Your questions don't make much sense. Do you mean flooring rather than roofing? What is the span, spacing and location of the joists?

The dead load of tile flooring should not be a factor in the choice of bearing wall studs.

All of the information you need should be provided in the local building code so if you do not understand that information you need to hire someone to design the structure for you.

Yep, a professional should be looking at the engineering of the structure, but for what it's worth, our second story FLOOR joists are trusses measuring like 2 X 10s, enough space for stuff like flexible ducting, light cans, and in our case R19 insulation (the second story normally stays shut off (except this morning I have a house full upstairs sleeping).

Bevangel wrote:
"In answer to your original question, my two story house was designed to allow HVAC to be run between the floors. The space between the floor is 18 inches."

So if I have 12' cealing in each of my two story house I will end up with 12' + 1'5' + 12' + whatever the height of the roof/attic turns out to be.

I get confused with srercrcr's following answer:
" our second story FLOOR joists are trusses measuring like 2 X 10s, enough space for stuff like flexible ducting, light cans,"

10" or 18"? Wich one is the correct answer?

The correct answer depends on the design, which in turn depends on the designer, who depends on the requirements. The requirements come from the prospective homeowner, the building codes, available materials and skill sets, cost constraints, and local practices.

In other words, there is no standard or universally correct answer.

There is no absolute set answer. The necessary joist height (thickness) depends in part on the distances you are trying to span. It also depends on the load you will be putting on the joist, how widely you space your joists, and the type of wood your joists are made of. A good grade of 2X10 lumber might be used for joists spaced 16 inches apart where the longest clear span is no more than say 15 feet. If you needed to span a room that is 20 feet across with lumber of the same quality, you might need to use 2x12s that were spaced no wider than every 12 inches.

You mentioned a 7000 square foot home. I would lay odds that you intend to have large open rooms downstairs. That fact alone means you will need thicker joists. The whole issue of space between the floors for can lights and ductwork is secondary compared to making sure that your joists will support the load that is put on them.

This is NOT an area that a novice should be playing around in. I have an undergraduate minor in physics and despite having recently read far more about joist loads and span table that I ever wanted to, and I can barely comprehend the subject. I would NEVER trust myself to actually try to determine the proper size joists for a house without help from a structural engineer. And frankly, I rather doubt most architects - who are only required to take a very minimal amount of physics to get their degrees - really understand span load tables either. I suspect (pray) they confer with qualified structural engineers whenever they are designing anything out of the ordinary.

But, let me give you two examples and tell a cautionary tale...

In our current house, the joists are 2x12 lumber. (I know b/c several years ago I repaired the sheetrock in the dining room after we had a leaking pipe.) However, our current house is a very simple rectangular shape of only 1900 sq feet. It has a central staircase with rooms on either side. The joists between the 1st and 2nd floor run perpendicular to the staircase and the widest room they have to span is only 14.5 ft wide. The HVAC unit also sits in the middle of the house (behind and just to one side of the staircase) so the AC ducts run parallel to the joists (i.e., each duct lies between two joists) to reach the rooms on either side of the staircase. A soffit over my kitchen cabinets houses a duct that runs perpendicular to the joists from the HVAC unit to the back of the house. Parallel ducts T out of the top of this to reach the rooms that are at the back of the house. No holes had to be cut thru the joists to accomodate the ductwork. Meanwhile, all our upstairs plumbing is centralized over a single "wet wall" so all the drainage pipes run straight down. Again, no need to cut holes thru those 12 inch joists. The only holes drilled through our 12 inch joists are little tiny ones that accomodate electrical wiring.

Now the second example and the cautionary tale...

Our new home - still under construction - is a 3200 sq ft house with a much more complex design with much larger rooms downstairs. I will also have two large cast-iron bathtubs upstairs and wanted to be sure their weight, when filled, would not be a problem. My architect and his engineer specified 18" deep open-web trusses spaced 16 inches on center.

To save money however, my builder elected to use 18 inch i-beams joists made of oriented strand board (OSB) instead. (Actually OSB is stronger than real lumber when used for ibeams.) The engineer at the lumber store where my builder bought his lumber package told the builder he could place safely place the OSB ibeam joists 24 inches apart.

I was out of town for several days while framing was going on so first time I saw the i-beam trusses, my builder was already framing up the second floor. I immediatley insisted that he stop what he was doing and verify with his engineers that he would be able to cut large enough holes thru the ibeams to accomodate my HVAC ducts BEFORE he went any further.

I should have just told my builder he had to follow the specs but I was trying to be reasonable. I told him that IF holes could be cut through those ibeam joists large enough to accomodate HVAC ducts without compromising their structural integrity, he could go ahead and use them. But, if not, he needed to back up, take down the second floor framing, and replace the ibeams with the open web trusses specified in the plans.

