Caulking for Wooden Deck

I believe that the gap is there for a purpose, to allow for expansion and contraction (like an expansion joint). Normally it is not a good idea to fill this area. I am not an expert in this field however, it is just what I understand to be true.

If the wood has cracks water will leak into the cracks and freeze. When water freezes it will expand. This will make the cracks larger over time it will split the wood completely. I would recommend filling the cracks while the weather is still good. Do some research and decide for yourself what is best for you.

As the gaps are there to help with both expansion/contraction and with rainwater drainage issues, perhaps it might be better to leave them alone?

The others are correct in saying it is for expansion and contraction of the wood. If you do fill between the boards. If the fill gets hard and is not flexible as the wood contracts the paint will crack along what ever edge of the board it pulls away from. Some of it over time will fall away. As the board expand it may make the boards buckle up. If you use a soft flexible material to fill the joints. The paint will flake off under expansion and contraction. The surface of the filler will bulge and bow cracking the paint.

I recommend just painting the boards.

If you are just wanting to make it easier to paint, save yourself the trouble and get a thick nap roller or a 6" brush and apply generously.

If you want to resurface the deck and require a smooth finish, use a Portland cement filler and a trowel. This stuff is kind of expensive but it is resistant to moisture and used as a floor leveler in bathrooms, etc.

Note however that this sets up quite hard and is not supposed to be sanded after it cures. So when you trowel it out, you need to feather the edges so as to not have ridges. It sets up in about 10 minutes and drys in an hour or two depending on conditions and thickness. So, only mix up what you can use fairly quickly.

BTW, most of the expansion and contraction mentioned has to do with water absorption, not temperature. If you seal all of the water out, it will cause the wood to contract, leaving cracks. So you might have to apply a thin second coat to fill the cracks after a month, and then paint. Splinters will be non-existent but it converts a wooden surface to something like concrete over a wooden floor. Still subject to cracking, but you have to decide what you want. You can always lay outdoor tile over it if you have used the Portland cement as a prep.

I don't think the tongue on T&G flooring is more than 1/4 inch long. That's a lot of warp. If pulling all the deck up and renailing is out of the question, caulk it with silicone, or something flexible.

Paint won't adhere to silicone, go acrylic

paperguy could you elaborate a little. Do you mean that you have T@G decking that has pulled apart so badly that the tongue has pulled out of the groove and there is a 1/4 inch gap to boot?

Or have the boards separated 1/4 inch but the tongue is still in the groove?

Personally i've never seen T@G used as decking because of the water drainage problem.

Lastly T@G does mean tongue and groove, correct?

The decking is on the porch of a 100 year old house. It is tongue and groove fir. There are no gaps through the decking... the tongues are still in place. The previous owner used a hard filler that has cracked out of the gaps in many places. I'll pressure wash out the remainder. What I'm hoping for is a caulk that will move with the wood and keep dirt from collecting in the spaces between the boards once the fresh paint seals them. Thank you all for your help!

2 pack epoxy 100% polyamide cured is probably your best bet if you do not want the black coal tar epoxy - no solvent and no contraction - exterior durability excellent. Put epoxy in the crack then force rope/string in. Finish with epoxy to smooth finish.

Why not use the old shipbuilders technique. They used jute "tow" and tar and this was hammered into the cracks with blunt chisels. Jute or other natural fibre - probably rope would do and instead of tar you could use an epoxy mix. I would suggest the polyamide type rather than the amine cured type. If you can get some coal tar epoxy then you would do well - it has excellent flexibility. Problem with coal tar epoxy is overpainting - the bitumen used can bleed through the paint.

In olden days, we, sailors used to fill the gaps in the wooden deck planks of navy ships with oakum leaving just about 1/3 of an inch that was finished with tar (bitumin). Both have to be renewed at least once a year.

Not sure what kind of decking you are talking about. If it is outdoor decking (ie. patio) then by all means don't fill the gaps. You need to provide a way for the water to drain after a storm. If you are talking about boat decking or other surfaces that need to shed water, I would select a silicone. Silicone has the best properties in terms of weathering, adhesion and flexibility. Silicone is difficult to paint so I would select a color matched variety and apply after painting.

As a boat builder, I would recommend a light base of cotton string or jute in the bottom of the seam. This is to keep the caulking from sticking to the bottom. Don't use silicon or polyurethane. Both are unsuitable in this application. Use a polysulfide caulk like 'Boatlife^®' , available from Marine suppliers. It is more flexible than polyurethane and has better adhesion than silicone. If you want to do a real good job, make sure the bottom of the seam is covered with fiber, masking tape the edges, fill the seam with polysulfide, smooth with a gloved finger, and give it a light misting with water. When the caulk has skinned over, remove the tape. Now you have a Pro job that will last 20 years.

PS; For the smaller cracks that you can't put fiber in the bottom, just make sure not to completely fill the gap. This is so the caulk is not forced to flex in two directions, which will cause seperation. And Polysulfide comes in colors, and may be painted after a full cure, which water accelerates.

On sailboats with teak decks polysulfone is used to caulk the gaps. Teak usually has to be primed because it is so oily but I don't think you will have to prime the fir, just make sure it is dry.

Polysulfone never completely hardens so it will flex for a long time. You can't use polysulfone with plexiglas though. Now that you need to worry about that for this application...

Another caulk that might work well in this application is Sikaflex. I don't know much about it but my father loves the stuff.

You can get polysulfone in normal caulking tubes at West Marine or other ships stores. You can paint and sand the stuff too. Might be ideal for this application.

GA, But Sikaflex, as good as it is, does not flex enough. 3M 5200 is of similar qualities. It can even be used to join planks that will later be steam bent. Marvelous stuff.

A two part poly-sulfide is used on the missile blocks for nuclear reactor decks, over a rope caulk.

We are on the same page however. This is another case where the public doesn't know what's out there. Ya can't get it at Home Depot, and if you ask, DUH?

At professional plumbing supply stores, a product referred to as "dumdum" (sometimes called plumbers putty but there are various types of plumbers putty). It comes as one pound rectangular blocks. It NEVER dries hard and takes paint like a magnet. Material that was used 40 years ago that I had to remove was as soft as the day it was first used. I even considered reusing it except there would have been a lot of imbedded paint. It is messy and a little stringy to work with and requires a day or so to skin over so you can lightly sand to feather the edge.

I redid my front porch last year and used it to bridge larger gaps at the edges. In the board gaps I chose to use a little harder setting painters putty (White Lightning brand Fix'n Finish from a dedicated paint supply store). A year later(one winter cycle) and I don't see any cracking. In fact there is slight bulging which tells me it is still soft enough to deal with expansion. The only areas I see cracks are those where I used wood putty to fill gouges and broken out board edges. I figure there will probably be a few areas that will show a little cracking, but another coat of paint after two years should take care of that.

Be sure to scrape out the gaps well and blow out with compressed air. I was amazed how much dirt was compacted in the gaps. For the rest of the porch decking, I used a heat gun and removed ALL the paint and belt sanded to bare, flush. As bad as the paint had flaked after 100 years of over-painting and the boards cupped, there was no other acceptable way. Before using the putty, I primed the gaps, just to keep any future moisture from soaking in if cracks appear. It is now flat as a pool table and unlikely to warp anymore. After it was done, my Father, Brother and the mailman (he sees more porch floors than anyone) complemented the job.