I have to replace the main beam in a deck at my house. The deck is 10 x 23 and the beam is 6 feet from the wall with the other side of the deck supported from the wall. The deck joists are sistered to the floor joists with lag bolts (about 3 feet of sistered joist section). As per the above there is 4 feet of cantilevered joist going past the beam and 5 feet of cantilevered beam that connects to another beam.

The beam span is 18' and the original beam is a 13.5 x 6.75 glulam. It failed due to dry-wood termites and water causing the laminated sections to separate right over the column connector. I think that a previous owner may have had a plant right over the beam where it joins the column and when watered steady stream of water ended up on the corner of the beam. It never sagged in the center. The deck is ~35 years old and was built with the house. For good measure I am also replacing the posts, joists (many are pretty rotten) and deck boards (which where still there after 35 years so very rotten).

Anyways this size (13.5 x 6.75) while available and not terribly expensive (~$400) would be very hard to get into position to lift into place (lifting wouldn't be that hard). The issue is that the back of the house is situated over a very steep hill (practically a cliff) and not accessible by heavy equipment.

A few alternatives I am considering.

  1. Making a built up beam using 3 2x14's and a 3x14 (13.25 x 7) how would I calculate if this is equivalent to the 13.5 x 6.75.
  2. Adding an extra post (the soil is very hard but I could hit bedrock or a huge boulder within 24" or so).

I am mainly am looking for input on number 1 above.

  • It bothers me that you’re calling the beam a “13.5 x 6.75”. Don’t you mean 6.75 x 13.5? It’s a standard glu-lam right?
    – Lee Sam
    Commented Dec 6, 2020 at 23:09
  • Boise Cascade makes I-joists for open but protected outdoor installations. Does this deck have a roof? If so, check with your lumber supplier and see if an I-joist would be easier to move into position? Commented Dec 6, 2020 at 23:14
  • Yes it was a 6.75 x 13.5, its a standard glu lam. I ended up replacing the glu lam with another glu lam of borate treated alaskan yellow cedar (it was expensive). The lumber yard actually had a structural engineer that looked at the plan and confirmed that the beam was correctly selected according to code. I flashed the new beam with galvanized steel flashing to protected.
    – user103870
    Commented Dec 8, 2020 at 5:02

3 Answers 3


Download a free pdf Wood Deck Construction Guide from the American Wood Council. It is based on current codes and standards and can help with all aspects of wood deck construction in the absence of specific direction from your local AHJ (Authority Having Jurisdiction).

It will not address engineered construction members (like laminated beams) though...

  • Thanks for the comment, I previously reviewed the deck construction guide as well as the current code and local codes and surprising the decks construction meets even the current code apart from just a few minor things (PT lumber within 36 inches of the dirt for local codes is all I noticed). I have a feeling that if I talk to "AHJ" they are going to say replace it with exactly what is in the original approved plan, and I guess thats the right answer. I supposed I should change the question to ask whats the best way to move a 600 pound beam down a steep hill without equipment.
    – user103870
    Commented Jul 14, 2019 at 23:25

A 6.75 x 13.5” glu-lam that is 18’ long can support 900 lbs. per foot if it’s spruce or pine and 1,000 lbs. per foot if it’s Douglas Fir for an 18’ span.

Likewise, a 2x14 spf spanning 18’ with a grade of “Select Structural” will support about 200 lbs. per foot.

A 3x14 spf spanning 18’ with a grade of “Select Structural” will support about 300 lbs. per foot.

You can see how you’d need to multiply each member to get the equivalent beam size and adjust the post cap size accordingly.

I hope this helps, but there are other considerations too:

  1. the code requires 2 deck restraints (one at each end of your deck) similar to https://www.fastenersplus.com/Simpson-DTT2Z-Deck-Tension-Tie-Zmax-Finish?gclid=Cj0KCQjw7YblBRDFARIsAKkK-dJZrRsiFDrb6UevCNA1fWsgoBllbcaFbSIxYM08Q-lVaduQDD0EbOYaAvekEALw_wcB

  2. I don’t understand the cantilever load and how it’s being included.

  3. Size of posts. Posts are sized based on load and length of column. If your deck and this beam is extended over a cliff, then the posts could be over extended due to their unsupported length.

  • 1. One side is retained with an engineered bracket (original to the house) in a perpendicular wall. I did add a tension tie on the other side. But I think it isn't code required since there is no ledger board the deck joists are sistered into the floor.
    – user103870
    Commented Dec 8, 2020 at 5:04

I built a similar deck 25 years ago. It is 16' by about 16' (irregular shape). I used two 2 X 12 ( southern pine ) at the center. They are not sistered , one on each side of 4 X 4 posts. It has work well , no bounce or spring ,which I do not like. I did not follow any code .

  • 1
    I'm glad your deck has worked well for you, however I don't think "I did not follow any code" is good advice to be offering to others. I understand that many DIYers will sorta-kinda follow code where it's not too expensive or difficult (and they're not getting inspected), it's not good advice to "recommend" doing so.
    – FreeMan
    Commented Dec 7, 2020 at 13:14

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