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Looking for help understanding live and dead loads and the mathematical equation for what beam to use or help with what beam we should use. We are trying to span 20ft between kitchen and living room in an old house no true good foundation.

Beams will rest either through the floor on poured footings or plates directly on the ground. The floor is not great and the whole house is three different home addons. Located in Texas so no real need for snow load. Live load I believe is 40psf.

Any knowledge you can throw my way will help a lot. This is my first remodel.

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    You need to get this calculated and checked by a professional engineer who can sign it off for insurance etc. – Solar Mike Jun 17 at 18:25
  • "or plates directly on the ground" What? As solar mike said, you need someone with the proper knowledge to engineer this. – Alaska Man Jun 17 at 18:28
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    There are several people 'round here who have answered these type of questions with very knowledgeable sounding answers. Maybe one of them will stop by to give you a pretty good "guestimate", but nobody's going to insure you without a structural engineer's sign off. Heck, you probably won't get a construction/remodel loan (if needed) without that. – FreeMan Jun 17 at 18:31
  • Yes plates or footers but the foundation is a bad pier and beam and it's well awfull. Just imagine hungover bar patrons building onto your house to pay off tabs, it's not great not great at all. I asked cause me and my brother in law are doing most of the work and installing a beam and building false walls is something we can do. We are trying to get budget information as this is for out mother in law and funds are limited. – McBelac Jun 17 at 19:30
  • Which way do the roof joists/trusses span? Do they rest on the new beam or run parallel to new beam? – Lee Sam Jun 18 at 4:09
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I’ll make some assumptions and you can correct me if I’m wrong.

The roof Live Load is 40 lbs. per square foot (psf).

The roof Dead Load is composition shingles (4 psf) plus roof sheathing (2 psf) plus framing (2 psf) plus ceiling insulation (12” = 3.5 psf) plus ceiling joists (0.5 psf) plus ceiling material (1/2” gypsum board = 2.5 psf) plus miscellaneous lights, ducts, (1.5 psf) for a total Dead Load of 16 psf.

The load equals Live Load plus Dead Load for half the span...the other half rests on the exterior walls. So:

40 + 16 = 56 psf

12’ + 6’ = 18’

56 psf x 18’ = 1,008 per linear foot (plf) resting on ceiling beam.

A 6x12 can support about 360 plf depending on the species and grade. That’s not close so try glu-lam. It would take a 5 1/8” x 18” glu-lam to span 20’.

That also seems unreasonable. I suspect the roof spans from the wall between the bedrooms and kitchen rather than to the exterior wall.

Therefore, the load is: 56 psf x 12’ = 672 plf. A reasonably sized cut timber would still be too small, so use a 3 1/8” x 19” or 5 1/8” x 15” glu-lam for a 20’ span.

This may seem excessive, but what makes it so large is the live load of 40 psf. (I live where it’s 25 psf.) If you can verify the live load that could help a lot.

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