09 PM

raftlar ties????

AndyGump 08-09-2012 04:53 PM

O.K. you are in my neck-of-the-woods, the garage is not a living space of course unless you intend to convert it to one so the normal insulation codes you would need to comply with in living space does not apply.
If you are trying to keep out excessive heat from radiation off the roof deck you might look into installing a radiant barrier on the rafters.
That would probably just consist of a paper product that has an aluminum foil on one side and you can just staple it to the underside of the rafters.

By the way, I for one do not subscribe to the notion that comp. roof shingles are compromised by a non-vented attic space.

Andy.

cjkb 08-09-2012 05:02 PM

Thanks. Garage is just used for storage and has a small workbench area. Don't even use it for cars anymore. It does get pretty warm inside, but it is in large part due to the 16x7' uninsulated aluminum garage door that faces east and heats the space up quickly. especially on days like today where it is pushing 100F ! In the past I considered insulating it with polystyrene sheets, or getting a new one. I might bump that up my priority list.


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