What type of beam to span 20ft?

Question

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.

Lee Sam · Answer

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.

What type of beam to span 20ft?

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