Structural Questions

[thomcarol]

We are about to start building our house and I wanted to verify some info I have received. We are building a house similar to the 20x30 1 1/2 story on the site except we are expanding it to 24x40 and enclosing the entire 2nd floor like I have seen some people do on this site. We are also raising the walls from 10 ft to 12 ft, with 1st floor ceiling at 8ft and the 2nd floor with a 3 ft knee wall. We are digging for 24 piers and using 6x6's coming out of the ground. We plan to attach tripled 2x12's beams running the length (40ft) on top of the 6x6's forming 4 beams running the length of the house, and using 2x12's for the joists. We are balloon framing and notching out the studs for the 2x4 supports so the second floor can sit on them. Over the living/kitchen area we will use 4x12's at 24 or 36 in oc. The roof is going to be 12/12 and living in Alabama, there is really no snow load to speak of. We were planning to use a 2x12 ridge and 2x10 rafters at 16 oc as we do get wind anywhere from 30-60 mph from time to time. I have gotten various opinions on the sizing of our ridge and rafters so I would appreciate your ideas. Usually builders around here don't use collar ties and I don't know if we need them on our house. My main concern is that we are using the correct size of lumber in the foundation and roof. I know that since we have widened and hieghtened the house we need to take extra steps to make sure that it is sound. Thanks for all of your advice in advance.

[MountainDon]

The 2x10 rafters are fine, assuming you use something like #2 SYP on 16" centers. The rafter calc says with a DL (dead load) od 15 psf and a LL (live load) of 70 psf they can span up t0 13'9". So that should withstand the winds. DO use collar ties; doesn't matter what others do. Others are not always right. Collars ties help hold the roof together in those winds.  The ridge board is of suitable size for rafters... it is not a load carrying beam.

BUT, there are other questions this raises. 

1. Do 2x10 provide enough space for roof insulation? Is there still room for venting with insulation? What type of insulation are you thonking of? ...

2. The biggie... Your plan does not include rafter ties.

Loft floor joists 8 ft up from the bottom end of the stud do NOT count as rafter ties when there is several feet of wall above the loft floor.

There is danger of the outward horizontal force from the rafters pushing the wall tops out and causing ridge sag.

Have you read through AJ's project thread? Al and Robins 20x30 1 1/2 near Lake Eufaula, OK   We beat him up pretty bad over his roof/rafters. Sorry Al, but you know we have your best interests at heart.

Back later.... I have to go install more foam insulation.....

[Squirl]

How do you plan to support the second floor?

[thomcarol]

The second floor will be supported by 4x12's that are spanning 24 ft and sitting on 2x4's that have been notched into the walls.

I read AJ's post and it seems like we have the same problem. It looks like my two options would be to either put up a ridge beam or raise the 1st floor ceiling and take my knee wall away. I have noticed a few projects on here that have a small 1ft knee wall and no other rafter ties, would that be sufficient enough?

[MountainDon]

Everybody who builds more than a simple one story with ceiling joists that rest on the top plates of the walls, and do double suty as the rafter ties as illustrated in the image I posted in my last post, encounters this problem. Some owner-builders solve the problem correctly and some do not. Designs that use a joist dropped down the stud, leaving an upper kneewall all run the risk of wall top spread and even stud splitting where the ledger is notched into the stud. The IRC says a builder can not do that, can not move the rafter tie down the wall and have it count. The IRC does give the builder the option of using an engineer to design that section. Most engineers, the ones I have talked and corresponded with, would not approve. Not all owner-builders listen, not does every project here represent best practices in all aspects.


... back to work; that was a short lunch....  back later...

[Squirl]

I am a bit limited by my reference resources.  I tend to go to the AWC calculator first to calculate if a span will work.  A Select Structural SPF 2x12 spaced 12" o/c can span a max of 21'-6".  Do you have some reference to spanning a 4x12 23'-5" spaced 24" O/C? Do they sell select structural 4x12s?

[UK4X4]

Using I-forte software  using 40 live and 12# loads

4x12 southern pine grade 1 will be OK at 12OC

but fails at 24OC due to 0.789" flex in the middle

If you use 30# for live it passes just ...with 0.59" of flex- some would say 1/2" of movement makes for a bouncy floor

I'm presently designing a 26 x 36 and have a central glulam and two posts for my second floor with 4x10s on 36OC

You can do it using I-beams - but then you loose the look

[thomcarol]

So, if we were to put in a ridge beam instead of a board what size would it need to be to span 40', and would there need to be internal supports? As I understand it the weight of the roof would then transfer to the ends of the house, so would extra allowances need to be made in the structure to handle that weight other than the supports that the beam sit on?

