Span Tables in the IRC for exterior decks

[Glenn]

Thanks, Francis.

The span tables in the IRC are not for wet use. That said, the only result of slightly overspanning is too much deflection. The tables are based on L/360 deflection, which is pretty minimal considering decks don't have any brittle finish beneath them like a plaster or gypsum ceiling below, nor do they typically carry the amount of furniture that causes the serviceability issues from a rocking grandfather clock when the kids run across the room. There was some serious discussion as to if that deflection limit is even an appropriate deflection for decks, and during testimony when it was proposed to specifically add "decks" to the load table next to "floors", there was enough doubt as to what it should be that it was not added. That said, all the new tables in the 2015 are based on L/360. Let's just remember that overspanning does not cause structural collapse. Deflection is the controlling limit when it comes to span.

The 2015 IRC has joist and beam span tables specifically for decks and deck environmental conditions. It also has limits for various post cross sections. I put a lot of work into new deck provisions for the 2015 IRC, but so did many others along my side. The AWC, NAHB, NADRA, CLMA, SST, OMG, and representatives from about six different ICC chapters (and sorry for those others that didn't come to mind right now). It was teamwork and collaboration at it's finest. I hope you think it produced quality code. It sure wasn't easy, but it's how code should be developed.

 

[Glenn]

PS. I've started my 2015 revision to my ICC deck code book/commentary. It provides every single possible IRC code that affects decks. The 2015 edition will be so much more helpful with all the new provisions. Sorry for the plug, but it's on topic at least. The plan is to be on the shelves next summer. More details here: www.deckcodes.com

 

[fatboy]

So, Glenn, based on the 2012 IRC, relevant sections unamended, what span charts should/could I use?

 

[Francis Vineyard]

Published 2008; http://www.deckmagazine.com/books/structure--using-the-prescriptive-residential-deck-construction-guide.aspx

 

[fatboy]

That is great for reference, and no offense to my pal Glenn, but it is not the code, unless it has been adopted as such.

 

[jar546]

R301.1.1

R301.2.1.1

R317.3.2

R401.1

R402.2.2

R402.2.3

R502.2

R502.3 (especially this one. Do you need an invitation?)

Table R503.1 three times

R602.3

Table R602.3.1

R602.10.6.2

R611.9.2

and so on

and so on

and so on

and so on

In all, 28 times the WFCM AF&PA is referenced in the 2009 IRC

So again, if you don't have this, I don't know how you are legally doing your job.

The IRC is for dry use only and there is a difference in spans, up to more than 2' with SYP 2x12's from the IRC to the actual span.

Will it mean an automatic structural failure? Probably not, but that is not the point. This is where knowing your job and continuing to learn in your job comes into play. This is basic knowledge for framers and inspectors, sort of like Kindergarten.

 

[SCBO1]

Does anyone use the span tables in the IRC to verify framing member spans during plan review or inspection?

Yes, ceiling joist and FJ on house plans

Decks are checked by using the AWC dc6 guide including piers. Some piers here are required to be made bigger by using the guide based off the load that each post is carrying.

Pc1

 

[Min&Max]

Where does it specifically say that the IRC span charts cannot be used for exterior applications?

 

[mtlogcabin]

That is great for reference, and no offense to my pal Glenn, but it is not the code, unless it has been adopted as such.

It does not have to be adopted. You as the building official just have to "approve" the document for use in the design for residential deck use. Similar to the different I-Joist span charts that are not adopted

R104.11 Alternative materials, design and methods of construction and equipment.

The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code. Compliance with the specific performance-based provisions of the International Codes in lieu of specific requirements of this code shall also be permitted as an alternate.

 

[ICE]

An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code.

Thats an open ended statement. Apparently a building official can decide to accept whatever based on his/her knowledge. I've met a few that I wouldn't trust for that.

 

[jar546]

Where does it specifically say that the IRC span charts cannot be used for exterior applications?

It doesn't and doesn't have to. You as a professional should already know that based on your knowledge of construction. Decks themselves are not prescriptive by design and therefore not covered in full, prescriptively in the IRC to begin with.

 

[jar546]

Thats an open ended statement. Apparently a building official can decide to accept whatever based on his/her knowledge. I've met a few that I wouldn't trust for that.

It does not matter whether or not there are a few people that you would not trust, it is the reality of the situation. We are given authority that we must understand first.

 

[mtlogcabin]

We are under the 2006 IRC. On our web site we link to the DCA6 manual for reference when designing a residential deck. Whey would I "approve" the use of DCA6 since it is based on the 2009 IRC which the state did not adopt? Because R104.11 authorizes me to do just that when the legislature is not keeping up with the construction industry as it relates to design and materials used.

There are many organizations out there that can and should be used that are not referenced by the adopted code. Builders Guide to Cold Climates is an excellent reference book which is published by the Building Science Corporation but you will never find it referenced in the I-codes.

