I designed a plan with 18' spans and took it to a potential builder. He told me not to exceed 16' or there would be a huge cost increase. Is this true? All of the rooms are square and hard to furnish. What kind of cost will engineered joists be compared to regular joists? Any other cost saving design limitations I should watch out for?

One can certainly purchase standard 2x stock in lengths longer that 16'. Most lumber yards can provide up ro 22' lengths. The problem is that spans that begin to expand beyond 16' feet will provide for a certain amount of "bounce" or deflection at the second floor level. That can be fairly undesirable in the finished product. Changing the size of the floor joist from, say, 2x10 to 2x12 can add significant cost. Changing the spacing of 2x10's from 16"o.c. to 12"o.c., while achieving a certain amount of increased stiffness, can add to the cost. While even the newer engineered lumber has it's limits, those limits are likely beyond what you're looking at, and changing to the engineered lumber for joists, even at an decreased spacing (12"o.c.) might be the best bet.

If you're still in doubt with your builders opinion, get a second opinion from a local architect, builder, or engineer.

What I see here is an old time builder, ignorant of new methods and materials, and a designer who knows little of materials and design protocols. This is a recipe' for disaster in the final product.

Using TJIs, and sometimes with open web floor trusses, it is possible to have longer spans at less net cost because some beams can be eliminated and labor cost can be reduced. In other situations, cost can go up significantly. It takes experience to know when and how.

The best you can do, with your limitations, is to go to the local lumber supply that sells engineered joists and show them what you are trying to do and they may be able to help you with engineering software and cost estimates. But be prepared to follow through and buy from them, they shouldn't be expected to do this work for free.

Also check with the building department where you plan to build and find out if TJI is allowed for the first floor. Highland park, or maybe it was Lake Forest, did not allow TJI's to be used for the first floor, but was OK for the second floor. Seemed a bit strange to me, and not sure if that still holds true today, but 6 years ago, thats how they were here in IL.

I agree with the last two replies. I would deffinatly consider TGI joist. 18' for a TGI is nothing and as a draftperson i am using more and more TGI continualy. good luck!

T J I
not "G"

Truss Joist I-beam