Building a new one story house and should close on the construction loan next week.
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Due to the slope of the lot, there will be a nice tall crawl space under most of the house, varying between somewhere around 5’ at one end to at least 8’ at the other end. I plan on having the foundation contractor pour a rat slab for the crawl space. There is enough room to put the furnace, water heater, duct work, and lots of storage space left over.
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I see two possible answers to supporting the floor.
Option 1. two rows of support posts in crawl space supporting floor beams made up of two or three 2×12’s. That would leave a maximum span of less than 12’ for the floor joists, either 2×8 or 2×10 on 16″ centers
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Option 2. one row of support posts supporting one central floor beam – truss or I joist. That would mean a span of about 16’ for each floor joist – either 12†or 14†I joist on 16″ centers?
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So, two rows of posts, 2×12’s and 2×10’s or one row of posts with truss and I joist?
Which makes the most sense from an economic stand point?
Is there a really good reason to go with the more expensive answer anyway?
Or, is it all what the framer’s would rather work with that matters more?
Replies
That sounds almost like a basement rather than a crawl space. Less than 3 feet of additional excavation and you'd have a really nice basement. It would be really cheap additional space and might be worth giving it some consideration.
It would have made a good walk out basement and I gave it a whole lot of consideration. However, it would have taken a lot more foundation work, and the dollars are just not there.
Paul, I have a "crawl" that goes from 5'6" to 6'2"...money was our issue, too. It pizzes me off, how stoopid cheap I was, every time I bang my head into a pvc drain line or a duct box...Don't your plans have a floor framing pan? "But to be honest some folks here have been pushing the envelope quite a bit with their unnecessary use if swear words. They just put a character in to replace a letter. But everyone knows what they're saying." Sancho FREE CAROLE
>> Option 1. two rows of support posts in crawl space supporting floor beams made up of two or three 2x12’s. That would leave a maximum span of less than 12’ for the floor joists, either 2x8 or 2x10 on 16" centers <<
A span of 12' or less is reasonable for 2x10s. Much over that the floor will bounce. Here, we don't use 2x8s for primary floor systems, so IMO that is not an option.
>> Option 2. one row of support posts supporting one central floor beam – truss or I joist. That would mean a span of about 16’ for each floor joist – either 12” or 14” I joist on 16" centers? <<
The thing you need to understand about I-joists is that they are not much stiffer than dimensional lumber of the same size, but the good news is that they are available in taller sizes than common dimensional lumber - 2x12s or whatever. You can get a real lumber supplier (not HD or Blowes) to design and price you out a floor system of I-joists. Ask for a minimum of L-480 design value. I'm gonna make a SWAG that a 16' span with 12" I-joists would suck.
>> Which makes the most sense from an economic stand point? <<
My experience tells me that 2x10s and the extra beam and piers will be the most economical, but we cannot do your homework for you. You really need to figure it out yourself. The I-joist supplier will do that part of it for you, but you still need to figure the concrete footers, masonry for the piers (I assume it is masonry) and the dimensional option.
BTW, here we would most often use triple 2x10s for beams and space no more than about 6'6" OC. This kinda crowds up your storage space though... With 2x12s you can go somewhat over 7'. It all depends on what spans are being carried and what loads might be carried down through the walls to the floor system supports. Is the roof to be stick framed or trussed?
Also, BTW - don't you have a set of plans? Aren't they engineered? If it were me I wouldn't be "winging it" on stuff like this, Better to get a design professional and let him take the responsibility. Here, once a set of plans is drawn by a designer (not an architect) it costs about $800 to have them engineered - well worth it IMO. You could easily blow $800 on an overdesigned floor system too. Anyone can look at a span table, but there is a little more to it than that.
Another thing to think about when selecting material and layout for a floorsystem over a crawlspace is insulation - if you are planning on insulation the floor. For example, if you have a 19.2" layout you may have trouble finding batt insulation to fit in-between the joists - although they do make it. Likewise standard 16" OC insulation may not fit snugly between 16" OC I-joists. This is a real problem with open webbed floor trusses. OTOH, if you are going to insulate the crawl perimeter this isn't an issue.
Edited 1/13/2007 4:15 pm ET by Matt
The designer is charging me a lot more than $800, but that is fine. I just want to know which option makes sense and why before I sit down with him again and discuss some of these details. I have no guarantee that the designer will come up with the more cost effective answer, quite often they just go with the way they've always done it.The perimeter of the crawl space will be insulated.I do not know if the roof will be stick built or truss, another discussion for the designer and I to go through. Again, I want the most cost effective answer, and it is starting to sound like trusses will be it. I'm not planning on using any of the attic for storage space anyway. The house is essentially a big rectangle with the front and back porch sticking out. So, the roof is going to be a lot simpler than most of them being built these days.thanks for taking the time to answer my dumb questions!
The $800 is above and beyond the designer fees. It is for a PE - a professional engineer licensed to do engineering in my state.
You are going about it the right way though... get an idea of what you are going to build - then you, as the builder, decides what methods and materials are appropriate for the project, and the designer "roughs in" these on the drawing, and then licensed professionals do the floor calcs, etc. where he basically just adds his notes to the drawings and then stamps it (maybe the last step is not required where you are). If you use an engineered floor system (I-joists and an engineered roof system (trusses) you may not need the PE as the suppliers engineer it for you - once you commit to buy from them they supply the stamped floor sys and/or roof sys drawing. Sounds like you need to make the same decisions for the roof.
Be warned that suppliers can be slow in pricing our stuff for you. Since you say it is a mostly a simple box, they should be a little quicker. Can't hurt to check more than 1 supplier too.
As I said, if it is to be a stick built roof you will almost certainly have load bearing walls and/or load points that carry down through the walls too the foundation. If you have no intention to have permanent stairs up to some second floor storage/bonus/whatever area, then roof trusses will probably make the most budget sense. You can still get storage roof trusses and have access via a pull down ladder-steps.
BTW - on the project I am currently working on for the 2nd floors I priced out Metal plate floor trusses, I-joists and Trim joists - and that was the order of cost as they came in; Floor trusses the least and Trim-joists the most, which shocked me that the floor trusses are cheaper, especially since one supplier told me no-way they would be cheaper. The $$$ figures don't lie though - I think it has partially to do with the crazy fluctuations in lumber prices we have been seeing. I really wanted the floor trusses so I could easily run mechanicals and plumbing through there. Worked out great, but not a consideration with a crawl space. I really think that 2x10s will be cheapest in your case, and as far as I'm concerned, there is no long term down side just so you keep the spans short; say < 13'. If you look at span charts they may allow up to like 14'10" but the floor performance will suck. But like I said, you gotta price out your different scenarios.
BTW - if you want more spacing between the crawl space piers that support the beam(s) look into using LVLs for the beams. It will be more $$$ though. Or, how about LVLs just in the 8' tall area?
At my house, I have a 7' walk in crawl space - I specifically bought a down slope lot so the house would be set like that, and I like the look of no front porch railings, and a somewhat low foundation. The front of the crawl is about 2.5'.
The reason I asked about the plans is that you said that your bank loan is nearly ready, and normally the bank wants completed plans before they want to write the paper.
I thinking I must be a building geek - if I can voluntarily spend 30 minutes on Saturday night talking (typing) about floor systems ;-)