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Fixing a Rotten Tongue and Groove Porch Floor

Tongue and Groove Porch Floor

It seems like every tongue and groove porch floor suffers from the same problem. End rot. It makes sense that the ends of the boards exposed to the elements will take in water and begin to rot so fast it makes you want to pull your hair out.

On my house, we have a huge wrap around porch which is one of our favorite parts of the house, but with over 1,500 square feet of porch flooring that left me with plenty of rot over the years. In this post, I’ll show you not only how to repair rotten or damaged tongue and groove porch flooring, but also how to prevent the rot in the future.

The Wrong Way

I see this cheap way of repairing all the time and it drives me nuts. Folks will chop off the last 2-3 feet of the boards that are rotten and then replace just that part with a new board. Don’t do it!

The notorious “End Rot” typical of every porch

It looks shoddy and it never lasts. I know that you are saving on materials by doing it this way, but really the main cost of repairing a porch floor isn’t some extra tongue and groove flooring. It’s the all the labor that’s involved.

This cut and patch method creates yet another joint where water can get in and cause issues. It’s a joint that can separate causing gaps and lifted parts of your flooring. Ultimately, it’s a cheap fix that never pays off in my honest opinion so skip it and do it right.

How to Repair Porch Flooring…the Right Way

If you’re going to do this the right way, and why wouldn’t you, then you need to completely replace any damaged boards. “But Scott, how do I avoid the rot from coming back again?” Wait, I got you covered. What’s I’m proposing will work to prevent rot even in the rainforest if you do it right!

Traditional porch flooring is typically 1×4 but some rare applications are 1×6 in size so check first. If you’re replacing porch flooring on a house built before 1930 there is a good chance that the flooring may not be exactly the same size as what is currently available. If that’s the case you may have to either modify the new stuff or simply rip out everything up to a certain area rather than lacing in new boards amongst the old.

The new stuff can vary in size from the old stuff by as much as 1/2” typically so do some checking on your house especially if it is older.

Tearing out the rotten porch flooring with a crow bar

Step #1 Tear Out

Tearing out that rotten porch flooring is best done with a big crow bar. The first piece may need to come out in pieces or you can rip it out with a cat’s paw or smaller pry bar, but once that one is out you’ll find that most porch flooring is installed with just one nail per joist through the tongue of the board. This is called blind nailing and it’s how we’ll be putting things back together later.

Put the crow bar below the board and yank it up to pop the boards loose along the length. It won’t take long but it can be back breaking work if you got a lot to do like I did.

Step #2 Layout

It’s imperative that you think about two major things when it comes to layout. First, are you tying into an existing floor that needs to match. If this is the case then you’re probably best to start at the existing flooring and follow that spacing. You want to avoid a board that has to be ripped down more narrow than the rest in the middle of the field. It looks amateur. If you have a narrow board on the outside at the end that is fine, but in the middle? No way.

Second, using a framing square you need to consistently keep your boards perpendicular to the house. Porch flooring was design to run straight from the house to the outside of the porch at a 90º angle (this allows water to follow the grain of the board and slope of the porch as it flows out and off your porch). Installing boards parallel to the house is a bid “No, No.”

As you begin installing be mindful that even a spacing error as small as 1/16” can add up to over 6” out of square over 100 boards. Check each board as you install and make sure you are keeping it square.

Nailing through the tongue on each joist leaves hidden fasteners and secure boards

Step #3 Installing Porch Flooring

When installing your porch flooring you don’t have to worry about the length of the boards because at the end we’ll be trimming everything evenly. So for now just make sure that all your boards are long enough to have a 1-2” overhang. It doesn’t matter if the outer ends are slightly or even greatly out of alignment at this point.

Whichever way to decide to install always setup so that the tongue is visible. This allows you to set a board in place and then blind nail one nail through the tongue into each joist. No nails showing makes for a cleaner and more water tight installation.

Push the boards tight against the house, giving an even 1/16” gap between boards for expansion and then nail it in place using a stainless steel 2 1/2” 15 ga. nail. Repeat ad-nauseam and keep the pattern steady.

Mitrered porch flooring joint
Matching the angle of the remaining boards for a tight fit

Bonus: Lacing in Boards

If you’re only doing specific boards rather than a whole section then see my previous post Invisible Repairs For a Hardwood Floor on how to remove individuals boards and install only select boards. In that post I talk about hardwood flooring, but the same technique applies to porch flooring since it’s also tongue and groove.

trimming tongue and groove porch floor boards flush
Use a circular saw to trim the ends of the boards flush

Step #4 Trim to Fit

Once all your boards are installed you need to trim the ends to make everything line up. The easiest way to do this is by running a chalk line across the outside edge where you will leave a 1/2” to 1” (totally depends on your preference) overhang. Mark the chalk line and then using a circular saw cut everything to match that same line. If you have some posts or railings in the way you may need to use a jig saw or hand saw the finish some of the tighter spots.

Step #5 Sand High Spots

Despite you best efforts you may have a few areas where the porch floor has some lips between a few boards. If that is the case then give them a little sanding to level things out. Any lippage like that can be a trip hazard and you want to resolve this before you start painting.

