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RolHammer 05-25-2013 03:00 PM

Questions about a suspended floor
Hi - new to the forum.

My 20x20' garage is my woodworking shop. It's got a central floor drain, to which the floor slopes in 4" over the ~10' from the walls. Clearly, a fair bit of slope and a pain you-know-where for a woodworking shop. I've mostly just sucked it up and dealt with it, but recently decided enough was enough and it was time to solve the problem. I considered a number of approaches, but as 'reversibility' was the #1 requirement by SWMBO, I've settled on a suspended floor approach. As my experience with this particular kind of work is limited, I thought I'd run my plans up the flagpole here to collect some thoughts.

Here's the currently planned approach:
-Establish a flat, level plane around the room's perimeter with a set of rim joists.
-Frame a 16x16" box around the floor drain, height such that its top is coplanar with the rim joist tops.
-Run a beam between the middle of the box and the side rim joists, top set so it's also coplanar. This bisects the space into two 18.5'x9.25' areas.
-Run joists from the front and back of the garage to the central beam, connecting them with joist hangers.
-Span tables call for the joists to be 2x6, set 12" OC. So, rim joists will be 2x6 also. We don't have 20' dimensional lumber here, so rim joists will be two 10' 2x6's sistered with a third 2x6 centered over the joint as a 'mending plate.' The space at each end will be filled w a piece of 2x6.
-The central beam will be doubled 2x6s, connected to the floor drain box by double-joist hanger. Because everything hangs off this beam, it will also be supported from below by shims every two feet.

So, first question: Am I on the right track with the above?

Now, the above plan makes the assumption the room perimeter itself is flat and level. (I know, I know...) It, not surprisingly, is neither of those two things. The right front corner is highest, with the left front corner 1/4" lower, the right rear corner 1/2" lower, and the left rear corner 3/4" lower than it. Now, I realize this is probably not unusual for a concrete floor and well within the realm of shim-ability. Question is: should/can I?

So, second question: Can the rim joists of a suspended floor be shimmed flat & level, or are they required to be in intimate contact with their bearing surface continuously through their entire length?

RolHammer 05-26-2013 08:14 AM



oh'mike 05-26-2013 08:32 AM

You have a plan----I have not checked a span chart, but what you have described seems sound--

Use plastic shims--and pin then un place with a dollop of builders adhesive.

Get the new floor level---don't be concerned of the old rim joists do not match up perfectly with the new ones.

joecaption 05-26-2013 08:37 AM

#1 Do a moisture test first.
If there's been water coming in under the doors, the wall or rising through the slab it's going to cause mold and fungus under the floors.

#2 Please post some pictures.

oh'mike 05-26-2013 08:57 AM

Good thought,Joe---

RolHammer 05-26-2013 01:20 PM


Originally Posted by oh'mike (Post 1187893)
I have not checked a span chart, but what you have described seems sound

Thanks for replying, Mike. Using the AWC online calculator: using 2x6 No. 2 SPF as floor joists set on 12" centers with a deflection limit of L/360, live load of 50 psf and dead load of 20 psf, maximum joist span is 9'6" with a minumum bearing length of 0.52 in at each end.

These are quite probably worst-case numbers and I'm likely actually looking at live loading more along the lines of 40 psf and dead load around 10 psf. But, even with worst-case I'm within the maximums with planned spans of 9.25" and bearing support length of ~1.5" at each end being supplied by metal joist hangers. That being said, I plan on belt-and-suspenders-ing it and also shimming from below at the midway point of the span. This should ensure any bounce or sag is reduced.

BTW, PL200 was what I had around and was planning on using to lock the (composite) shims in place. Glad to hear I was on the right track with that too. :thumbsup:

RolHammer 05-26-2013 01:21 PM


Originally Posted by joecaption (Post 1187898)
#1 Do a moisture test first.
If there's been water coming in under the doors, the wall or rising through the slab it's going to cause mold and fungus under the floors.

#2 Please post some pictures.

Good thoughts. I'll do that and post back with the results here. I'll also take some snaps and put them up too. :)

oh'mike 05-26-2013 01:34 PM

If this room is going to be a dedicated work shop---consider some floor outlets for power to the tools.

To keep dust out of the outlets, I suggest you use the above floor outlets that have the opening on the side------or recessed boxes with a whip so you can mount outlets to the stationary equipment.

You will enjoy having a shop set up properly.

Also, Screwing down the sub floor will give you the option of removing a sheet in order to add wiring at a later date.

RolHammer 05-26-2013 02:57 PM

S'funny Mike - it's like you're peeking over my shoulder at the plans. :laughing:

(Which, BTW, I'm taking as further confirmation I'm not out in left field with all this.)

oh'mike 05-26-2013 03:30 PM

Hells bells---I wish I was helping----I love woodworking and would love to be setting up a new shop====

RolHammer 05-27-2013 07:38 AM

OK - did the plastic sheet test according to the ASTM standards. Zero noticeable condensation on the plastic's underside by sight or touch, no noticeable darkening of the slab.

Would I be good to proceed, or do you figure I should get some calcium chloride tests and quantify it?

Background: slab (and house) are ~20 yo. I did struggle with rain driving under the door during bad storms, but after systematic experimentation found the best results with one of those glue-down rubber garage doorsills. That made a 90+% improvement. Having a garage door company into ensure proper fitment also helped immensely. Water under the garage door's no longer a problem.

oh'mike 05-27-2013 09:14 AM

I think you are good to go---If possible, add some floor vents to allow some air circulation.

You might also consider adding a waterproof barrier at the door so if driving rain does get under the door,it will hit a water proofed stopper.

PVC trim board or even a slice of synthetic deck board, siliconed to the floor might be all it would take.

RolHammer 06-01-2013 04:59 AM

OK, framing's nearing completion. The subfloor sheathing's not far off now.

My plans are, if possible, to not use anything over the subfloor itself, like say vinyl tile or whatever. I'd like to have access to the screws so I can pull a sheet if I needed/wanted later on - trying to look ahead with this and allow myself room to accomodate downline changes, as well as being able to access the space if circumstances demanded it.

So, I eventually settled on Crezon, owing to its hard-wearing phenolic resin top coat. Local place here has it in 1" 4x10' sheets. Being such an exceptionally well-paintable surface (supposedly 2-3x more durable than a similar coat on another exterior grade plywood), the intent is to paint it with a good quality deck/porch paint. Probably in a lighter colour for reflectivity.

Questions for this phase are:
1. Edges: I'm gonna either bevel them a tad or round them over to ease sheet to sheet transitions. As mentioned above, joists are 12" OC. Is screwing every 12" around the perimeter sufficiently close for smooth transitions?
2. Sheet gapping: Crezon's not T&G. Is an expansion gap b/w sheets required? (I understand a 1/8" gap is typical for ply subflooring.) Or, is fitting them tightly fine? Clearly, a gap of that size would be less than ideal, considering the overall plan.

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