Should I leave a gap when installing new plywood over rotten hardwood floors, and do I need to use caulk between the sheets and walls?
Asked by Dex Van Meegen 
6 months ago
Last Updated: May 14, 2024
So, we made the call to ditch the carpet and go with LVP instead. When we took up the carpet, we found out that the original hardwood was in bad shape and the floor joists had some rot. So, we’re going all in and replacing everything. My main question is about laying down the new 3/4 plywood – I’ve read to leave a 1/8 inch gap. Should I also use caulk between the sheets or between the sheets and walls? Thanks for any advice you can offer!
I really hope this is a joke, because replacing the subflooring is just the tip of the iceberg when it comes to your problems.
There are just so many issues in this picture.
I would definitely attach the sheets to the joist and consider using tongue and groove plywood. Also, make sure to install a rim joist on the cinder block front wall or very close to it. ‘t forget to lay down a plastic vapor barrier on the dirt while you have the easy access to it all.
You might want to do some research – it looks like a vapor barrier could be necessary in that area. If the wood is rotting, it could indicate moisture coming up from the ground below the floor.
It might be a good idea to have a vapor barrier before installing the flooring. Also, be sure that the flooring is offset and consider gluing it to the floor joists if you feel like it.
In cases like this, the vapor barrier should actually go beneath the plywood. I suppose you could attach it to the joists before laying down the plywood, as long as the joists are pressure treated. Otherwise, I would recommend placing it directly on the ground in the crawl space.
Yeah that’s exactly what I was thinking. I apologize if I didn’t explain it well before. The vapor barrier over the soil would most likely be the most effective option.
I just wanted to confirm because many people think The Vapor Barrier should be placed beneath the flooring or underneath the padding on top of the subfloor. However, that’s only applicable when you have a concrete subfloor.
Wow, are those 2×6 joists? Seems like quite a long span for them.
Hey, maybe we should add some quarter round and caulk there. Just kidding! Definitely call a professional for this issue.
If I were you, I’d start by cleaning up that crawl space and putting in a six mil vapor barrier to manage the moisture there. You should also double-check the span of the joists. It might be a good idea to excavate and create an access point so that when the plumbing needs fixing, you’ll be ready. It’s a big project but now is the time to set things right. Make sure any support posts have concrete footings too.
There’s a lot happening here, so if you follow through with what you’re saying, it will deteriorate again. As mentioned earlier, wood in contact with the ground without a vapor barrier will lead to the same issues. You should contact a foundation contractor.
Do you know where the wood is touching the ground?
Haha… It’s just a temporary support someone put in.. I’m pretty sure it will be taken out when they strengthen the floor.. no need for a contractor
I simply mentioned it, right?
Is saying nothing will decay
Wait, , are you saying she mentioned it’s already decayed? Have you read it carefully?
Pointed out that the exposed ground provides moisture, even if not in direct contact
You mentioned, , that even after repair it would continue to rot
Agrees and mentions that a vapor barrier will quickly fix that issue. What’s the other problem?
That’s all I have.
Are those 1×4 boards?
Concrete blocks lose their strength when placed sideways.
Is that a 4×4 nail attaching a 1×4?
Is the 1×4 just sitting on a rock?
Doesn’t it turn into mud?
Hey Gary, Mo mentioned all of this.
Absolutely 👌
Hey , were you aware that this house was constructed over 80 years ago? Everything was considered acceptable back then.
Hey , the dimensions are 2×8 and the answer is yes to all your other questions. The house dates back to 1900 and everything is original. Interestingly, the floor joists are not secured at the ends, they are just sitting there. I plan to fix that when I renovate. Also, the house is situated in a flood zone, so during heavy rain, the yard gets flooded.
It is recommended to opt for 3/4 tongue and groove subfloor panels. Apply liquid nail on the joists, nail it down, then come back to screw it with 1 3/4 inch subfloor screws. It might seem like a tedious process, but the results will be beneficial in the long term.
Adhesive and ring shank nails work well and tongue and groove is not essential
I never claimed it was a requirement. It’s more of an improvement than a necessity.
Makes a good point – when properly glued and secured with 3/4” ring shank galvanized nails, T&G flooring will remain stable. Attempting to remove it may result in some of the joists coming up as well. Additionally, it’s crucial to include a vapor barrier over the dirt surface.