 

[SCBO1]

In the "ol" days, UBC days you had to figure your own spans by using a formula I have since forgotten. The jurisdiction I worked for, we would calculate the spans and put out a builders guide then adjacent AHJ's would call or get their hands on a copy which was fine by us. We were all on the same page at some point in the area.

The IRC like a calculator made it easy for everyone. Then came the span adjustments and you would have to decide if and when you would enforce the the new spans. Adopting the new spans or making it your policy to enforce the current span information IMO is the AHJ's call unless restrictions are in place by a governmental entity.

pc1

 

[steveray]

TABLE R502.3.3(2) CANTILEVER SPANS FOR FLOOR JOISTS SUPPORTING EXTERIOR BALCONYa, b, e, f

Sooooo.....This exterior balcony span table does not cover wet use? Must be good in SoCal only.....

 

[Glenn]

That is great for reference, and no offense to my pal Glenn, but it is not the code, unless it has been adopted as such.

No offense at all. The link you're referring to is my article discussing the AFPA DCA-6 (which is not my work). My book IS the code, and my commentary of it (which is not). It does explain the dry use only for the IRC tables.

...sort of like Kindergarten.

Not that simple, and not cool to talk to your professional colleagues that way, but since we're talking like we're in kindergarten now, I guess we should all go back together for a bit.

Yes, the WFCM is an approved referenced standard. So let me break out my copy.

Chapter 2 is "engineered design" and where you find wet use reduction factors. I certainly hope you aren't referring to engineering practice as kindergarten... so I'll just stop there. Chapter 2 is off limits to anyone but someone licensed in the state to perform engineering.

Chapter 3 is "prescriptive design". The limitations of use in the general sections don't shed any light or give any assumption that the prescriptive tables are based on wet use, quite the opposite. None-the-less, the provisions under "floors" points you to span tables. Guess what... they are identical (with very slight variation here and there) to the IRC tables (comparing documents of equivalent design values for southern pine). The WFCM prescriptive provisions do nothing more for wet use joist spans than the IRC. However, it does have lots of other cool and helpful stuff in it and I would recommend purchasing a copy if you work in wood frame construction.

Well.... turns out that AFPA employs engineers. So based on the engineering chapters they did engineer tables for decks in wet use and published them in their document specific to decks. Those spans are NOT what are in the IRC or the WFCM.

Fatboy, here is your path to using the DCA 6 or the 2015 IRC. Let the applicant decide.

1) Prescriptive code does not provide spans for deck joists. Which means an alternative is necessary.

2) You are allowed to approve alternatives based on engineering or testing.

3) The applicant does not have site-specific engineering or testing. They are more than welcome to go get that on their own....or...

4) You could suggest that you would accept the DCA 6 provisions or the 2015 IRC provisions. Why?

5) The IRC clearly respects and recognizes the AFPA as the source for wood engineering. They have engineered another document specific to the conditions you need to review. That document was the basis of the 2015 IRC provisions. You approve the engineered tables in the DCA 6 based on this knowledge.

If the applicant does not want to use the DCA 6, they are more than welcome to procure their own engineering and testing. They will probably choose the DCA 6.

Let's be helpful to each other, not demeaning.

 

[SCBO1]

Kindergarten, I remember my superman lunch box, girls kissing on me and pulling out the beach towel for a nap....Ahh... those were the days.

Oh ya, getting along with people was taught! I know I don't contribute anything here but some here do!

Thanks, Glenn for the information, and yes, lets be helpful to each other! A lot of us in kindergarten wanted to be Police or Firemen, sure not a thankless building inspector!

pc1

 

[Min&Max]

It doesn't and doesn't have to. You as a professional should already know that based on your knowledge of construction. Decks themselves are not prescriptive by design and therefore not covered in full, prescriptively in the IRC to begin with.

First you attempt to chastise those of us who have been doing this job for decades for what YOU deem to be poor code enforcement for allowing decks to be built in a manner they have been constructed for many more decades(see quote below). You then proceed to comment that based on our knowledge of construction it doesn't have to be in the code for us not to allow it. Well here's a news flash, based on knowledge of construction which is based on observed past performance over the last 40 years, with zero deck failures in this jurisdiction, I am more than comfortable using the IRC span tables for floor joists while maintaining a minimum 40psf load capability. "Not the point. Not the point at all.

Do you want to continue to set precedence with picking and choosing what codes you enforce.

That excuse is as bad as "I've been doing it that way for 30 years" which we hear all too often."

 

[JBI]

Not for nothing, but the treated lumber manufacturers can provide you with span tables for their products.

The tables we have always had in the ResCode are not intended for treated lumber, which was traditionally 'pressure' treated and incised, as the process historically denigrates the strength of the lumber.

All standard design methodologies have a 'wet service' factor which reduces allowable spans in wet service conditions.

Also, look at Chapter 5 and the floor diagrams... do you see any exterior floor structures shown? Answer - No, you don't.

The 2015 ResCode will fix some of that by expanding the deck provisions, but it still does not adequately address the pier foundation prescriptively.

Understand where the Codes come from, not just what (you think) they say...