[MountainDon]

There is so much going on with this I am not sure what everyone is referring to; roof, second floor, main floor? 

I would suggest taking a step back for a moment. The right way to design this is to start with the roof and work down through the structure to the ground and the foundation. That will get everything working together and nothing having to be torn apart and rebuilt. We hope. Drawing a floor plan is the easy part; designing the support structure is a little harder, a little more technical. Owner-builders can do it, some need more assistance than others to keep from making design errors that can affect safety or durability.


Regarding the roof; a ridge beam that did not have any mid supports while spanning 40 feet is going to be big. I'm not going to try calculating that. That's what the engineer at the LVL manufacturer gets paid to do. It's included in the cost of the beam. Yes the gable walls would need column supports built into them. The load from the columns needs to be safely transferred to the ground. A phone call to a lumber yard should get a price and a size; they probably will NOT have any info on how to design the support structure for the load, though. There are some folks here who can assist with that.

You may have no snow but a 12/12 pitch roof of that size will be a big sail as far as winds go. Wind design speed is 90 mph unless local conditions call for more. A strong wind blowing on the long side and roof is going to be generating some large forces that will have to be resisted by the foundation. The leeward side will get lighter and the windward side will press down harder. Personally I believe pier and beam foundations are completely inadequate for a structure of this size. When you look at one half the total roof load being transmitted down from a one piece ridge beam only supported by the gable end walls it gets even worse, IMO.  Piers tend to act as individual units. A full perimeter foundation offers much greater stability than any pier and beam foundation. The continuous footing would be tied together with rebar and give a rigid base for the home to rest on. It helps to spread the load out over the entire footprint. Twelve inches deep is probably good for your area, but that would have to be checked.  The short perimeter wall could be concrete block or concrete placed in formwork.


Question?  We readers / helpers have no idea about your level of expertise; can you elaborate?  None of us are engineers, a couple people here are actually skilled carpenters, some of us come with some knowledge of design.


Are you familiar with the IRC? Even if your project area is not subject ti inspections, there is a lot of solid information in the IRC. The methods described there can be used and the structure would be approved without requiring any special engineering. A foundation like a pier and beam is not covered by the IRC. Techniques not covered by the IRC would require an engineer to be permitted. This is not to say that anything or everything built in a non-code approved manner will immediately fall down.

[thomcarol]

If we were to reduce the size of the house to 24x32 with a 10/12 roof and put in a loft instead of having a second floor , that would allow us to put in the rafter ties and reduce the stress on the roof and walls. Would this be doable with a post and pier foundation?

[Don_P]

That's a full question. Some random thoughts,
Non engineered pier and beam is limited in most places to 120-200 square foot outbuildings. An article in our local paper years ago from the building inspector said that our greatest wind damage is from debris from these buildings and the buildings themselves crossing property lines and damaging adjoining properties.

The first question would be, why are you wed to this foundation? Structurally it doesn't get much worse. It is not fun to work under a completed building repairing these. This is a full sized, permanent, habitable structure that people will be running to for shelter in bad weather, as opposed to a small, temporary, structure where failure is less important. At your shallow frost depths, a full perimeter, legal and code compliant, foundation is not that expensive by comparison and stands a much better chance of surviving nature's blows.

Shallower pitches extert more horizontal thrust on the walls than steeper pitches... lowering the pitch exacerbates the thrust problem on the kneewalls. A ridgeBEAM supports the roof so that it doesn't thrust horizontally against the kneewalls. You don't want a 40', or a 32' beam clear span. The ridgebeam would require a central support of some sort. A beam doesn't get twice as big when it spans twice as far, the span has doubled and the load has doubled, working against a much longer lever arm... it gets several times bigger.

A full second floor makes for a much stronger building than a big open cathedralled room. At 24' wide attic trusses on top of 8' walls would do this with no flimsy kneewall projections or ridgebeam and provide a strong engineered roof and second floor.

[thomcarol]

I got a quote today on how much it would cost to put in a crawl space for a 24x32 house. Labor to dig, put rebar in and pour the concrete is $500. Materials were quoted at around $1500. We need to find a mason to put a couple of layers of block on and we'll be good to go. We should have the foundation poured by the end of the week and soon to be on to building. Thanks for everybody's advice.