Here, a 45-year retired carpenter, simply looking to help you save time.
T&G has been considered an industry standard for around fifty years now. It helps prevent squeaks, airflow, humidity, and mold.
We actually only construct around 200 houses each year using that subfloor method. It’s been like that for about 13 years now. If you want to avoid floor squeaks down the line, that’s the way to go. The real issue isn’t whether the nail moves or not. If it gets wet or a dog does their business on it, it won’t warp and start squeaking. Quality and necessity play a huge role here. You can choose what you think is necessary, but I’ll always go for quality.
I’ve been dealing with a squeaky second floor, they just glued and nailed the subfloor. Trust me, use screws instead!
Yeah, I agree with everything except the recommendation for Liquid Nail. I’d personally opt for a proper subfloor adhesive like Rainbuster 345.
Yeah there are plenty of those. I’m down south where we refer to everything by specific brand names. For example, any type of soft drink is a Coke. Circular saws are Skill saws. Reciprocating saws are Sawzalls, and so on.
If it’s glued with PL 400 construction adhesive and secured with galvanized ring shank nails, it won’t squeak. One of the major causes of squeaking is when bridging or cross bridging rub together.
Hey Ben Neidlinger, do you have any advice for these guys?
Just finished replacing the subfloor in one of my bathrooms.. not the most enjoyable task but the results were well worth it. It’s a great feeling to walk in and not feel like I might sink lol
For the subfloor, go with 3/4 inch plywood or OSB with tongue and groove on the longer edge. Be sure to use subfloor adhesive along each joist as you go. There shouldn’t be a 1/8 inch gap at the tongue. That’s intentional. Secure with 2 3/8 inch nails or screws. Best of luck!
Looks like there are some larger issues at play in this situation
Just make sure to pick up tongue and groove sheeting that’s 3/4 inch thick, you’ll be all set
Do you know where your concrete foundation is located? It’s essential to know before beginning any construction.
Forget what I said earlier, I see you’re only working on part of the subfloor. But definitely make sure there are no gaps at all. You might want to reinforce a few joists next to the existing ones. It looks like they might be 2x8s, so maybe consider reinforcing with a 2×10 or 2×12 and ensure the crowns are facing up (the straightness of the new joists…if they’re curved, have the curve facing up).
Dave Young
You’re gonna want to add some vapor barrier in there. It’ll help prevent your new lumber from rotting, like the old stuff. Plus, it helps with Radon control.
The real question is, why the rot? It indicates a story of some sort. If you’ve come this far, I suggest contacting someone about encapsulation or a vapor barrier for your crawlspace. It’s hard to assess from here, but your joists seem quite close to the ground, so you might need to remove some of the soil. Maybe just encapsulation will be enough, but you don’t want to repeat this work in a few years. Also, check your downspouts.
Consider purchasing the kreg ripcut or accucut if you’re looking to add a new tool to your collection.
Hey , which vapor barrier would you suggest for a 780 square foot house that we plan to turn into a rental in a couple of years when we buy a bigger house?
Lambeth I believe the standard is 6 mil plastic, although it could be 10 mil (not entirely sure)
Sure thing, I’ll make sure to follow that advice. I’m currently focusing on redoing the floor joist and subfloor one room at a time, so I’ll start by laying it down in this room and then continue to add on as I work on each room
Lambeth no need to stress about cutting the plastic to fit, just roll it out as quickly as you can to the next room and leave it there until you move on to the next floor
Back in the day, the old lumber was more resistant to moisture and rot compared to what we use now. The lumber available today won’t last long in the current crawl space conditions.
Before deciding on a cover-up, address the mess and ensure the area meets safety standards.
I am changing everything out. The house was constructed in 1900 and this is all original. I’m focusing on renovating one room at a time
Lambeth make sure you clear out all the clutter from this area and lay down a minimum of 6 mil plastic for vapor and moisture protection on the ground. Make sure to seal it properly at the foundation and any openings to prevent radon gas from seeping into your home. Have you thought about insulating this area? Closed cell spray foam could fulfill all of these requirements and should be done by a professional. It’s just a recommendation. But it’s really important to seal off this space and install proper footings and piers as needed because a piece of wood or a cinder block in the soil won’t suffice. Regarding your question about the plywood, use tongue and groove for tight joints and secure each piece with screws and glue. Vinyl plank flooring is popular now, but any imperfections in your subfloor like cracks, gaps, holes, or uneven surfaces will show through over time. It looks great, is relatively easy to install, but it’s unforgiving if not done correctly. Best of luck.
