Rafter ties again

[bill1952]

If just a rafter tie, IRC says min 2x4 max 24" o.c. and that "other approved rafter tie methods shall be permitted". Would a 2x8 48" on center fit that?

I've often seen rafter ties - usually uninsulated garages - routinely 4 or 6' on center, and often just a 2x4 or 2x6. I wondered how it complied with code, or just didn't.

The other curiosity is the common use of vertical hangers - like a 2x4 or 1x6 nailed to rafter and to rafter tie or sometimes a ceiling joist below. I know it's been done a lot but don't see where the IRC allows it (without an RDP seal). Not to mention nails in the ends of a board in tension seem dubious.

 

[Yankee Chronicler]

The 2003 IRC said, "Rafter ties shall be spaced not more than 4 feet (1219 mm) on center."

Good thing I checked. My fuzzy recollection was that the IRC used to allow rafter ties at 6 feet on center.

 

[bill1952]

Did I misread the 2024 IRC? Was sure it said 24". Your 2003 reference explains a lot and recollection of 6' may explain more. Wonder why it's changed? I think I have heard of failures but from fasteners pulling out.

 

[bill1952]

Here is text from 2024IRC.

R802.5.2.2 Rafter ties.
Wood rafter ties shall be not less than 2 inches by 4 inches (51 mm × 102 mm) installed in accordance with Table R802.5.2(1) at a maximum of 24 inches (610 mm) on center. Other approved rafter tie methods shall be permitted.

Would love to see the reason from the proposal that changed from 48 to 24. I think I tossed my old CABO one and two family dwelling code or I'd look there.

 

[Yankee Chronicler]

Would love to see the reason from the proposal that changed from 48 to 24. I think I tossed my old CABO one and two family dwelling code or I'd look there.

The 1992 CABO can be downloaded from The Internet Archive. Here's what it said:

R-702.2 Framing details: Rafters shall be nailed to ceiling joists to form
a continuous tie between exterior walls where joists are parallel to the
rafters. Where not parallel, rafters shall be tied with a rafter tie, located as
near the plate as practical. Rafter ties shall be spaced not more than 4 feet
on center. Rafters shall be framed to ridge board or to each other with
gusset plate as a tie. Ridge board shall be at least I-inch nominal
thickness and not less in depth than the cut end of the rafter. At all valleys
and hips there shall be a valley or hip rafter not less than 2-inch nominal
thickness and not less in depth than the cut end of the rafter. Hip and
valley rafters shall be supported at the ridge by a brace to a bearing
partition or be designed to carry and distribute the specific load at that
point.

Ends of ceiling joists shall be lapped a minimum of 3 inches or butted
over bearing partitions or beam and toenailed to the bearing member.
When ceiling joists are used to provide resistance to rafter thrust, lapped
joists shall be nailed together and butted joists shall be tied together in a
manner to resist such thrust.

2006 IRC requires rafter ties to be a minimum of 2x4 lumber but doesn't specify a maximum spacing. It does specify that collar ties must be not more than 4 feet on center.

2009 IRC, 2012 IRC, and 2015 IRC were the same as 2006.

2018 IRC reorganized the rafter tie and collar tie sections but still did not specify a spacing for rafter ties.

IRC 2021 introduced the maximum 24 inches on center spacing for rafter ties.

 

[Yankee Chronicler]

The copy of the 1992 CABO code available from the Internet Archive is missing the first page of Chapter 1. It's blank. If anyone has a copy of CABO 1992, I would appreciate it if you could scan it or photograph it and send it to me so I can fix my copy.

Thank you.

 

[Yankee Chronicler]

Did I misread the 2024 IRC? Was sure it said 24". Your 2003 reference explains a lot and recollection of 6' may explain more. Wonder why it's changed? I think I have heard of failures but from fasteners pulling out.

Based on what I found in CABO 1992 (and I'm old enough to have been working under CABO for a good number of years) I believe my recollection of 6 feet on center was faulty.

The 2024 IRC requirement of 24 inches on center is problematic in that many roofs are framed 16 inches on center. If the rafters are 16 inches on center, ties can't be 24 inches on center. Requiring ties at every rafter seems excessive. They could be 32 inches or 48 inches on center.

