Deflection Calculator

[joathout]

Hello Everybody!

I just bought a 1962 raised foundation home. Tore out the linoleum first thing! I have a question regarding the deflection calculator. Spacing between joists only goes up to 24". I have 3.5" x 5.5" joists on 48" spacing, 4'8" between supports. The calculator puts out .031" deflection. Since this is only the deflection along the LENGTH of the joist, would my deflection just be twice the .031"?
Also, sub floor planks, t&g, measure 1-5/8" x 5". Would a simple beam calc work for the deflection between joists? Should I include, in that same calculation, the plywood that should be used on top of the planks? I appreciate all and any help. This is my first attempt at doing ANY new flooring project so any tips especially for newbies would be very helpful.

Thanks
Jim

[Houston Remodeler]

Jim,

We don't have plywood that can span the joists and meet spec. You'll be adding more joists to the system. Reduce the spacing to 24 inches and see if that works. But since you're adding joists, I'd be looking very hard at 16" OC spacing instead.

[Carbidetooth]

I'm guessing this is left coast 2x6 tongue and groove planks over the post and beam joist structure. If the subfloor planks are in good shape, this can usually be made to work with tile.

In the case of planks, the Deflectolator won't be much help. I've plated this kind of subfloor with plywood to stiffen things up. I imagine it could be done with 1/2" but my go-to is 3/4".

Photos will help us help you.

[cx]

Welcome, Jim.

What Peter said. If you'll add a geographic location to your User Profile so it remains permanently in view it'll help in answering some types of questions.

Generally, if your 4x6 beams are of good grade and species and are supported every 7 feet or less, they are adequate structure. If your decking is nominal 2x T&G material, the addition of nominal half-inch plywood will make the subfloor suitable for a ceramic tile installation.

My opinion; worth price charged.

[wwhitney]

Hi Jim,

Peter and CX answered your practical questions. For your questions on the deflectolator:

Originally Posted by Jim

I have a question regarding the deflection calculator. Spacing between joists only goes up to 24". I have 3.5" x 5.5" joists on 48" spacing, 4'8" between supports. The calculator puts out .031" deflection. Since this is only the deflection along the LENGTH of the joist, would my deflection just be twice the .031"?

Yes. The calculated deflection will be proportional to the joist spacing (and inversely proportional to the joist width). So if you enter 24" joist spacing, you can double the answer to get the result for 48" joist spacing.

Originally Posted by Jim

Also, sub floor planks, t&g, measure 1-5/8" x 5". Would a simple beam calc work for the deflection between joists?

With the understanding that the calculation is academic, as the design criterion for deflection between the joists is not the same as the criterion for deflection along the joists, you can do this with the deflectolator. You just need to know that the relative deflection (in the form L/whatever) will depend on the span L and the joist height h as a function of (L/h). Since you can't enter a 4' span or a 1.5" height, you can instead enter, say, a 12' span and a 4.5" height to get the same answer.

Of course, you also can't enter 5.5" wide joists at 5.5" on center, but you can enter 3" wide joists at 12" on center, and then multiply the resulting deflection by 1/4.

Cheers, Wayne

[joathout]

Wow! Such quick response time. Thanks guys. Yes, I guess it is called beam and post. I don't know if these photos will really help. Would I lay the ply so the face grain is running perpendicular to planks? Also, I am going to check for high spots on the subfloor before putting down plywood. When I glue and screw down the plywood won't it tend to follow any contour that the planks exhibit already? I'm not saying they are bad, just a thought.

[cx]

Jim, you want all layers of structural subflooring installed with the strength axis perpendicular to the joist structure.

Yes, your plywood will tend to follow the contours of your plank subfloor. The thicker the plywood, the less it will conform.

I would recommend you not try to glue the plywood to your plank subfloor.

My opinion; worth price charged.

[Carbidetooth]

Looks to be in good shape. If you were a glutton for punishment you could rent a floor sander and knock down cupping and such, but not really necessary. If you've got knots or other big obstacles grind them down. All nails & staples flush or removed. I vacuum after so I can get a good look at every sq.ft. Only floors I've encountered that really needed sanding were somehow exposed to big moisture...like a flood.

Ply perpendicular to joists, so parallel to planks. I try to land seams in middle of planks just to tie together as best you can. Offsetting sheets at their ends is a good idea too.

The approach is to screw the ply down like crazy. 6" spacing perimeter, 8" field or a little tighter. I try to hit meat in center of planks and steer clear of the tongues if possible. Rattle gun and deck screws will be your friends. I like to use 3/4" 'cuz screws have less tendency to drive through. If you have entry doors with thresholds this may say something about what thickness you choose. Can't just cut those doors like interior ones and most people like rugs in front of doors to boot.

You really just have to get this done and see what your floors are like afterward. Dips get filled after ply, if needed. I think you'll be kinda amazed at how this tightens things up. Do a jump test before and after for visceral verification.

Oh, and add some long deck screws through planks into joist before ply.

FWIW and from a seat-of-pants engineering perspective, the subfloor is likely strong enough as is, but not stiff enough to carry a tile assembly. Which is a good way to think of it...an assembly.