Two days later, in one of many many lies he told me, my builder said he had checked with his engineer and that the HVAC ducts would be "no problem"... that I should "trust him" and "let him do his job" because he was a professional. Foolishly, I did so.

Later - after my house was fully framed, sheathed, roofed, shingled, sided with Hardie, had all windows and doors installed, and the exterior was painted - I learned for the first time what my builder meant by "no problem." It meant he had unilaterally decided that he would move the two HVAC units and ductwork up into the attic so holes would not have to be cut thru the ibeams for the ductwork!

He completely ignored the fact that this decision meant the HVAC would no longer be in the heated and cooled envelope of the house which would increase my utility bills AND require him to purchase larger and more costly HVAC units; that running flex duct down from the attic to the first floor rooms required chases which took significant amounts of space out of each of my bedroom closets; and that positioning two HVAC units and a spiderweb of flex duct in the attic made it completely impossible to ever consider putting in an attic bedroom - which was the whole reason I had elected to have a 12/12 stick built roof in the first place!

But my cautionary tale continues...

It then turned out that the necessary holes for the plumbing drainage pipes was too much for the joists! In some cases the joists span distances of 25 feet. Once the plumber cut a 4 inch diameter hole through several joists to accomodate a toilet drainage pipe, the joists started bowing inward.

I noticed rather quickly that the sub-floors on the second floor were no longer level. My builder tried to tell me that it was "normal" for the subfloors to bow a little but that putting the final layer of flooring on top would correct the problem! HAH! By this point, I was long past trusting anything the man said.

We are building out in the country where no inspections are required except for septic systems but I had included in my contract a provision allowed me to call in an independant third-party inspector at any point. The contract stated that the third-party inspector's finding and recommendations regarding the safety or structural integrity of any element in the house would be controling. In other words, if the inspector said something needed to be done, builder had to do it at his cost.

The inspector required my builder to go back and put an additional 18 inch i-beam joist between every pair of existing joists that had had a hole cut thru it. So now the joists are spaced every 12 inches instead of every 24 inches and the second floor no longer bows.

I don't know what it cost my builder to put in the additional joists and then to replace the drain pipe he had to cut... but I'd lay odds the total cost was more than if he'd used the open web trusses in the first place.

In case you're wondering, we ultimately fired the builder and filed suit against him after even more issues reflecting his incompetance and dishonesty. Now I'm the GC to finish the build... and believe me, I'm relying heavily on my third-party inspector to help me get things done right!

The bottom line, simply by asking "10 inches or 18 inches? which one is the correct answer?" you reveal that you have a lot to learn before you proceed. You might start by reading this link.

Here is a link that might be useful: Tutorial on joist design

Bevangel,

I read your long post with great interest. I am impressed with your thoughtfulness. Thank you very much for the advice.
Your post will help others too.

right now I am designing the home in Autocad 2010. I was thinking If I leave room for 18" joist then it can't go wrong but if I leave room for 12" only then something can go wrong.

Yes, you are correct, I will have a lot of open areas, long span in my first floor.

since you are a lot more experience than I am let me ask you another question.

Do you think it is wise to use an owner builder program advisor to help me build one's home or just give it to the GC? Of course I want to save money but despite of my technical background I am not fully confident in my knowledge, skill, and experience of home building.

with owner-builder type program, I think, I can save $100K in $700K house, a 14% saving. I will put in about 20 hrs./week for a year.

Do you know subs? Will they give you priority or put you at the back of the list?

My thought is that you will not save $100k. My builder charged $80k on a $600k build - and there of course is consulatant costs. I'd expect to pay $20k to a consultant. There are lots of things the supervisor did that were included in that $80k - probably another $20k worth. So in retrospect, I probably could have saved $40k by GCing myself with a consultant.

This is assuming the consultant would essentially give me builder prices from all the subs.

Now - if I looked at every dollar and did competitive bids on everything, I could have saved more. It is tough to know if this would have been worth the trouble. Most subs seemed quite reasonable.

I'll just throw my $.02 about the original question. The vast majority of houses (around here) have attic run HVAC ducts. Is that ideal - no. Is it reasonable - yes. Whether it is worth doing I-joists between floors to keep the ducts in the envelope depends on many factors - especially climate. It is problematic enough to get the ducts between floors that (around here) to get ducts in envelope means foaming the attic.

Why does this matter so much? Well - I think your distance between floors is going to be higher if you want the ducts in this space - that just makes sense. That makes your stairs cost a bit more, your siding cost a bit more, more stairs to climb etc.

I'm going to guess that 12 inches is a good average build and you will need to go to 18 for ducts in that space. If you are doing ducts in the space, your vents will obviously be on the floor and you must plan for a significant chase from the basement/crawlspace.

So the first question is - where do you live and where do you plan HVAC duct work?