My detached garage is 20' x 20'. The ties are 20-foot 2x6s at 4 feet on center. The roof slope is 3:12 or 4:12, the garage has been there for about 50 years and has seen some VERY heavy, wet snows, and there's no evidence of the bearing walls splaying. 24 inches on center seems like overkill IMHO.

 

[steveray]

R802.5.2​

Where ceiling joists run parallel to rafters and are located in the bottom third of the rafter height, they shall be installed in accordance with Figure R802.4.5 and fastened to rafters in accordance with Table R802.5.2(1).

you cannot fasten them to the rafters if the IC spacing is different…

 

[bill1952]

I guess I look askance when I see a 24' 2x4 every 2' used as a rafter tie. Even more curious when overhead door is suspended from it. I guess it does its job of countering rafter thrust but those nails at the end (between 3 and 34 by the table -: still trying to envision 34 16d nails in the end of a 2x4) or the end of the 2x4 seem very stressed.

And still wonder about the vertical hangers. I guess for a rafter tie the gravity load is not significant - at least until someone throws a sheet of ply atop them and starts storing stuff.

Mentioning ridge beam above, I was asked if that could be dropped an inch or two. 5 1/4" LVL, can't get ridge vent and shingles to work if the beam has to cover entire plumb cut. One tip I found online was deeper LVL than required for load and chamfer top edges between rafters.

 

[wwhitney]

2018 IRC reorganized the rafter tie and collar tie sections but still did not specify a spacing for rafter ties.

2018 R802.5.2.2 specifies that rafter ties be installed "at each rafter." 2021 changed that to maximum 24" o.c.

It seems to me the whole idea of Table R802.5.1(9) "Rafter/Ceiling Joist Heel Joint Connections" implies that each rafter requires a rafter tie or ceiling joint. As that table about the nailing connection refers to "rafter spacing" not "rafter tie spacing".

If you were allowed to install rafters at 16" o.c. and rafter ties at 32" o.c. (every other rafter), and use the nailing pattern shown in the table, it would make no sense to still require that same nailing pattern if you then switch to rafter ties at 32" o.c. Half the nails in each connection would be sufficient to resist the thrust with twice as many rafter ties. So I infer the table is based on one rafter tie per rafter.

Cheers, Wayne

 

[bill1952]

it would make no sense

Doesnt stop some people.

I get they feeling that other than recognizing they importance of resisting rafter thrust, this is not a section of they code that has been exhaustingly reviewed by engineers. Probably because not many builders would mess with stick framing like this but would go to trusses. iirc the code still allows balloon farming - or did till recently, long after nearly everyone had gone to platform framing. I wonder when they last proposal for a change to balloon framing requirements was submitted.

 

[wwhitney]

If just a rafter tie, IRC says min 2x4 max 24" o.c. and that "other approved rafter tie methods shall be permitted". Would a 2x8 48" on center fit that?

Here's one possibility that I see as being defensible as prescriptive: size the rafters for 24" o.c. and install in pairs at 48" o.c. Use a single 2x8 rafter tie between each pair of rafters, with the Table R802.5.1(9) connection from each rafter into this double-sized rafter tie, based on 24" o.c. rafter spacing. Size the roof deck to span 48", or install bonus rafters in between the double rafter/double-sized tie assembly so the roof deck actually only spans 24" o.c.

Cheers, Wayne

 

[bill1952]

Interesting. I've been intrigued by replacing rafter ties with a horizontal beam at top plate, with ends of beams tied togerher neat ends of walls. Requires an RDP in many or most jurisdictions I suspect but on the same basis, seems simple to grasp that if 24" o.c. is for minimum wall of 2x4, some greater spacing ought to be fine for 2x6 or larger plates. Kind of not intutitve that same span between ties applies to single or double top plate.

The particular project that promoted the question has 24' 2x6 ties at 4' centers and passed inspection. The owner planned to sheetrock and dump cellulose on it. I suspect a contract issue of whether they builder actually agreed to build it to accomodate a ceiling, which would require 2x8s 16 on Center - some grades and species - no storage.