I’m not planning on insulating it. I’ve heard that sand can help with moisture control. Would it be beneficial to lay it down before the barrier? Also, is there an affordable dehumidifier available that I can place there? My house tends to hold water during heavy rains because it’s in a flood zone. Do you think the barrier will hold up against the water pressure from outside trying to seep in?
Lambeth if flooding is an issue, it’s probably best to seek help from someone experienced to prevent any damage to the foundation. Having wood in the soil doesn’t seem ideal. What about plumbing problems? Would you have to tear up the floor? It doesn’t look like there’s much room to access it. Shouldn’t plumbing be insulated? Even though it’s an old house, there are quite a few things that need fixing.
Lambeth what did people do before they knew about radon gas? Did they have any solutions in place?
Mana a few people passed away or fell seriously ill because of it. Damaged lungs and cancer.
What if we simply fill it with concrete… any thoughts on that?
Seems to think having bottomless pockets is a good idea. After roughly calculating the cubic yardage, it’s actually not a good idea.
If you decide to fill it with concrete, keep in mind that regulations require 130mm of packed hardcore, 25mm of sand, a membrane, 100mm of reinforced concrete, another membrane, 70mm of insulation, another membrane, and a final 100mm of concrete. Failing to include the membrane will lead to damp issues, skipping insulation will result in cold temperatures, and missing hardcore will cause cracking.
No way…absolutely not. If you fill it with concrete and have a plumbing or framing issue later on, you’ll end up spending tens of thousands of dollars to fix it. Avoid doing that at all costs.
Oh my goodness thank you
Thanks , now I know I’m facing the same issue and thought that solution would work. Thanks for the information!
No worries, !
What region is this house located in? I typically work on houses in towns with a long history, over 150 years, and haven’t encountered this style of construction.
I’m situated in southwest Missouri. I think it’s pretty standard for houses of this age to be like this
Lambeth I’m currently in Tennessee, but originally from New England, and I can confirm that old houses up north have basements while houses in this area are often on dirt and cinder blocks, haha
Lambeth I’m here in Southeast Kentucky and every home I’ve come across looks exactly like this. I used to think this was typical until I saw the comment.
The older portion of our house is similar to this. I live in southeast Missouri.
California has a lot of this happening. I’ve been able to make a decent income by replacing foundations that resemble this one under houses in this area. Placing a vapor barrier directly on the ground, sealing it to the foundation with roof tar at the edges and footings, and adding proper venting can prevent this issue from recurring. The lack of clearance under joists and absence of venting leads to this problem.
Texas also has these.
Before laying down any subfloor, it’s important to address these foundation issues.
I have this type of “foundation” in my house and a vapor barrier on the ground can really make a big difference. By the way, have you thought about using pressure treated or coated lumber? Living in a wet area means the air is always damp. Additionally, my house has some extra concrete footing in the middle to support a beam under the joists. If you’re in a cold climate, it’s a good idea to insulate your pipes. If I were replacing more floor joists, that’s definitely the perfect time to pour concrete into your “foundation”. It’s difficult to do it any other time and it will help the house stand strong for another 100 years. Dealing with an old house means any renovation you start always becomes a big project.
The additional supports seem to have been installed to minimize the flexibility in the floor. While necessary, they’re not quite right. 2*6 floor joists shouldn’t span 12 feet unsupported. Remove and replace worn-out joists and hangers with new ones. Install new supports. Tongue and groove subfloor is the way to go, not 3/4 plywood. There’s a significant difference.
It seems like this area was originally put together in a haphazard way. I’m not exactly sure where in the country this is. The hardwood has rotted due to consistent exposure to moisture over time or the lack of a moisture barrier. Call me a perfectionist, but I would suggest tearing it all up and starting fresh.
Adding a vapor barrier is essential to prevent potential radon exposure. You might want to test for that first with a charcoal block to see if there is an issue. At the very least, I would recommend laying down cement footings and a vapor barrier.