To help answer the question, ducts in the envelope in my mixed humid climate reportedly save 6% on HVAC costs. For a well insulated 7000 sq foot house, you are probably looking at $2000 a year IN MY CLIMATE. So the savings would be $120/yr. Sources : Energy consultant for Energy Star program. There is a lot of wiggle room in that $2000...

So many things depend on where you live that you need to remember that on these (absolutely wonderful) forums.

"I was thinking If I leave room for 18" joist then it can't go wrong but if I leave room for 12" only then something can go wrong."

And if the span is large enough even 18 inches may not be enough.

The joist depth is not an arbitrary number.

Joist span tables are available from all the manufacturers that supply joist material (2x lumber) or engineered joists (standard trusses, i-joists, gluelams, etc.).

There are some issues with engineered joists.
The deflection limit is 1/360 of the joist span.
For long spans this results in floors that feel bouncy, even though they are strong enough and meet the code.

Smaller deflection limits are common on long spans to produce a more solid floor.

Putting the HVAC equipment in the attic to serve the upper floor is the standard approach in my part of the world.

The alternative approach (when the attic is not available) is to feed the upper floor vertically from the basement at multiple locations. I have never seen main trunk ducts run horizontally through a second floor since that usually requires expensive custom open web trusses. I-joists can only accommodate high-velocity HVAC systems sometimes used for renovations.

The attic approach requires that the attic space be insulated at the rafter plane so it will be included in the conditioned envelope of the house. Frankly, I would do that even if the HVAC equipment were not located in the attic space.

I can only say that I wish I had gone ahead and used an owner builder advisor program instead of hiring our snake-in-the-grass builder! DH and I considered doing it ourselves. We even took an 18 hour evening class on how to be your own contractor from a fellow who works with Owner Builder Network but who definitely did not push that particular program in his class.

I felt I had the personality to OB and knew that I was willing to learn all that I needed to know to do it. But, at the time we were starting our build, I was working 70 to 80 hour weeks on a regular basis. DH had more time available, but he just wasn't that interested in house-building details. While I soaked up everything said in the housebuilding class and enjoyed every minute of it, DH was bored within ten minutes and only kept going the entire six weeks to make me happy. Just, different personalities and different interests.

So, after taking the class, DH and I decided that, for us, it would be worth it to pay a professional builder to do all the worrying and scurrying and planning and dealing with subs and materials and details. So we went that route.

But then I very quickly wound up spending way more than 20 hours per week just trying to keep tabs on our builder - while I swear, he was doing everything in his power to keep me in the dark! Ultimately, building has actually been less stressful since we fired him. But maybe only because things slowed way down at work and I now have the time to devote 30 to 40 hours per week to housebuilding tasks and studying up on all things related to housebuilding.

I think if you were to find a good honest builder - and despite my experience, I do still believe that there are many many fine builders out there - he will be worth every penny you pay him. If you wind up with a snake-in-the-grass (and unfortunately, there are a lot of those out there as well) then you'll wish you had gone the OB route. There are risks either way.

While saving money is obviously an inducement to owner-build, a better reason to do it is if you honestly believe you can build a better house GCing it yourself. After all, no professional builder will ever care as much as you do that your dream home is built right.

Only you can assess whether you have the personality, desire, time, and commitment to OB. But yes, if you go that route, I would definitely pay for the guidance offered by a good advisory program. But, check them out in advance the same way you would a builder. Talk to other OBs who have used the program IN YOUR AREA and make sure you know what you're supposed to get for the money you pay. Also find out just how helpful the advisor you will actually be working with has been with previous clients. How much you get out of the program is going to depend heavily on just how good your particular advisor is.

What is an "owner builder program advisor"?

munzer1-

We are also building a larger home with large spans (20') in some areas. Our designer was the world's worst and her "enigineering" was so faulty that even the county refused to give us a building permit until they had reviewed a set of independently engineered drawings. There are engineers who do just this for a living. We found ours through our local building supply houses. You may want to take a copy of the floor plan to your local building supply house and see if they can put you in touch with an engineer who can then tell you what you will need.

Macv-
I believe the Owner Builder Network that was mentioned in an earlier post is a national outfit that works with owner-builders to construct their new home. They provide a consultant & 3rd party inspector as well as volume disounts on materials (like a regular GC would get), & help with scheduling and pulling permits. They can also help with financing. All for 4-5% of the cost to build the house. I've not worked with them, so all this info came from their website.

Here is a link that might be useful: Owner-Builder Network

I believe the Owner Builder Network that was mentioned in an earlier post is a national outfit that works with owner-builders to construct their new home. They provide a consultant & 3rd party inspector as well as volume disounts on materials (like a regular GC would get), & help with scheduling and pulling permits. They can also help with financing. All for 4-5% of the cost to build the house. I've not worked with them, so all this info came from their website.

They are not national. They appear to have exactly six offices, all of which are in Texas. Those offices are independently owned, which indicates they are franchised.

All from their website.