can I span 20 feet on my deck

[Bart Kiefer]

Hi everyone,
I have a question and everywhere I have looked I can't seem to find an answer. I have a deck that is already built and we are putting a roof over it. The question I have is can I span 20 feet for my trusses to connect to? I'm wanting to maximize my view that I have of a lake. I'm thinking that a 2x8 flitched beam would do, however I"m getting a lot of mixed feed back on it.

Primarily there will be only dead load on the beam with a little bit of live load, I'm thinking of putting a swing under the roof back close to the cabin so a good portion of the weight will be carried down thru the walls of the cabin.

I've some some people telling me to use an lvl, but it will be exposed to the elements and I'm afraid that the beam will just fall to pieces once the water/humidity get to it.

Does anyone have any thoughts? I could post a crude drawing if it would help. Thanks in advance.

[Scott T Smith]

Bart, I think that you're going to need to use an LVL for that span, in order to meet code. Alternatively you may be able to use (3) 2 x 12's SYP ganged together, but the LVL would most likely be superior. You can always skin it with a more attractive wood in order to hide the appearance.

[Bart Kiefer]

Scott,
thank you for your reply, i agree that an LVL would be better suited, however I"m just worried that it will delaminate whenever rain and the humidity get to it. Any thoughts on protecting it? As for it being more attactive to hide the appearance I don't think my dad would really care. He is just wanting some shade to sit in.

[Tom Sontag]

Your question is very specific and exactly the kind of questions structural engineers answer daily. A very experienced contractor mind be able to give you an answer too, but unless one of these types posts here I sure would not let the internet answer your question for you. They have no incentive to answer you either since it could have legal consequences. Do this right and go to someone willing to put skin in the game: a contractor, an LVL beam manufacturer, or equivalent. Code requires the calculation use a minimum number for live load, so your expectations there should not be a factor.

In short your question has a calculable scientific answer and everything else is just a guess.

[Brett Bobo]

Hi Bart,
As a structural engineer, heed Tom's advice. Yes, it's fairly straightforward to provide a beam size for your application but you'll be best served having a local supplier design and supply the required header size. Regarding the exterior application, there are treatments for engineered lumber, including LVL's, and there are adjustment factors for the design based on the exposure/moisture content. Additionally, there may be other components that require modifications and/or need to be considered as well, e.g. column sizes, header to column connection details, foundation requirements, etc. Typically, in longer spans such as this deflection becomes the governing factor in the design. Granted, deflection is just a serviceability limit state but there are code requirements.

[Bart Kiefer]

All,
Thank you for your replies, I will have to get with an engineer to see what he/she advises. Thanks once again.