Bracing floor from below

I was working in the crawlspace of a house I own and discovered that the subfloor (shiplap running diagonal to the joists) is badly rotted around the chimney. The rotted wood is powdery dry now because I corrected the moisture problem several years ago. The floor above the rotted subfloor is ceramic tile, possibly over durock (cement board) underlayment, but possibly over plywood. It doesn't matter either way.

Can anyone suggest some sort of material that I could put on top of plywood patchs to fill the voids when I press up from underneath? Something about the consistency of joint compound would be ideal. I actually considered using joint compound, but decided against it because the area did have a moisture problem once. Probably never will again, but still...

Thanks,

Stan

Maybe a cement based tile thinset

Do not feel bad I have a place in this room that I have a water leak when it rains that I seal then wait a month and it leaks again that has rotted out an area. I have been waiting till it gets a bit warmer to go under and jack it up. Not a fun job but hope to jack it up seal it with that new spray foam and hope to find the $$ to do a metal roof in the area to maybe stop the problem. Short of cement injection one like you have is hard to do much with, with out ripping it apart and starting over again.

Light weight concrete or floor underlayment would be my choice if there is no flex in the floor because both will support heavy load but will crumble if flexed. If the floor gives and flexes,you can buy portland based products that use latex liquid additive instead of water.

The correct way to fix it would be to remove everything down to the framing and reconstruct it however it would probably be a lot easier to patch it. I worked on a house last year that had a plumbing leak flood under the house for a very long time. Half the floor joist and subfloor were rotten from the humidity. The customer had just had a new floor put in and was unaware of the condition of their house under the floor. They had me go under the house to install and electrical outlet and I found the condition. When I gave them the options of how to fix the floor they wanted me to patch it because of the new floor. I went in and added extra floor joists and cut strips of 3/4 plywood and installed it under the subfloor cut crossgrain to re-enforce it. I cut strips of 1x2’s to hold it up. From your description there seems to pieces of the subfloor missing. You could cut small pieces and glue in with liquid nails to fill the voids before it's re-enforced.

It's called "floor leveler". You should be able to get it at Home Depot or any flooring store. It comes as a powder you mix with water to get the consistency you want.

If you want something stronger and more water resistant than floor leveler, I suggest "Durham's Rock Hard Water Putty", available at most hardware stores.

Bondo is flexible, waterproof and adheres well to most everthing solid. Scrape away the rotted sections and fill with Bondo. If I remember right Bondo makes a waterproof wood filler.

Try some Durabond.
Durabond Is joint compound but is a setting type mud. It is not water proof but it is highly water resistant. Really sticks and is pretty strong. Gob a bunch of it on your plywood then jack it up into place from below till is shmooshes out the sides.
I use a lot of the stuff for taping sheetrock in bathrooms and for filling metal corners because it's so strong and water resistant.
Get the stuff in the brown bag not the Easy Sand in the white bag. A lot of retailers will try to fob off the Easy Sand but it is not as strong, not as sticky as Durabond.
The 90 minute setting time will give you a longer working time.
It's kind of a cheesy way to patch things but I do understand your dilema if you have tile already set.

From a structural stand point I agree with:

a) remove the floor and do it right

b) put some backing material underneath it and support it on the joists

Any sort of "patch" will not do anything to stabilize the rotten area. It is still possible for someone to break through at some point.

I would consider hydraulic cement such as Quickcrete Hydraulic Water Stop cement that is used to patch cracks in foundations, because it expands a little. It would provide the support that you are looking for.

Even a mortar mix would do what you need if your just patching it like you say. My concern would not be the subfloor but the joists.If the floor has dry rot you can pretty well bet the joists do also and it just keeps on spreading.I would check those joists good and if they show any sign of dry rot ,i would either replace them one at a time from below and cut out replace subflooring as you went back to good wood,or at least treat them to stop the rot.I dont recall the name but theres a product used on wooden boats thats supposed to stop it and replace rotted wood.You need to stop the rot or it simply keeps spreading.Even without the added moisture from a leak the humidity in a crawl space is generally pretty high.best/ easiest way would be to go in from above of course,but you can replace subfloor from below with enough work.Back when i was plumbing it was not unusual at all to find the floor rotted around a toilet.With enough time and patience you could pry out the rotted pieces, cut the nails, and slide in new boards. but it sure is a pain. If just the tops of joists are rotted ,you could also cut just a partial joist . Hold house up on jacks,cut the top half or whatever is rotted out of joists,replace subfloor( most often 1x8)then scab 2x4 or 2x6 in where you made your cut and hold them in place with plywood on sides.again not easy but it has been done.generally not too hard getting the subfloor out except on ends where its nailed under wall plates,but if you get those ends out you can generally reach in with a sawzaw and cut nails off the drive your new board in.Once you get those in place then slide in your joist scabs and secure it all with screws.You will most likely find a layer of tarpaper or something above the subfloor ,leave that or replace it or your tile will condense moisture bad above the patch,and it will rot even faster. hope this helps.

