plywood moisture content

[marquette]

i have read many of the post here over the last couple of years about fiberglasing over P/T wood. i have read the this artical http://glen-l.com/wood-plywood/fiberglass-plywood.html and it seems like a fairly good test. the bond between fiberglass and wood seems pretty even at 65% moisture content and declines at 95%. i assume the bond varies in strength based on absorption into the wood. the question i have is how to measure the moisture content? do i just trust the lumber yard? if i buy it at a big box store how do i know i have waited long enough for it to be at 65%? will the wood re-absorb moisture from the air if the humidity is high for an extended period? we steam wood to bend it so wood will absorb water from the air. is there a simple test to determine moisture content in plywood? i have read on posts here everyting from buying wood a year in advance to let it dry to stacking it properly and blowing air over it with fans or heaters. all of which are good ideas but how do you know when it is dry enough. is it done by a known weight of a 65% 4x8 sheet or the SWAG method or what?

 

[92excel]

Re: plywood moisture content

the question i have is how to measure the moisture content? do i just trust the lumber yard? if i buy it at a big box store how do i know i have waited long enough for it to be at 65%?

You can buy a moisture meter. I know you can get them at home chepo. You can pick up a cheap one for like 20 bucks.

 

[sschefer]

Re: plywood moisture content

You can buy a moisture meter. I know you can get them at home chepo. You can pick up a cheap one for like 20 bucks.

Lot's of experience with this - If you buy your wood from a reputable yard they will have a moisture meter with probes to check the overall moisture content of the material. It needs to be checked at several points and then averaged.

The cheapo units at HD only check surface moisture content and even the better ones with spike style probes still only get you a rough guess.

Think of it this way, if your surface MC is 15% but the core MC is 70% where is that core moisture headed when it begins to hydrate out.

 

[oops!]

Re: plywood moisture content

with out a gauge....your guessing.....but....if your guessing....put heat and a fan on it......and after a month when you think its dry......dry it some more till you KNOW its dry

 

[marquette]

Re: plywood moisture content

using one of the moisture meters from home depot could i measure along a freshly cut edge to tell core moisture? are the meters usually found in the tool dept or somewhere around the lumber?
we have a lumber yard here in minneapolis that claims they have dried P/T wood. would i be out of line to ask them to verify the moisture content on each of the sheets i buy or do they do that on a regular basis?
what is the cost between a good moisture meter and a cheap one? the lumber yard gets about 3 times the price per sheet compared to the big box stores. i look at tools as an investment. if after 2 or 3 boats i reach a break even point then having my own good meter could make sense.
thanks for the good information.

 

[sschefer]

Re: plywood moisture content

Most lumber yards don't have the facilities to properly dry lumber, that takes a kiln if they intend to dry at a rate consistent with high sales volumes. They usually just keep it in a semi controlled environment (covered area) and let it air dry.

A better yard that is charging a premium for dried lumber will sticker the lumber, conduct routine moisture checks and adjust the environment according to the amount of dryness that is recommended as optimal for the particular type and species of wood. They usually don't buy in large quantities nor do they sell that way either. These are your custom wood distributors.

We're talking pressure treated lumber here. Go buy it, find a place to lay it flat and place stickers between it and other sheets. If you're resting spot for this wood is concrete then lay a piece of Tyvex moisture barrier down first, then some 1x2 stickers then a sheet and more 1x2 and the next sheet etc. Turn the sheets once a week.

During the winter months you won't see a huge drop in moisture content unless your storage space is heated. In the mid to late spring, you're biggest change will occur and in the summer when the humidity rises the moisture will return.

My rule of thumb is to buy fresh lumber and note the weight of the wood when I buy it. Its moisture content is highest then and it should be fairly heavy. When it's dry there will be a significant loss of weight which is very noticable. This usually occurs after about a month in mid to late spring like conditions and I usually find that the wood is at about 14-17% moisture and ready to work.

If you want a good meter get a Lignomat.

As a side note: Most all P/T wood contains copper so contact with bare aluminum will create zinc oxide and destroy the aluminum through electrolysis.

The way around it is to use Storm Flash, GAF-ELK or other self adhesive vulcanized flashing in between the wood and the aluminum wherever contact is made and use 18-8 Stainless fastners with a coating of zinc based anti seize compound. That still won't prevent leaching caused by a good soaking on a rainy fishing day so you need to paint your bilges too.

