Filling cracks in pressure-treated lumber

[DaveC426913]

TL;DR Summary

Can I fill the cracks in my PT 6x6s to discourage frost heave?

I'm building a low wall out of 6x6 PT lumber on my front yard. The lumber has long splits in it, up to an eighth of an inch wide.

These will fill with snowmelt in the winter, freeze, and surely widen the cracks even more. Will this eventually cause the beams to disintegrate?

I built walls 15 years ago, but they were of those fake 3x5 rounded railway ties, that aren't pressure-treated. So those have been rotted by ground-contact and bugs. Ideally, this time will be the last. I hope these new walls will outlast me.

Can I fill them with something to discourage further splitting? Maybe some white glue mixed with sawdust?

@phinds

 

[phinds]

White glue is certainly not the answer as it will degrade over time w/ moisture. I tried Titebond III in one outdoor application and was very disappointed when it failed after only one season so even though it's touted as waterproof, it's not, fully. Of course I DID put in on red oak, so the porosity of that wood allowed more moisture access to the glue line than would be the case with other woods but had it been truly waterproof that would not have mattered.

I believe, but have not researched it, that there are epoxies that are genuinely waterproof so that's what I'd suggest you look for (and possibly mix w/ sawdust or fine shaving like from a lathe).

Also, if you don't care about looks just fill the cracks with roofing tar which is definitely waterproof. Since it's in your front yard, I'd think this would not be acceptable.

 

[hutchphd]

In my experience (Maine and upstate New York) the effect of freezing expansion on wood in this use is seldom a problem. I always put the growth rings so they make a "roof" and not a "cup" but nothing otherwise. I think there is enough flex in the system to not worry about lasting mechanical damage from water expansion...and remember the checking is due to shrinkage initially.
I have never had a problem but will happily stand corrected by someone who has !

 

[jrmichler]

Wood has three different rates of expansion with changes in moisture. Longitudinal is lowest, circumferential is highest, and radial is roughly half of circumferential. If the moisture content decreases, and the center of the tree is inside the timber, you will get radial cracks.

Dry wood (less than about 10% moisture) cannot rot. Saturated wood does not rot. That's why wood fence posts rot at ground level, but not deep underground or in the air. Pressure treating is the solution for wood in contact with both ground and air.

How's this for a shrinkage crack? The disk is 14" diameter. My wife wanted a wood disk for an art project.

 

[phinds]

Wood has three different rates of expansion with changes in moisture. Longitudinal is lowest, circumferential is highest, and radial is roughly half of circumferential.

All true but irrelevant to this thread. Dave's not asking about wood movement due to change in ambient moisture conditions, which is what you just described, he's worrying about ice forming in the existing cracks and then expanding and cracking the wood further and what he might do to stop that.

 

[Rive]

Can I fill them with something to discourage further splitting?

As far as I know these things fillings (especially anything not wood-like, like epoxy) will likely just make this worse since it won't have the same movements as the base material, so every time the wood gets wet/dry or hot/cold and expands/shrinks the filling will just split the wood further.

By my experience about wood and weather the key would be the regular treatment of the wood instead. It is a bit of an outdated and not really acceptable practice anymore, but I've seen here some 50+ year old fences: the traditional treatment was to paint them every year with the worst kind of waste oil (burnt, spent, carcinogenic, almost black, quite poisonous thing...)

Some people here still prefer used cooking oil (the worse the best) compared to modern chemicals to preserve the woodwork in the attic. They swear that it kills all the bugs.
To be honest I think that stuff would kill everything

So maybe you better stick with the 'regular' part

 

[Guineafowl]

Use a two-part epoxy type like this one ^^

The weak part is the join to the wood which will eventually fail and allow the filled segments to peel out. Some staples or panel pins (galvanised) hammered in before the filler will help anchor things.

You can grain the filler with a segment of hacksaw blade.

 

[phinds]

As far as I know these things fillings (especially anything not wood-like, like epoxy) will likely just make this worse since it won't have the same movements as the base material, so every time the wood gets wet/dry or hot/cold and expands/shrinks the filling will just split the wood further.

I disagree. The crack area is very narrow and won't expand/contract very much at all.

@Guineafowl 's point about it possibly falling out is a good one, but given the irregular shape of the cracks I doubt that's really a problem in this case.

 

[hutchphd]

As far as I know these things fillings (especially anything not wood-like, like epoxy) will likely just make this worse since it won't have the same movements as the base material, so every time the wood gets wet/dry or hot/cold and expands/shrinks the filling will just split the wood further.

By my experience about wood and weather the key would be the regular treatment of the wood instead.

Additionally any applied filler will age very differently from the unfinished wood even if it initially matches. Trying to surface seal timbers on the ground is not likely a successful enterprise anyhow.
On above ground cedar I do apply a "soft"sealer concocted from linseed oil melted paraffin and mineral spirits which I bet it is similar to Thompson's water seal. It makes it patina nicely and seems to keep it happy. But I don't think pressure treated lumber cares.