Thank you all that posted here. Very helpful.

 

[Yankee Chronicler]

The copy of the 1992 CABO code available from the Internet Archive is missing the first page of Chapter 1. It's blank. If anyone has a copy of CABO 1992, I would appreciate it if you could scan it or photograph it and send it to me so I can fix my copy.

Thank you.

Cancel this request. I found the 1992 CABO book in my code archive and I was able to reproduce Page 1 and splice it into the PDF copy on my computer.

 

[steveray]

Interesting. I've been intrigued by replacing rafter ties with a horizontal beam at top plate, with ends of beams tied togerher neat ends of walls. Requires an RDP in many or most jurisdictions I suspect but on the same basis, seems simple to grasp that if 24" o.c. is for minimum wall of 2x4, some greater spacing ought to be fine for 2x6 or larger plates. Kind of not intutitve that same span between ties applies to single or double top plate.

The particular project that promoted the question has 24' 2x6 ties at 4' centers and passed inspection. The owner planned to sheetrock and dump cellulose on it. I suspect a contract issue of whether they builder actually agreed to build it to accomodate a ceiling, which would require 2x8s 16 on Center - some grades and species - no storage.

Thank you all that posted here. Very helpful.

Concept is sound…if the wall can’t bow out…the ridge can’t come down, opposite or a ridge beam. Similar to the roof or wall diaphragms that do more of the work than most people know.

 

[rktect 1]

Rafter ties are spaced in an equal amount as the roof rafters. They are not collar ties which are specifically spaced 48" o.c.

 

[bill1952]

Rafter ties are spaced in an equal amount as the roof rafters. They are not collar ties which are specifically spaced 48" o.c.

That seems to be the case under 2021 and 2024 IRC as far as rafter ties are concerned, unless it's an "other method that shall be permitted". Rafter ties on max 48" centers before 2021.

Never lived anyplace where collar ties seemed required. Seemed like a hurricane and tornado thing.

Still curious about the practice of supporting a ceiling tie from the rafters or ridge board. It seems common but I don't see where that additional load on the rafters is accounted for.

 

[SCBO1]

Rafter tie: Min 2x4, Maximum 24-inch spacing, lower third (IRC 2024 R802.5.2.2)

Collar tie: Min size 1x4, Maximum 48-inches spacing, upper third of the attic space (IRC 2024 R802.4.6)

Not seeing any change unless the 2024 code has new verbiage?

 

[bill1952]

I may have miss understood Yankee's recount of edition by edition. Thought earlier editions were max 48" o.c.

 

[Energystar]

I thought the purpose of rafter ties is to reinforce the rafter to ridge connection against wind loads. This is a key structural connection between two diaphragms much like wall to floor or rafter to wall. Rafter ties are used in lieu of rafter hangers at the ridge.

 

[wwhitney]

I thought the purpose of rafter ties is to reinforce the rafter to ridge connection against wind loads. This is a key structural connection between two diaphragms much like wall to floor or rafter to wall. Rafter ties are used in lieu of rafter hangers at the ridge.

You have described collar ties. Rafter ties or ceiling joists are used to connect the lower ends of the rafters to create a simple triangular truss with the rafters.

Cheers, Wayne

 

[Energystar]

Oops, sorry.

 

[bill1952]

I thought the purpose of rafter ties is to reinforce the rafter to ridge connection against wind loads. This is a key structural connection between two diaphragms much like wall to floor or rafter to wall. Rafter ties are used in lieu of rafter hangers at the ridge.

Wayne is correct but I would phrase it as rafter ties are to resist rafter thrust - the force axial to the rafters that tried to spread the walls out. Rafter ties in bottom third of rafter; collar ties in top third. Also primarily for gable roofs.

 

[Sifu]

The copy of the 1992 CABO code available from the Internet Archive is missing the first page of Chapter 1. It's blank. If anyone has a copy of CABO 1992, I would appreciate it if you could scan it or photograph it and send it to me so I can fix my copy.

Thank you.

Interesting, mine does too. We must have gotten it from the same place.