I'll stop for fear of causing a rukus over which type of wood to use. I have Tinny's and I use Exterior Ply and have a couple of low RPM fans/bilge blowers to dry my boat out between uses. That's just what works for me in my climate in Northern California where humidity is rarely a factor.

 

[marquette]

Re: plywood moisture content

are there 2 standards for measuring moisture content in wood? i went online and loked at some moisture meters including the Lignomat. none of them measured more than 40% and most including the Lignomat were in the 5-25% range. the reason i ask is that the test data was on 65% and 95% moisture. if it is all the same standard and you beleive the test data the wood doesn't have to be very dry for fiberglass to stick. they did not encapsulate the wood with fiberglass to trap moisture inside the wood so that may be another issue all together. thinking about this out loud 65% doesn't even sound like it was very dry. if the wood wasn't going to be fully encapsulated when the weight of a sheet of P/T dropped by 1/3 it should be dry enough for fiberglass to bond. i'm thinking of something like a casting deck where the underside would get be left uncovered so air could get to the wood and it could continue to dry and a layer of cloth could be put on top to provide a smooth void free surface. something like a stringer or transom that gets fully enclosed where there is no place for moisture to escape would probally require a much lower moisture content no matter what kind of wood that was used.

 

[sschefer]

Re: plywood moisture content

marquette: I think that's solid logic about about encapsulation. I don't know of multiple standards for moisture content in wood products. The 65% number would be extremely wet and 100% would mean surface water or possibly even submersion. No wonder it didn't stick...LOL.

I suspect that 14-17% is the baseline and 28-34% is the equivilant of the 100%. That would make better sense and get us back to a single standard.

 

[marquette]

Re: plywood moisture content

sschefer- if you were to guess what would you say the average moisture content would be of a sheet of P/T at the average big box lumber store? because if you handle several sheets of it you can feel the moisture coming off it in your gloves. and if you saw it as soon as you bring it home it binds the saw and the sawdust is wet. why would someone doing a test want to increase the moisture content to beyond that level? it would make no sense to go to a test level that isn't practical or common.

 

[tashasdaddy]

Re: plywood moisture content

I never use Pressure treated, it will never completely dry out. the chemicals are not friendly to metal fasteners. to me there are just too many draw backs to PT.

 

[sschefer]

Re: plywood moisture content

I never use Pressure treated, it will never completely dry out. the chemicals are not friendly to metal fasteners. to me there are just too many draw backs to PT.

I don't want to hijack this thread but yes, I agree. Lot's of folks will aruge but not me.

 

[sschefer]

Re: plywood moisture content

sschefer- if you were to guess what would you say the average moisture content would be of a sheet of P/T at the average big box lumber store? because if you handle several sheets of it you can feel the moisture coming off it in your gloves. and if you saw it as soon as you bring it home it binds the saw and the sawdust is wet. why would someone doing a test want to increase the moisture content to beyond that level? it would make no sense to go to a test level that isn't practical or common.

From what I understand the test was done as a comparison to a dried piece of P/T. The point they were trying to acertain was at what moisture content did adhesive quality deteriorate. IE, was it at 65% or 95% above that of a realtively dry piece of P/T lumber.

The lumber that is delivered to a big box store is the bottom of the food chain stuff. They treat it, ship it and hope that by the time it reaches the store it is in a condition that is usable. Many times it is not but most folks that shop for lumber at big box stores either don't know or don't care. To make it usable we have to do what the lumber yard didn't. It needs to be properly stickered for air circulation and then dried to a point that it is usable.

I think what we all have to come to terms with is that your average P/T lumber was never designed for precision millwork or cabinetry. It is designed to withstand long periods of contact with soil. We're trying to use it for something it was not deisgned for.

 

[Hanr3]

Re: plywood moisture content

IF your worried about moisture content, let it dry for 2-3 days. That's it.

There seems to be some misunderstanding about pressure treated plywood.
Maybe this will help.
Pressure treated gets its name because of the method the wood is treated. Plywood sheets are put into a container and sealed, a liquid chemical (CCA) is pumped into the wood and penetrates every cell. Once the wood is saturated the pressure is removed and the excess is vacuumed off. Then the wood is kiln dried to roughly 19%. The kiln drying process bonds the chemicals to the cells of the wood. This is a permanent bond and not even a complete water soaking will reactivate the chemicals. Hence why pressure treated wood is not considered a hazardous material. While the CCA is still in liquid form it is a hazardous material. This checmial reaction also makes the wood easier to paint/stain and retains the paint/stain better over regular untreated wood.

