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How do you determine the load of a roof and what method to support the load?

How do you determine the load of a roof and what method to support the load?


Old 07-30-10, 07:04 AM
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How do you determine the load of a roof and what method to support the load?

Hey all,

I had a draftsman draw up some plans for a front porch and after submitting the drawings to the county for plan review and permit acquisition, I'm starting to second-guess the plan details.

To explain the porch, it's going to be a 8'x20' front porch. The porch will be secured to the house using a 2x10 ledger board. The frame of the porch will have double-rim joists (2x10) on the three sides, with 2x10 joists running 12" O.C. to the ledger. The porch will be sitting on six columns, each column being a PT 6x6 sunken 24" into the ground; sitting atop 10" of concrete and backfilled with more concrete. The porch boards will be Azek PVC boards.

Now, in the drawings I had Vinyl columns with metal inserts used to support the roof. There were six columns, each being a load-bearing column capable of supporting 14,000lbs and positively connected to the rim-joists using 900lb updraft kits. Also, I think it's important to note the columns are set directly above the footers, so the weight is being transfered directly to the ground and not run through the frame of the porch.

Since I've turned in my plans I've visited a showroom and learned of Timbertech's Radiance composite railing system and I prefer the matte look of the finish to the high-gloss sheen of the vinyl railing. My concern is the Radiance 12' 5"x5" cover fits over a 4x4 post. Since my plans are being submit to the county for a 6x6 column that supports up to 14,000lbs, will 4x4 columns be ample substitution? Six vinyl columns have a combined load of 86,000lbs, but the only thing it'll be supporting is a roof, lighting, shingles, etc. Doesn't that seem like overkill?

To detail the roof - The header is going to be using three 2x8 which will be supported by the columns and tied into the house. The headers will be holding the weight of five factory-engineered trusses. The roof will be covered in 3/4" plywood with paper and shingles on top. That whole construction isn't 84,000lbs of weight. I live in Northern Virginia and we had 2ft. of snow this past winter at one point, but even then it's not 84,000lbs...

So, what do you guys suggest I do now? How do I calculate the load of the roof and, with that, how should I find a good alternative to the vinyl columns? Should I use a PT 4x4? Should I look at getting rust-resistant loli columns? Is there another option... or should I just stick with the vinyl and deal with the unsightly contrast between sheens?

Here are some of the drawings...

Thanks in advance for any suggestions you may have.
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Old 07-30-10, 08:25 AM
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That's not up to you to do those calculations. That's why you hired someone to draw the plans. The building dept. engineers will tell you if they aren't correct. If you don't want to depend on them, hire an architect to review the plans.
Old 10-01-12, 11:25 AM
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Load other than Static Load

While the load given to you may be high for your static load, it may be calulated taking into account non static loads such as wind and surface area and 100 year factors for snow. Also it is typical to double actual load to account for unusual circumstances, such as a limb or other object hitting the roof.

If you want to change from a 6X6 to a smaller size, I recommend you find out if there is either an engineered lumber 4X4 that is equivalent or a steel column that is equivalent in load capacity. Also note that you not only should ask about diameter, but also what steel wall thicknesses are available. Also remember that the attachment of railing brackets to steel will need to be drilled and tapped and done with machined screws or self tapping screws. Depending on the steel thickness will determine which option is better IMO.

BTW, I am not a structural engineer, but have found if you study what you think are good recommendations and then take that to a Structural Engineer, he may charge less and you will get the information that is most useful to your point of view. Hope this helps!
Old 10-03-12, 11:22 AM
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It should be noted that in terms of supporting your roof; you have four columns and not six. The two columns in the front will help support the porch deck and they will give some very minimimal support to the roof but not enough to matter. The trusses will bear on the two sides of the porch and the loads will be carried horizontally on the (3) 2x8 heads to the four corner posts. The load bearing capacity of the other two columns should not be considered in terms of the roof load.

You should also consider the fact the floor/deck of the porch and the people on it have to be carried by the posts as well (and maybe the wall of the house). In general a floor load will be double what a roof load is. The floor load would probably be somewhere near 7 to 8 tons maybe more. I know you will probably never get anywhere near that much weight on the deck but you still have to design for it.
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