Perhaps I didn't explain what I did very well. I cut away as much of the rotten wood as possible and replaced it with new wood. Then I essentially constructed a second subfloor under the old one. It would have been better to demo the floor and rebuild it however to really do it right it would have been better to tear the house down and build a new house. Nobody will ever fall through the patching I performed on this house. All plywood and timber I installed to re-inforce this floor was done with treated wood. Any floor joists which appeared to be failing had a new boards placed on each side, bolted with a threaded rod.

I second the thinset mortar.

what you did will probably work,BUT, youve greatly reduced your strenght for one thing(it may not be needed though).lets disect your patch, first you bolted through a rotting floor joist.this adds no strenght whatsoever ,once the middle fails bolting is no longer structuraly sound.Your scabbed on boards will be simply laying there.A much simpler, stronger way would be to simply remove rotted joist(s) and replace it(them).Thats easy to do,simply take a saw and cut it and pull it out. then you could take a skill saw,set it for the correct depth and cut out subfloor,or oak floor ,or durock whatever needed replacing.simply slide in new subfloor,then either replace floor joist(s)or use a built up joist made of plywood.that would be much much stronger than the original floor joist.its dificult only in that your often hampered by room to work.all that being said your patch should work if you actually got back to sound wood.I dont know why you should need any cement patching though if you cut out the rotted subfloor,cutout the rotted joist,it should be the same as original as far as thickness goes.

(quoted from post at 07:50:54 01/26/12) Perhaps I didn't explain what I did very well. I cut away as much of the rotten wood as possible and replaced it with new wood. Then I essentially constructed a second subfloor under the old one. It would have been better to demo the floor and rebuild it however to really do it right it would have been better to tear the house down and build a new house. Nobody will ever fall through the patching I performed on this house. All plywood and timber I installed to re-inforce this floor was done with treated wood. Any floor joists which appeared to be failing had a new boards placed on each side, bolted with a threaded rod.

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Don't let them get you down Steve. I understood exactly what you were doing from the first post. You would need to write a book in order to find and cover all the possibilities that people will bring up, from things that they think are hidden between the lines.

I understood your fix; my comment about patching was all the other response of either epoxy or some sort of mortar to fill in the rotted area. That sort of "patch" does not return the strength and the the loads to joists. If I noticed this I would probably fix it like you just described until I had the time and resources to do it properly.

I guess I should have put and "OR" between option a & b....

I have heard that the treated lumber to will corrode nails and bolts. Your work looks good.

Dusty

Hi bjb,

I think you misunderstood my question. My intention was always to repair the damaged areas as Stephen Newell described in his drawing. I asked for, and got, suggestions about a patching material with the consistency of joint compound to fill in the voids in the subfloor so that there will be 100% contact (and support) from the plywood bracing. The idea of of hanging any kind of patching material from the underside of the damaged floor without bracing it structurally is moronic. I don't know whether to be amused or offended.

I know how to repair this damage correctly, and if it was your house and you were doing it I might be tempted to take the high road with my advice, too. But I'll never get my money back out of this place. Not only is it currently worth not much more than some people on this forum spend on a vehicle, but I've already spent more on repairs this month than I'll make in rent in a year---so a repair that will take care of the problem for the life of the house is going to have to be good enough.

Stan

I don't know of any liquid product that you could use in this application. There is an epoxy product made to fill rotten wood that is similar to bondo and I believe it's expensive. The problem is working from underneath trying to get the stuff or bondo from sagging or falling out would be the difficult at best. On the house I worked on I used my sawzaw and a chisel and removed as much of the rotten wood as I could and filled the void with new wood, gluing it and nailing it to the original oak finished floor. I used the sawzaw on angle where I was only cutting through the subfloor. It might be easier if you had one of these dual saws. I've never used one so I can't say.

Hi Stephen,

The idea I had in mind was to put the filler substance on top of the piece of plywood and press it into place, then secure it with 1x3 nailers. If it were sufficiently plastic in its initial state, the excess could just be squeezed out the ends.

I'm trying to avoid having to use power tools to cut the rotted subfloor back to the joists for two reasons: 1)If I shake tile loose it will be an incredible PITA to replace them, and 2)Getting anything in and out of the crawlspace is very difficult. The opening in the poured concrete foundation is narrower than my shoulders, so I can only go in or out by putting my arms out in front of me (like I'm surrendering) and putting them through first. Then, the height of the crawlspace is too low for me to move on my hands and knees, so I have to move in a low crawl. It's not the worst crawlspace I've ever worked in, but it's in the top ten.

Thanks for your advice.

Stan

That sounds like a plan. I was picturing a bigger spot to patch. The house I worked on had a place about four foot diameter that was especially bad.

I thought I had it bad getting under the house I worked on. It had a access hole about 20" square in a hall closet I used to get plywood and timber through. Then in some places there was only about a foot between the ground and the floor joists but mostly had about 2' clearance.