Preventing Porch Flooring Rot

“You still haven’t answered my question about rot, Scott!” Ok, ok here are the steps I use to make sure my exterior tongue and groove porch flooring doesn’t rot.

Step #1 Use Treated Lumber

Most lumberyards will offer tongue and groove porch flooring in regular pine or pressure treated pine. The pressure treated costs more but lasts longer. It’s not bulletproof, but rather than rotting in 2-4 years it will last 10-15 years so that is a good place to start, but we’re gonna take it even further because we want a 30+ year porch floor, right?

preventing rotten porch floor boards
Applying BoraCare prior to installation with a pump sprayer

Step #2 Treat with BoraCare

My favorite DIY treatment to prevent rot is BoraCare. Your mix it with some water and spray it onto all sides of the wood with a pump sprayer. Once it’s dry you install the wood as usual and prime and paint it. No muss, no fuss. This stuff protects against wood destroying rot as well as termites and other pests with a 30-year warranty. Don’t skip this step!

Step #3 Glue the Ends

Once your install is finished and you have trimmed the ends of your porch flooring so everything is lined up and pretty you need to come through with a wood glue like Titebond III and smear it into the end grain of all the porch floor boards.

End grain is like a super highway for absorbing water which leads to rot. By sealing the end grain with wood glue prior to painting you put a roadblock on that superhighway and stop the wood from absorbing water. You have just prevented rot.

repaired tongue and groove porch floor
The finished product!

Step #4 Keep it Painted

One last thing, if you keep things painted over the years you will protect your investment and make it last longer. A porch floor gets a lot of traffic so you need to keep up with the maintenance on that to prevent any pooling water or resolve issues before they become big problems.

My favorite paint for porch floors has been SherCryl by Sherwin Williams. It sticks to wood like white on rice and can handle the heavy foot traffic porch floors get.

Final Thoughts

If you’ve got rotten porch flooring like I did, I hope this post has given you the confidence to repair it. The video below will show you the whole process that I went through along with some surprises I encountered and how I dealt with them.

Don’t be intimidated by repairing tongue and groove porch flooring. It can be a lot of work, but doing it yourself can save you tons of money and be immensely satisfying work once you get to sit back and enjoy the view from a solid, rot-free front porch.

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5 thoughts on “Fixing a Rotten Tongue and Groove Porch Floor

  1. Anyone have any experience applying marine varnish to a tongue-and-groove porch? The porch in my case is covered. I am seeing a lot here about painting all sides. I had not planned on marine-varnishing all four sides.

  2. Do you recommend caulking the tiny gaps between boards before painting, or before repainting? We had one entire porch floor ripped out and replaced about six months ago. We live in a 1915 house, and the floor was damaged enough that we decided to replace rather than repair since the new material wouldn’t match the dimensions of the old material. I wish I had seen this earlier, so I could have asked our contractor to add glue to the ends of the boards. Once installed, the floor was painted with Sherwin Williams porch paint, which the painters and the paint box itself claimed was a primer and paint in one. I said I wanted it primed first, but they didn’t listen or forgot. The boards were pretty tight at first, but after the winter, there are small gaps between many of the boards from contraction. What is the best thing I could do at this point to help the longevity of the floor? Should I caulk between the boards, then prime and paint again? We live in a very humid climate, and I’m worried if a little water ends up between the boards, it will take a while to dry. Should I add glue to the ends of the boards, even though they’ve already been painted once? I’d be grateful for your advice. Thank you!!

  3. I have been working on porches for 40 years. Check out my blog to see some that I have been involved with. oldhouseporches.com I have seen them rot away in less than ten years. The Boracare is a good idea but I have been successful without it. Inspect your flooring regularly and add Boracare anywhere you see rot starting. You will see discoloring or soft spots. A porch floor needs painting two or three times as often as the house as it gets so much weather.
    Every porch floorboard should be painted on all four sides with deck and porch enamel before installation. The treated stuff actually states that on the label. People don’t read labels today. If both sides are not painted on exterior wood then it absorbs moisture more on one side which makes it warp. After installation add another coat to the surface. Do not use any house primer.
    The old timers caulked every tongue before they installed another board. That makes the deck absolutely waterproof and keeps water out from between the boards. If water sits between the boards it’ll rot them. I disagree with leaving a 16th of an inch between boards for expansion. The boards won’t expand unless you don’t put paint on them and they get wet. It is new wood so the boards will contract as they dry so you don’t want any extra space or the space between the boards will be too big. I have two douglas fir porch floors on our house that have lasted for over 30 years. When the ends get rough I sand them and then put another coat of paint on them. I use treated lumber for everything except the porch floor.

  4. Great job and a much needed project for all historic homes which normally have a porch or two that need maintenance. I will have to redo mine very soon as the previous owner did just what you mention not to do, repaired just the ends and cut boards and the entire section is bad so I can attest to doing right the first time, treating it with the Bora and then having a porch that will last. Thank you for explaining all the processes and steps and offering this up to other do-it-yourselfers.

  5. Is it true Scott that porch floor rot can also be contributed to no ventilation beneath the boards? I also have a large, wrap-around porch and the flooring needs to be replaced, so thanks for the great tutorial! I plan to install closable louvered vents in the hand-cast block perimeter to help with ventilation.

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