Glassing over Timber Decks

I have laid beech timber decks and want to glass over them - Is it possible to just remove the seams and fill and then glass over with expoxy resin and matt or do I have to lay plywood first and then glass over that?

 

Solid wood will contract and expand, and fiberglass, no matter what resin you use, cannot resist the wood's movement. The bond will be lost eventually unless an intermediate substrate such as plywood is laminated between.
The fasteners used (such as ring nails or bronze staples) move a bit within the softer plywood, and if the bond between the ply and the hardwood is of something like #5200 adhesive, quite a bit of differential movement can occur without any loss of adhesion.
I've never heard of beech being used for decking----- are you sure you don't mean larch? Or is there a down-under variety of beech that makes for a good boat wood?

Alan

 

I imagine you may be referring to queensland beech - a well known hardy deck timber.
The few samples I have seen are quite 'oily', and may not be a good candidate for epoxy anyway. May need to check that.
If it is queensland beach, what is the main resons for epoxy?
A good timber preparation should perform beautifully with all the attractiveness of timber, and relatively little maintenance.
If you epoxy, you also have to varnish (due to UV), which would be even more work than a proper timber finish.

 

Glass over wood, is allways bad. Thats my opinion, but I have NEVER seen this work out the way the owner wanted it to. In addition it allmost allways seems to promote wood rot faster then if you simply left the wood completely unprotected, with no coating at all.

My opinion.
K9

 

Glassing plywood or cedar strips such as are used to construct lightweight hulls (even forty footers) is a successful method because the wood species and dimensions are not dense enough or large-sectioned enough to pull free of the glass/resin layer that skins the wood. Not so for large-sectioned dense woods. There are practical limits for wood thickness and density (species) beyond which solid wood cannot be successfully glassed over, and even then, it becomes necessary to completely seal the opposite (inner) side with epoxy.
In this case, a plywood intermediate layer "marries" two dissimilar kinds of materials (different in expansion coefficient) together. The ply (say 1/4" or 3/8" e.g.) is mechanically fastened to the solid wood in addition to using a non-hardening adhesive like 3M 5200. The ply in turn is a perfect base for epoxy/cloth. This is tried and true technology. The deck in question will not be prone to rot simply because it has been sealed. In fact, it is less likely than before to allow the conditions that promote decay, namely moisture trapped in voids and fissures. Of course, care must be taken to ensure that all hardware is properly bedded and holes sealed. Many a deck has been made watertight and structurally sound by plywooding and glassing.
Kay9, I would not agree that glass over wood is always bad. Sometimes, it is very good. In the case of this beech deck, you are correct. Wood can't both expand/contract and remain adhered to a material that can't expand and contract. Something has to give. Either the glass will fracture or the bond will let go.

Alan

 

Epoxy coatings and epoxied sheathings over wood or wood products (sheet goods) have practical limitations.

Solid lumber ideally should be an 1" or less in laminations. This prevents moisture gain in a joint, where epoxy is used as an adhesive, from sheering the bond. Softwoods are more tolerant of slightly larger thicknesses.

Light weight sheathings serve as water vapor barriers and increase abrasion resistance only. It requires a fair thickness of GRP laminate to provide any real strength and compete with the strength of the wood, unless quite thin wooden pieces are used, in which case you're really building a GRP sandwich structure using a wooden core.

Some forms of wooden construction wouldn't exist without epoxy coatings, reinforcement and sheathings. Blanket statements about epoxy not belonging on wood is clearly not particularly well researched or understood. In this vain lets compare the usual suspects. Traditional oil coatings only keep out about 20% of the moisture vapor, which isn't especially good and you'll have lots of gain/lose to moisture content cycling. Varnish is much better at around 70%, polyurethane maybe 80% with shellac about 5% better still. The polyesters will get you just over 90% with limited peel strength and marine epoxy over 98%, so pick your poison folks.

The jury has been in on properly encapsulated wooden structural components. It does work and very well. From a statistical point of view and properly done, you'll not stabilize wood better with any other material.

To the original poster, you can sheath your decks, but you'll have to prep them properly. The best and recommended treatment is to fasten down a plywood substrate, in a bed of polysulfide or oil based bedding compound like Dolfinite. Sheath this with two layers of 8 ounce fabric set in epoxy or heavier lamination. Of course, the substrate would be fully encapsulated before fastening to the deck.

 

Covering Queensland beech - assuming it is smooth, attractive (being recently laid you would expct so) - with plywood, to epoxy over, is like covering polished marble with hand painted purple water based undercoat - an abomination in the site of Wooden Boat Gods!!!
This timber, with little more than annual scrub and recoat with a quality oil will last 50 years, be a delight to walk on, look amazing and make the boat truly classy.

 

It is assumed a problem exists, at least by me. I do agree that a newly laid deck ought to have been correctly fastened and caulked or else the beech could serve no purpose except as a structural component----- and it would have then been far more efficient to double-laminate a ply deck instead if that's the case. It would have been cheaper, faster, and lighter, if less attractive.
I'd assumed an old deck is being renewed and some practical considerations are emerging as the deck is being surveyed and its condition evaluated (recaulk with spot repairs or bite the bullet and commit to a serious rebuild procedure?).
Maybe Waverly could add some more details.

A.

 

I too made similar guesses at the current state of this deck. It's very likely that repairs and re-caulking will be less costly then a glassed over substrate.

If the deck was done (requiring replanking) I'd just yank up the whole thing, slap down some plywood over the beams and sheath this. The deck would be lighter, stronger and leak free. A veneer decking could be applied over the glassed substrate (if the laid look was desired), which would mimic the laid decking and serve as the wear portion of the surface, being renewed when worn thin, without affecting the leak free elements of the deck.

 

thanks alan, it's queensland beech which it widely used here but my decks are old and need work hence my enquiry to glass over them

 

Thanks for the advice and passionate comparision to marble - I wish I could take some photos and post for your inspection as it would make the situation a little clearer but your right of course and if it were possible I would prefer to keep the timber deck
Regards

 

of course and if it were possible I would prefer to keep the timber deck

Sounds like your choice is to completely rebuild the deck with wood , or rip it off and replace it with ply or glass.

Laying up a GRP + core deck on a very simple mold should take the least time , and require the least skills.

FF

 

I disagree about the least time and costs with a cored GRP approach.

The likely least cost, both in materials and labor is a laid deck.

The bottom line with laid decks are the condition of the seams and the remaining thickness of the decking material itself. If you have sufficient thickness then resurfacing, reefing out the seams and recaulking is the least costly way to go.

If the decking planks are worn thin and must be replaced, the cost would still probably be less then removing the deck, making a substrate and 'glassing that.

The only real cost you can control some what is the labor. If you do a fair portion yourself then you might be able to half the labor costs and still have the seams professional caulked.

If you're against reconditioning the deck planks, then a substrate (encapsulated plywood) and a fabric sheathing would be the tightest version of deck you can apply. This could be done over the current deck, though would add considerable weight or as a replacement for the current deck, which is lighter and stronger.

All this assumes the beams are in reasonable shape and the leaking deck hasn't caused fastener issues.

How much are you going to be able to perform yourself Waverley?