Expert: Bruce E. Johnson - 2/12/2010

Question
QUESTION: I am visiting my daughter in Australia.  She came here on a University Affiliation Program 14 years ago, fell in love with Australia and, then, an Aussie and never came back.  I retired the same year she left and have been traveling all over the United States doing major Handyman projects for all my relatives and friends.  My daughter, rightly so, feels like she has missed out on my Handyman skills so bought my wife and I a ticket to travel to Australia with the request that I build a deck for her new home while I'm here.  I have built the actual deck and placed the saddles for the roof but am in a quandary, now.  I have used hardwood (the lumber supplier assures me it is from sustainable logging) and am extremely impressed with how nice it is to work with.  I have been unable to find any Span Tables for the 18.5 foot width necessary to put a roof over the deck.  What size lumber do I need to span 18.5 feet using the Australian equivalent of Structural Select lumber?  She lives in Queensland so has Monsoons part of the year.  What is the minimum slope I can use on a lean-to style roof attached to the house on one side?  I sure hope you can help me on this.  My daughter and I will both appreciate any help you can give.  Thank you.  Stan

ANSWER: Hi Stan, or should I say G'day mate?  18.5 feet is a pretty hefty span.   Depending on what sizes are available I would say you would need at least a 2x12 or the Aussie equivalent for any pitch up to 3/12.  You can begin to decrease the size of the rafter the steeper the pitch from there.  4/12-5/12 2x10, 6/12 and steeper 2x8.  The flatter the roof the more gravitational pull on the center of the rafter.  Even without a live load the dead load alone will eventually cause the lumber to sag over time if it is undersized.  I'm sure you know about crowning your wood and it is imperative that the lumber be installed with the crown up in all instances.  This helps prolong the effects of gravity.  As far as pitch goes: the pitch needs to be relative to the roofing used. 90# Rolled roofing installed with the proper overlap and sealed at all the seams with a compatible mastic is good for shallower pitches of 2/12 or higher.  Less than 2/12 you need to have the roof done built up with hot tar. For pitches 3/12 and higher you can start getting into asphalt shingles..4/12 and higher wood shingles or shakes.  Consult local lumber yards for the roofing stats in your daughters area.   I'm sure the local customs will help determine the best type of roofing for the weather in that area.  I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com  

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QUESTION: G'Day to you too, Bruce,

I am no less than astonished at the quality and depth of your response!!  Thank you so much.  However, I must admit, right away, that you rained on my parade.  I was using calculators like the one at www.cwc.ca/Resources/E-Tools/SpanCalc.htm, www.timber.org.au/resources/Supp%204%20-%20Grp%20D_RG1_N1-3.pdf and www.awc.org/technical/spantables/tutorial.htm and had decided they told me that I can span more than 18.5 feet with Structural Select (SS# grade lumber or recycled hardwood.  I have assumed that Australia's grade F17 is equivalent to our SS.  #I need to say, here, that the Australian Span Tables seem to be akin to a State Secret.  I even went to the local building permit people in the town where my daughter lives and they told me that they have the tables there but they cannot show me.  They suggest that I buy the tables myself and I would, if I could, but I need to have the cut list ready to go to the lumber yard before I leave for Tasmania on Sunday so haven't the time.#  I don't know where I went wrong but I accept that I have and am going to ask you another question.  What dimension should I use for a 15 foot span?  I am using light weight, metal, sheet roofing over battens #purlins# so can go with the lightest actual loads.  If I do go to the 15' span, I will need to overhang a portion of the uphill end by about four feet.  I do appreciate how quickly you answered my first question and I wish I had stumbled onto this website much sooner in my planning.  I hope that you can answer this one quickly, as well, so I can get the list out on Saturday. #We're already tomorrow, here, so I only have about 24 hours.#  Thank you, again.  Stan that they have the tables there but they cannot show me.  They suggest that I buy the tables myself and I would, if I could, but I need to have the cut list ready to go to the lumber yard before I leave for Tasmania on Sunday so haven't the time.)  I don't know where I went wrong but I accept that I have and am going to ask you another question.  What dimension should I use for a 15 foot span?  I am using light weight, metal, sheet roofing over battens (purlins) so can go with the lightest actual loads.  If I do go to the 15' span, I will need to overhang a portion of the uphill end by about four feet.  I do appreciate how quickly you answered my first question and I wish I had stumbled onto this website much sooner in my planning.  I hope that you can answer this one quickly, as well, so I can get the list out on Saturday. (We're already tomorrow, here, so I only have about 24 hours.)  Thank you, again.  Stan

Answer
Hi Stan, I believe structural select is as good or better than #2 as I mentioned in my previous response.  Was that your question?  As far as size goes for a 15 foot span?  I thought I covered that in my previous answer.  I know 2x12 seems kind of big but you have to realize that the lumber must not only be able to hold up its own weight but also the weight of the roofing.   I did look up that span table reference you sent me and all I can say is they seem a little off to me.  But according to that table you should be able to use 2x8s for the roof system you want (honestly I was surprised at the spans they give for 2x12).  The flatter the roof the more live load on your rafter so if you have a pitch of some sort and build with the crown up then you should be okay with smaller lumber. You'd think that someone in Australia would have some advice as to the type of lumber and the grading.  But I think you are at least safe in using a southern yellow pine or douglas fir table.  If anything the harder wood will span farther because of its density.  I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com