The disadvantage to using p/t with aluminum is the corrosive nature of CCA and aluminum. Although as stated earlier, the chemical will not leach. The simple solution is to use a protective barrier between the two. Like a rubber coating to go between the aluminum frame and the wood deck. Check your local automotive store for a rubber coating in a spray can. Works great and will insulate any loose creaky spots between teh frame and wood.

As for durability, I made a pressure treated cellar door for my mother-in-law 20 years ago. It was never painted and is exposed to mother nature 365 days of the year. Right now it has 6" of snow on top. Other than some rust spots from the fasteners it works and functions as the day I installed it 20 years ago.

If you really want longevity, build it out of western red cedar. It will never rot. Marine grade is another grade of pressure treated.

 

[Hanr3]

Re: plywood moisture content

Here is some more information.
First link is about Marine Grade Plywood.
http://www.glen-l.com/wood-plywood/marine-plywood.html

"Marine-grade plywood is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, it should be pressure-preservative treated to an appropriate standard." In other words if your worried about rot due to moisture use pressure treated plywood. Marine grade plywood is NOT more rot resistant than pressure treated plywood, it is only a better grade of materials, in that it has less defects, gaps, holes, spaces, etc within the ply's.


Second link is about building boats with plywood. REad through the part about bonding resin to both untreated and pressure treated. The only difference to bondability is when the moisture content is above 65% saturation level. Actually higher than that, the testing was conducted at 65% and 90%.
http://www.glen-l.com/wood-plywood/boatbuilding-plywood.html

If your buying your plywood from the lumber yard, you shoyuldn't have any worries about moisture content unless is sat in the rain recently.

 

[marquette]

Re: plywood moisture content

thanks those are both good links to read. i have only ever used P/T from the big box lumber yards. the clause in there that the manufacture does not have to dry the wood to 18% after treating if waived by the specifier is probably how the big box stores keep the price down by not having it redried after treating. so it seems that determining moisture content is low enough on any wood to be fiberglassed is important to proper bonding. though it does appear from the tests that there is a fairly large window of % of moisture that is ok.
i have a sort of test going of my own. not scientific by any means. i needed to make a top for a poling platform last summer. i used a piece of 3/4 P/T that was a left from a sheet i had bought from Home Depot about a month before. the partial sheet had been sitting on edge in my garage after i had used what i needed. after cutting the platform to size i resined the edges and put a layer of cloth on the top surface. the bottom was painted with rustoleum top side paint. the top with the rustoleum w/non skid. i made no effort to judge the dryness of the wood since it is fully exposed and if i had a problem i could easily take it off and replace it. it has been sitting in the Texas sun and salt air for only about 6 months but has shown no sign of delaminating even with using the high pressure car wash on it after each time the boat goes out. i think by the end of next summer should be a good test of the bond.

 

[sschefer]

Re: plywood moisture content

Carbon Fiber is about as close as I get to glass work so I won't even begin to think I can advise on that. I would speculate that the proper application of the substance would have as much to do with it as would moisture content.

By the way, you are absolutely correct about the way big boxes get their lumber. It's all about quantities and convienence. Cutting corners to cut cost is profitable when the majority of the customers don't have a means to determine the quality of the product.

 

[mthieme]

Re: plywood moisture content

The originally post link referred to relative humidity not moisture content.

 

[sschefer]

Re: plywood moisture content

The originally post link referred to relative humidity not moisture content.

Humm, good point, it was. Then wouldn't it have as much to do with the process as it would mositure content of the wood. Like I said previously, I'm not into plastic boats, I'd rather play with tin toys. There is a place for this process there also so I'm pretty interested in getting to the bottom of this, (if there is such a place).

 

[marquette]

Re: plywood moisture content

the link says they pre conditioned the wood to 65% and 95% humidties. what does that mean? does it mean they took wood with an 18% moisture content and placed in a box (room) with 65 or 95% humidity? if so how lond did they wait to see if the wood absorbed moisture out of the air? or did they just do the fiberglassing in a room with controlled humidity? if that is the case wouldn't that just be a test to see how fiberglass works at various humidity not the type of wood?