Any wood near soil needs to be treated, but it will still rot eventually. It’s not advisable to place wood directly on dirt due to the risk of termites or water/moisture damage. In this case, I would recommend using cedar 2x12s for added durability. The current setup doesn’t seem to be properly supported, so it’s important to address that first before making any
I’m not a contractor, but I agree with what others have said. It’s important to remove that plumbing time bomb. I would suggest pouring some footers and placing your block on top of them. Insert a short stick of rebar inside the concrete-filled block and sink it into the footer to prevent movement at the cold seam. It looks like you may only be able to fit one layer of block with some type of girder in that space. You could potentially laminate together three 2x10s and span that gap with your new joists sistered in to provide support. That should do the trick.
It’s a good idea to leave a 1/8 inch gap between sheets of plywood when installing over joists. This allows for natural wood expansion and contraction. No need to caulk these gaps – it’s important to give the wood room to move! Remember to also leave a small gap between the plywood and walls for baseboards or trim later on. Make sure your plywood is securely fastened to the joists before installing your LVP flooring. Good luck with your project! – Robert
I believe after doing some research you will discover the importance of placing plastic on the ground beneath before sealing it with plywood. Also, ensure there is adequate ventilation in the crawl space. Covering plywood over a damp area like that with LVP can lead to complications. The vinyl will retain the moisture and transfer it all to your plywood subfloor. Just a heads up, the plastic should be placed underneath on the dirt, not over the plywood as that will exacerbate the situation.
It might be cleaner to staple it under the new joists for a neater look.
There is an actual subflooring available that easily clicks together.
To properly update the current code, it would definitely require a significant amount of time and effort.
In theory, it’s recommended to pour pads for each support post needed. These pads must have rebar reinforcement and should be 16″-20″ square, approximately 8″ deep.
When using 2×10 joists, support them with at least 4×4 spaced at 16″ o/c. …and don’t forget about cross blocking.
Adding a vapor barrier on the ground and a vented access hole with a mesh ‘critter grate’ for airflow is also important.
Insulating with R20-R28 before laying down 3/4″ T&G plywood is a good practice.
That’s just my personal approach, haha.
Our house has a suspended timber floor similar to this. I would suggest placing slats under the timbers next to the joists, then laying down a waterproof membrane and filling the space with either insulation board or rockwool. It may be time-consuming but will pay off in the long term with improved warmth during winter months.
Cover the ground with a vapor barrier.
If it were me, I’d divide the options and place a beam under those joints with a buddy to help. I’d attach another joist to each of those beams. Clean up the mess under there, lay down some plastic, and consider adding pea gravel on top. Then, I’d lay down 3/4 plywood or OSB. Not sure about anchoring to the foundation walls, but I might add another beam for support if needed.
Seems like most people agree that there are foundation issues that need some attention, but they are not too difficult to fix.
Our house from the 1850s was similar, but it had no support piers, only rough sawn timbers on the ground! We updated the plumbing and replaced it with a concrete slab. Make sure to use a 6 mil vapor barrier and create a stronger support system.
Definitely go for treated wood when redoing.
‘s treatment emits harmful off-gassing that is not suitable for indoor projects or safe for breathing.
What the heck is going on?
The soil caused it to rot because of the missing vapor barrier! Investing $10 now will prevent costly repairs down the line.
I agree with that.
Make sure to use a vapor barrier and consider setting some of those posts in concrete footings.
Make sure to address the plumbing issues before adding a vapor barrier over the soil, as it can get costly. Consider laying 6 mil poly and gravel to prevent future clogs that may require tearing up the new flooring to reach the plumbing.
It’s like the classic scenario of thinking it’ll be an easy fix, but it turns into a major project…
I totally understand the struggle with my 1910 house. Great advice in this thread. The silver lining is that you have the opportunity to make improvements now.
The gap is there to allow for expansion and contraction due to temperature differences inside and outside. I had a similar issue. Instead of replacing the entire board, I decided to sister another beam to mine and add more supports. I then placed 4 jacks under the house in that area for extra support and to raise the floor where it had sunk. Additionally, I crawled under the house to lay down a vapor barrier with black plastic. Since my house was built in 1950, those homes didn’t need it back then.
Old houses operate in this manner.
What kind of rat’s nest did you guys uncover and create?! Looks like we’ll need to install a ledger now that you’ve removed the subfloor all the way to the wall. Use PL400 under the sheets and screw them down securely. Then, get some OSB T&G.
For this situation, it is necessary to have a vapor barrier on the ground, with a minimum thickness of 6 mil.