Decks, Patios, Porches and Docks - Deck spans - tables vs lowes designer
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aldorr
10-01-07, 01:42 PM
I've been designing a deck, planning on 12' deep x 32' wide.
Deck is going to be about 9ft off the ground, so my plans were calling for:
5 6x6 posts, doubled-2x12s for the beam. Beam would be set at 10' from the house, allowing for a 2' cantilever. Then using 2x10s, 16" o.c. for the floor joists.
From all the span charts I've been looking at online, seems like this would be fine. However, after plugging everything into the deck designer on Lowes' website, their tool states that I need a 2nd beam, placed 5 feet out from the house or it says I need to use 2x12 joists instead of 2x10s.
Seems like overkill to me, but I want to make sure I'm not miscalculating something here. Anyone have any thoughts or suggestions?
(I should note that the house uses 2x10s for the floor joists, so I didn't think I could move up to 2x12s for the deck joists and still be able to easily attach the ledger board to the house, with carriage bolts.)
Thanks! - Al
Deck is going to be about 9ft off the ground, so my plans were calling for:
5 6x6 posts, doubled-2x12s for the beam. Beam would be set at 10' from the house, allowing for a 2' cantilever. Then using 2x10s, 16" o.c. for the floor joists.
From all the span charts I've been looking at online, seems like this would be fine. However, after plugging everything into the deck designer on Lowes' website, their tool states that I need a 2nd beam, placed 5 feet out from the house or it says I need to use 2x12 joists instead of 2x10s.
Seems like overkill to me, but I want to make sure I'm not miscalculating something here. Anyone have any thoughts or suggestions?
(I should note that the house uses 2x10s for the floor joists, so I didn't think I could move up to 2x12s for the deck joists and still be able to easily attach the ledger board to the house, with carriage bolts.)
Thanks! - Al
Concretemasonry
10-01-07, 02:34 PM
Both could be correct.
Design of a structure has two factors - strength and deflection.
Just because something is strong enough to carry the load, does not mean it is a good usable design for people.
There could also be a difference in the grade of lumber and the modulus of elasticity.
At 9' high you will still have some minor lateral stability that could be compounded (from a mental/comfort standpoint) by deflection or "springiness".
Charts are not always correct, but they could be the bare minimum, which is not always right.
Design of a structure has two factors - strength and deflection.
Just because something is strong enough to carry the load, does not mean it is a good usable design for people.
There could also be a difference in the grade of lumber and the modulus of elasticity.
At 9' high you will still have some minor lateral stability that could be compounded (from a mental/comfort standpoint) by deflection or "springiness".
Charts are not always correct, but they could be the bare minimum, which is not always right.
lefty
10-02-07, 09:37 PM
The Lowe's program is blowing smoke!
Using 2X10's at 16" O.C., at a 10' projection you are WELL under what they can span, unless you have a snow load issue that you haven't mentioned.
But why the 6X6 posts if you are only using a double 2X beam?? I would use 4X6 posts in that application. A double 2X is only 3" wide. A 4X6 post is 3-1/2" wide and will fully support it. The 6X6 posts are simply a waste of money in this case.
Using 2X10's at 16" O.C., at a 10' projection you are WELL under what they can span, unless you have a snow load issue that you haven't mentioned.
But why the 6X6 posts if you are only using a double 2X beam?? I would use 4X6 posts in that application. A double 2X is only 3" wide. A 4X6 post is 3-1/2" wide and will fully support it. The 6X6 posts are simply a waste of money in this case.
MissTFried
10-03-07, 07:37 AM
The only thing that is absolute when it comes to construction, is that nothing is absolute. There are always unknown variables that out of ignorance, someone or something can refute.
As you have already seen by your experience with Lowes, the charts that you have researched, and the answers you have and will recieve from the forum, there is always a difference of opinion when it comes to construction techniques. Many times, the difference in opinions are based on interpretation. Is eveyone recieving and understanding the same information ?
Also, remember that Lowes recognizes that there is a fine line when it comes to providing a service (the deck plans provided by the software program). They potentially have a huge liability risk if a system or method they recommend fails. They are a big target. So, when it comes to recommendations or advice, they will always err on the side of caution. You can bet that their software programs reflect that caution. Different software programs will come to different conclusions based soley on their programming.
What should you do ? Exactly what you are doing. Get different opinions, weigh the advantages, disadvantages and consequences, be sure you follow local codes to avoid a problem, and then make a decision.
As you have already seen by your experience with Lowes, the charts that you have researched, and the answers you have and will recieve from the forum, there is always a difference of opinion when it comes to construction techniques. Many times, the difference in opinions are based on interpretation. Is eveyone recieving and understanding the same information ?
Also, remember that Lowes recognizes that there is a fine line when it comes to providing a service (the deck plans provided by the software program). They potentially have a huge liability risk if a system or method they recommend fails. They are a big target. So, when it comes to recommendations or advice, they will always err on the side of caution. You can bet that their software programs reflect that caution. Different software programs will come to different conclusions based soley on their programming.
What should you do ? Exactly what you are doing. Get different opinions, weigh the advantages, disadvantages and consequences, be sure you follow local codes to avoid a problem, and then make a decision.
logcabincook
10-03-07, 01:44 PM
Your deck design sounds fine to me and very close to what we did. We beefed it up a little for snow load and because we overbuild everything, and the county's plan review added a few details. Ours is 28' by 14' with the posts at 12' and a 2' cantilever. When you submit your plans to the county, they should be able to point out any flaws in your plan. Our county is a stickler for weight and wind since the blizzard of 03 collapsed so many decks and roofs and we get hurricane force winds somewhat regularly.
Pendragon
10-03-07, 09:44 PM
The Lowe's designer may be using spans based on the lumber they sell.
Ratings for indoor and outdoor service are different.
For wet service (such as a deck), 40psf live load, 10 psf dead load, 360 deflection, a 2x10 SYP @ 16" oc:
Visually Graded
SS- 16'5" (SS = Select Structural)
#1- 16'2"
#2- 15'10" (#2 is probably the most common)
#3- 12'2"
Machine graded spans are slightly higher.
As the load requirements go up, the span gets shorter.
Regardless of these ratings *I* wouldn't span that distance with a 2x10 or a 2x12 for a deck, it's just going to be too bouncy.
Ratings for indoor and outdoor service are different.
For wet service (such as a deck), 40psf live load, 10 psf dead load, 360 deflection, a 2x10 SYP @ 16" oc:
Visually Graded
SS- 16'5" (SS = Select Structural)
#1- 16'2"
#2- 15'10" (#2 is probably the most common)
#3- 12'2"
Machine graded spans are slightly higher.
As the load requirements go up, the span gets shorter.
Regardless of these ratings *I* wouldn't span that distance with a 2x10 or a 2x12 for a deck, it's just going to be too bouncy.