I would appreciate any expertise regarding expected charges incurred for the engineering and drafting of plans (for gaining a building permit) of a concrete block wall garage with a flat roof supported by sistered 2×12 rafters. The building will be 24′ wide by 32′ long. The roof loading is 50 lb/sq ft, which requires a wet stamp – if it was only 30 lb/sq ft, I could go with a standard prescriptive method without a stamp. It is actually a woodworking shop, so there is only one 12’x10′ rollup door in one end. There will be just one 3’x4′ window on each side and a single man door on one side.
A roof loading of 50′ seems to be code in high snow areas, but I couldn’t find stock plans that are drawn to that roof loading. There are outfits on the Internet that advertise custom plans with an additional fee for engineered structures.
Any pricing information would be appreciated.
I should have added that the drawings should be as “bare bones” as possible and do not need to contain any electrical or plumbing information.
Thanks
Edited 7/5/2008 6:12 pm ET by CaseyR
Replies
why sistered 2x12 ?
i'd be looking at trusses.... like maybe steel bar-joist ?
supported by sistered 2x12 rafters.
What are sistered rafters?
The roof is not totally flat but will have a slope of about 1 in 24. I was told by an engineer that I talked with that doubling or tripling 2x12s would be the cheapest way to get sufficient strength to support the roof loading - sort of do it yourself beams on the cheap. I have not actually checked out just how much more expensive actual beams would be rather than the multiple two bys approach.I need the roof as low as possible to keep the county planning department less grumpy. The trusses I looked at were about a foot deeper than going with lumber.
"I was told by an engineer that I talked with that doubling or tripling 2x12s would be the cheapest way to get sufficient strength to support the roof loading -"So it sounds like you already had your engineering done, right? So why the question?This gets more confusing though since that statement is wrong, unless you get a good deal on dimension lumber and your labour, glue and nails is/are free
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Draw your own plans. Scan them to a PDF file and send them to five different engineering firms and ask for a quote.
Bob's next test date: 12/10/07
Casey
Try this span calculator for your 2x12 span.
Span Calculator
With a single 2x12 I came up with 19' 7"
Even with your 2x12 doubled I don't think you can span 24'.
I would look at engineered trusses.
Rather than cut your 2x12 for slope, you could get sloped insulation under a rubber membrane.
My 2 cents.
Rich
I too would look into engineered trusses or a engineered wood "I-beam" rafter such as Trus Joist or webbed "flat" trusses. If your not comfortable drafting your own plans look into hiring a local designer. Sounds like a very straight forward project, and if you go the truss route the truss manufacturer would be responsible for engineering if all they want is the roof portion. The building department is primarily looking for a system that will meet the criteria of your snow loads. Throwing in the double 2x rafters probably threw up a red flag.
If I buy I-joists at my lumberyard, he has engineering available through his supplier. I don't remember what brand he sells. I don't know if an I joist will do what you want, but you could find out for free. They just need some sort of simple drawing.
Same experience with CraigF. Needed a roof for 2 car carport without the center carrying beam. Decided to use LVL and got the manufacturer number from the supplier. The manufacturer wanted a simple drawing and type of roofing material and returned with engineering sheet that the building inspector accepted. It was free for the purchase of the material. Include: if you deck the roof with 1 or 2 layers of 3/4 inch plywood (glued and maybe screwed or use 10 or 12d nails) can you downgrade the lvl? Compare the costs. Hope this helps.
My problem with I-beams is that they look too much like kindling to me. I am in an area that is periodically swept by wildfires and I want something that is going to be standing after a firestorm sweeps through. I am planning on going with concrete blocks and green roof in large part because I feel it will be more fire resistant than standard construction, but I need something to support the green roof that I can afford. I think an over-engineered trippled 2x12 with an allowance for maintaining strength while getting charred in a fire might be the minimum that I would want to try around here.
you want green? easy engineering? go with parrallel chord roof trusses. The truss company does the engineering as part of the package.
Or the open web trusses you mention.
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Given that added info, you want LVLs and a 70# design minimum. 50# would account only for the green roof, but not for the water it can hold
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Pif -
The green roof I specified called for 3" of light weight growing medium which listed a saturated weight of 20 lb/sq.ft. I allowed an extra 5 lb/sq ft for the roof barrier and membrane and added that to the code required snow load of 25 lb/sq.ft to come up with the 50 lb/sq.ft. loading that I requested.
Well, if it beats code, it must be right, right?You know your climate better than i do.But regardless what number you are designing too, trussing or engineered will be far better for you than solid lumber on this
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I am not sure if fire insulated or metal roof would survive a structure engulfed in flame. If you want such survivability, maybe you should consider commercial type of concrete or light weight concrete roof poured on corrugated metal supports? Go from there with green roof as usual. I think for something like this you would have to pay for engineering. Good luck.
Yeah, I originally specified either concrete or metal roofing but I was told that I probably couldn't afford it. I did have the building engineered (see my other post in "Business") but it appears that I will have to start over with a new set of assumptions.
Locally, what you're asking for is $2/sqft +/-. It would save about 25% to draft it yourself, and probably another 25% to shop around and find a place that's a little slow, or that can throw a new engineer on it.
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