 

[DaveC426913]

By my experience about wood and weather the key would be the regular treatment of the wood instead.

The problem I'm trying to solve is not something I would expect a sealer would fix.

These cracks already exist - and they're between 1/16th 1/8th inch wide - so it's not like I'm trying to seal the wood itself against water incursion. I'm really just trying to stop water from pooling in them.

I'm assuming that water could fill these large cracks and, when it freezes, will physically push the cracks apart, splitting it (just like it does with rocks).

Now, it's possible that this will not make the cracks significantly worse over time. I'm not sure.

Let me rephrase: I won't attempt to fix a problem if it's not problem. In y'alls's * opinion is this a problem that could be - and/or should be - remedied?

*yes, y'alls's

 

[hutchphd]

The problem I'm trying to solve is not something I would expect a sealer would fix.

These cracks already exist - and they're between 1/16th 1/8th inch wide - so it's not like I'm trying to seal the wood itself against water incursion. I'm really just trying to stop water from pooling in them.

I'm assuming that water could fill these large cracks and, when it freezes, will physically push the cracks apart, splitting it (just like it does with rocks).

Now, it's possible that this will not make the cracks significantly worse over time. I'm not sure.

Let me rephrase: I won't attempt to fix a problem if it's not problem. In y'alls's * opinion is this a problem that could be - and/or should be - remedied?

*yes, y'alls's

I'all would not try fill the cracks. ff possible try to use fastenings that don't exacerbate the "splitting" (e.g. put spikes bolts etc so any compressive force helps). I believe the natural moisture content changes will produce more motion than any potemtial frost heave.
But if y'all are callin' me "y'all", how much freeze can there be?

 

[DaveC426913]

I believe the natural moisture content changes will produce more motion than any potemtial frost heave.

Sorry, frost heave was the wrong phrase. I"m not worried about movement; I'm worried about the lumber splitting until it disintegrates.

But if y'all are callin' me "y'all", how much freeze can there be?

Never mind water - here in the winters of The Great White North, we occasionally have to worry about the CO2 condensing.

 

[Rive]

Let me rephrase: I won't attempt to fix a problem if it's not problem. In y'alls's * opinion is this a problem that could be - and/or should be - remedied?

Unless you plan to give it a complete surface finish (strong enough to keep the moisture content inside the wood in check) I would not care about filling the cracks: but I would give it a surface treatment regularly, in a way that it can get inside those big cracks too (since that's also surface).

 

[DaveC426913]

Thanx all. I'll leave well enough alone.

 

[hutchphd]

Sorry, frost heave was the wrong phrase. I"m not worried about movement; I'm worried about the lumber splitting until it disintegrates.

Yeah I knew my use of the term was wrong...but I couldn't think of a better one...how about "internal frost heave"
The nice thing about the "dry ice snow" is that it doesn't leave puddles beside your boots...

 

[zul8tr]

Some Q's and suggestions:

1) what treatment type is on the label? I assume your 6x6 are at least ground contact general purpose unless you were able to get the next more severe treatment level up or have access to contractor treatment CCA high level dose the best. This CCA was banned by EPA years ago but contractors caan get it as well as Marine engineers doing docks and pilings, and electric companies for power poles, etc..

2) which way will the cracks be oriented horizontal plane or vertical plane? Or are the cracks on all sides?

3) I have had success with crack filling on PT and many other materials for expansion and contraction joints using SIKA polyurethane fillers, great stuff. Several different viscosity's available, just read the tube and select type for your use. They can easily handle what that wood will experience with cold and hot, snow and rain freezing and thaw. They have great adherence property on wood, metal, concrete, stone, etc.. I get mine at Lowes. Uses a standard caulk gun available in regular and big gun super duty size. Squeeze it in and final push in the cracks with a putty blade

4) If the PT is ground contact general purpose and not CCA treat all cut areas. I use brush constancy roof tar (thinned as needed with low cost paint thinner) on all surfaces that are cut or ground contact because today's treatments available at Lowes, Home Depot, etc are not as good as CCA and need help to last.,

5) If gravity works for your wall use spikes only if necessary, they promote rot. If you use spikes or bolts they need to be double hot dipped galvanized, or Stainless Steel, Yikes the cost!

6) Have you considered a full coating on the finished wall for additional protection, there are many available, I have favorites?

7) What are you doing for behind the wall drainage?

8) Post wall design details, happy to look at it, retired Civil Engineer on many diverse projects including retaining walls, building structures, foundations, groundwater, master drainage, water resources, flood control, hydraulics, etc.

 

[NTL2009]

Thanx all. I'll leave well enough alone.

