Floor joists

jel1988 · Feb 20, 2015

Which senario would result in a stiffer floor, (less deflection) 2*8's 16" on centre or 2*10's 24" on centre.

eddie48 · Feb 20, 2015

My calculations say use the 2x8s. That assumes that the load is evenly distributed and that the flooring is sufficently strong to avoid deflection between the joists.

oosik · Feb 20, 2015

To be quite honest - unless this floor is in some type of work shed or something like that - - I would recommend 2x10 or better yet 2x12 on 16" centers. Otherwise - go with 2x8 on 16" centers.

/pine · Feb 20, 2015

FWIW... Row(s) of "X" bridging or solid blocking help to distribute load...this will also stiffen the system regardless...

KennyG · Feb 20, 2015

Your 2x10 configuration has more weight carrying capacity. A 2x10 has about 65% more strength than a 2x8 and your increased spacing will only add 50% to the load. The 2x10 floor will be stiffer over the joists. HOWEVER, the stiffness between the joists will depend on your flooring material. Whether you use 2x8 or 2x10 and the spacing should be taken from a standard joist table which is readily available on the internet. The span is critical which you did not specify.

BHD · Feb 20, 2015

you will not even remember the cost of the deeper joists, in time to come but if you have sag or bounce you will kick your self ever time you walk across it or deal with the sag, for the few dollars you saved,

many years ago we added on to the church, I calculate out the size of joists for the span, and it was 2x8 I think, well later the plumber ended up gut about half of them by drilling out a large portion of the joist in the middle, later we remodeled some partly because of the sagging floor, (I had no idea that they nearly gutting the joists in the plumbing but in the end we sistered up new joists along the old ones, but even where they were not hack up there was more sag than desired,

I would suggest 2x12 lumber or the I joists, I do not think you will be sorry,

on a floor use 16" on center, your floor will thank you,

(I am not seeing you distance your spanning) which can influence your choice,

newbury · Feb 20, 2015

jel1988 said:
Which senario would result in a stiffer floor, (less deflection) 2*8's 16" on centre or 2*10's 24" on centre.

Yes, we REALLY need to know the span for a half decent answer. Are you spanning 4 feet or 14 feet?

DFB · Feb 20, 2015

newbury said:
Yes, we REALLY need to know the span for a half decent answer. Are you spanning 4 feet or 14 feet?

What he said! :thumbsup:

PILOON · Feb 20, 2015

Absolutely, the span is most important.
Also a few other factors: plywood sub, strand oriented chip board etc.
All being said the wider will offer more stiffness, in fact I am sure that 2 x 10 will 'out stiffen' 2 x 8 in just about any situation as long as you don't exceed the spec for the span .

ex: on a 8' span there will be no difference but at 14' there will be a noticeable difference.
At 16' neither is within norms.

Deere Dude · Feb 20, 2015

Just a slight hijacking here, but in the house I just bought has 2x10 joists @ 16" and is sturdy. But I noticed all the Xs with 1x4s in between the joists were never nailed or screwed in the bottom. Should I fasten them on the bottom?

Back to the subject.

Beltzington · Feb 20, 2015

jel1988 said:
Which senario would result in a stiffer floor, (less deflection) 2*8's 16" on centre or 2*10's 24" on centre.

If you have not considered it, engineered wood joists and flooring are the best available for stiff floors. It will be what I will be using in our new home. Here is one manufacturer, they also have calculators to determine size and placement for various loads. Good Luck.

AdvanTech Flooring, AdvanTech Subflooring, AdvanTech Floor | Huber Engineered Woods

jel1988 · Feb 20, 2015

Sorry about the span guys but I checked a floor joist calculator. At a span of 9' 6" 2.8's 16" on centre deflect .209 or L/ 544. At the same span 2*10's 24" O/C deflect .178 or L/641. In all cases 2*10's 24" O/C out perform 2*8's 16" O/C

PILOON · Feb 20, 2015

Deere Dude said:
Just a slight hijacking here, but in the house I just bought has 2x10 joists @ 16" and is sturdy. But I noticed all the Xs with 1x4s in between the joists were never nailed or screwed in the bottom. Should I fasten them on the bottom?

Back to the subject.

YES! they should be fastened top and bottom. If joist is twisted try to get them as close to vertical as possible for best results.

DFB · Feb 20, 2015

Deere Dude said:
Just a slight hijacking here, but in the house I just bought has 2x10 joists @ 16" and is sturdy. But I noticed all the Xs with 1x4s in between the joists were never nailed or screwed in the bottom. Should I fasten them on the bottom?

Back to the subject.

Ya normally with that style of cross bridging the bottoms are nailed after the sub floor is laid allows for the weight to push down on the joist crowns first, if they fit tight probably nothing has warped.

How old is the house?

Billrog · Feb 20, 2015

Deere Dude they are left that way by some contractors until the 2x10's shrink ( dry out) so now would be a good time to nail them solid but keep the 2x10's vertical don't hammer them until their leaning.

duke7595 · Feb 20, 2015

Beltzington said:
If you have not considered it, engineered wood joists and flooring are the best available for stiff floors. It will be what I will be using in our new home. Here is one manufacturer, they also have calculators to determine size and placement for various loads. Good Luck.

AdvanTech Flooring, AdvanTech Subflooring, AdvanTech Floor | Huber Engineered Woods

I'm sure you probably know this but make sure the crown is facing up on each joist. Also, for the few extra dollars it pays to install 16" O.C. regardless the
the size you choose, and no less than #2 grade lumber, hand picked if possible.

newbury · Feb 20, 2015

jel1988 said:
Sorry about the span guys but I checked a floor joist calculator. At a span of 9' 6" 2.8's 16" on centre deflect .209 or L/ 544. At the same span 2*10's 24" O/C deflect .178 or L/641. In all cases 2*10's 24" O/C out perform 2*8's 16" O/C

Good to know, my point was if it was a short span the difference would be very minor.

Billrog said:
Deere Dude they are left that way by some contractors until the 2x10's shrink ( dry out) so now would be a good time to nail them solid but keep the 2x10's vertical don't hammer them until their leaning.

AND if you are trying to nail in cumbersome and tough to get to areas and don't have a palm-air-nailer or similar you might want to get one. They work with even small air compressors like the 3-gal-13-hp-100-psi-oilless-pancake-air-compressor which has been going on sale for $40. I've bought 2.

woodchipper · Mar 14, 2015

Subject is closed, but as I just read the post, I will add that as mentioned, blocking between the joists every four feet will make this floor twice as strong, as the floor live load per square foot will be be distributed over a much greater area. The down force (moment of inertia) beeing transferred laterally to other members. Therefore solidifying the floor, and rendering it much less bouncy in the process.