I think you will be fine. You say cracks are 1/16" to 1/8", so round to 1/10" (0.1"). Ice expands ~ 10%, so we are talking maybe 0.01" expansion across a 6" timber. If I minded my decimal places, that's < 0.2%. I would think the wood will easily flex that much w/o damage, and return to the original shape at the thaw. It's not like inflexible stone that expands, does not flex back, then expands again with the next freeze/thaw cycle until there is a large crack.

Plus, the water/ice is not contained, so I imagine it would bulge out of the crack, into open space, as it expands, so not 100% of the expansion will work against the lumber.

Even with PT lumber, I'd be concerned about the ground contact. I'm picturing these are horizontal to form a wall, with soil mounded behind them? I would want some permeable fabric behind the lumber, back-fill with maybe 6" of round stone for drainage, and then the soil? Search what others are doing, but that sounds like a good approach to me.

 

[skystare]

Empirically speaking, no problem. I have a stone and timber raised flower bed with that type of treated wood. After 30 years, no change.

 

[DaveC426913]

1) what treatment type is on the label? I assume your 6x6 are at least ground contact general purpose

I did not know about rating before starting.
At least some of the lumber I've bought is ground-contact rated.

2) which way will the cracks be oriented horizontal plane or vertical plane? Or are the cracks on all sides?

I started off hoping I could orient the largest cracks on the sides, but gave up.

3) I have had success with crack filling on PT and many other materials for expansion and contraction joints using SIKA polyurethane fillers, great stuff. Several different viscosity's available, just read the tube and select type for your use. They can easily handle what that wood will experience with cold and hot, snow and rain freezing and thaw. They have great adherence property on wood, metal, concrete, stone, etc.. I get mine at Lowes. Uses a standard caulk gun available in regular and big gun super duty size. Squeeze it in and final push in the cracks with a putty blade
4) If the PT is ground contact general purpose and not CCA treat all cut areas. I use brush constancy roof tar (thinned as needed with low cost paint thinner) on all surfaces that are cut or ground contact because today's treatments available at Lowes, Home Depot, etc are not as good as CCA and need help to last.,

I'm beginning to think it would look terrible for all the effort.

5) If gravity works for your wall use spikes only if necessary, they promote rot. If you use spikes or bolts they need to be double hot dipped galvanized, or Stainless Steel, Yikes the cost!

Spikes. Dang.

7) What are you doing for behind the wall drainage?

Umm...

The beds are 8'x8' by 16" high. They have no floor, but are dirt all the way down. I will be installing soaker hoses.

8) Post wall design details, happy to look at it, retired Civil Engineer on many diverse projects including retaining walls, building structures, foundations, groundwater, master drainage, water resources, flood control, hydraulics, etc.

Thanks. This is part of my sketch:

 

[Wes Tausend]

Summary: Can I fill the cracks in my PT 6x6s to discourage frost heave?

I'm building a low wall out of 6x6 PT lumber on my front yard. The lumber has long splits in it, up to an eighth of an inch wide.

These will fill with snowmelt in the winter, freeze, and surely widen the cracks even more. Will this eventually cause the beams to disintegrate?

I built walls 15 years ago, but they were of those fake 3x5 rounded railway ties, that aren't pressure-treated. So those have been rotted by ground-contact and bugs. Ideally, this time will be the last. I hope these new walls will outlast me.

Can I fill them with something to discourage further splitting? Maybe some white glue mixed with sawdust?

Dave,

Frost expansion is a more minor problem with wood as opposed to concrete and asphalt. However some is present.

I believe the board cracks in you picture are largely caused by the unevenness of increased moisture in the bottom portion of the boards. If a suspended board lies face up for instance, rain falls on the face and migrates throughout the entire board. But being in the sun, the upper surfaces dries much sooner which causes an uneven shrinking.

The uneven expansion effect is not unlike a piece of thin plywood lying on a lawn. Normally, after a rain, the wood will initially swell more on the topside which causes it to curl downwards. Later, as the top dries in the sun, some of the moisture which has by now accumulated nearer the underside, causes the plywood to curl upwards. This is the same uneven effect that causes a dried mud puddle to break into cracked, curly segments. However, a thick board will crack before it curls.

If possible, I would seal the soil backside of vertical lumber used for a retaining wall, if that is what you are planning. Any excess humidity coming from behind such a wall will cause a greater expansion of the backside opposite the open face, which will exacerbate face cracking. Vertical placement will assist the face not absorbing standing water.

Offhand, when necessary, a possible exterior-grade wood filler, is to use catalyzed auto-body filler compound as is for sale at Walmart. I discovered this when I once applied some leftover compound to some plywood deck sheeting, since I'd used rough, C-grade sheets as a underlayment for planned patio stone. I noticed these sheets were collecting water in the knot voids and my concern for damage was an expected delay in finishing the project. The confounded delay finally took years and the raw, unpainted compound was still perfectly intact and stuck fine to the wood, seemingly unaffected by weather. The rest of the ply was shot. (My wife won the original argument and finally got her plastic wood decking.)

Good luck with your project.

Wes