Waterproofing Plywood. Sealing Plywood for longer life.

Plywood is an important and stable building material, comprised of many different veneers glued together at right angles. These layers provide dimensional stability, but leave the edges, which expose all of the laminate sections, particularly vulnerable to water damage. Different grades of plywood exist, using different glues to hold the veneers together, which themselves may be made of different materials (hardwood or softwood for instance). These offer varying degrees of water resistance, but waterproofing plywood edges, and to a lesser extent the board faces themselves, will significantly extend the life of plywood in any environment where water is present. This technique for waterproofing plywood edges is especially valuable for treating exposed cut edges of commercially finished phenolic faced plywood boards, such as Buffalo Board™. These boards are often used for extreme duty exterior applications such as trailer floors, and sealing the edges of these boards can significantly extend their useful life. Smiths CPES does not attack the phenolic surface of such boards, and offers high quality weather protection to the not protected surfaces of them.

How much water will plywood absorb?

As mentioned, plywood is sold in a variety of different qualities.

The following images show 5 sheets of a relatively cheap siding grade plywood, which is designed for exterior use to sheathe the outside of a house.

siding plywood waterproof impregnated with dyed cpes

The plywood has been stood in Smiths Clear Penetrating Epoxy Sealer that has been dyed blue. The bottom edge of the plywood sheets were placed in a shallow tray, and the dyed CPES was allowed to be wicked into the wood.

The CPES was allowed to cure and then the sections were cut open and sanded to show these internal sections.

The dyed regions show just how far the CPES has penetrated into the plywood, and clearly demonstrate areas of abnormal porosity (areas that are significantly darker than those around them). These areas of abnormal porosity will be the first to fail in exterior use, as they will readily wick up water and encourage the formation of fungal growth (rot).

Can we waterproof plywood with Clear Penetrating Epoxy Sealer?

We can significantly enhance the resistance of plywood to the ingress of water with Smiths CPES.

The photograph below shows another 5 sheets of siding plywood. These sheets were first saturated with CPES – which is naturally clear.

48 hours was allowed for drying, and then the sheets were placed in a shallow tray with dyed CPES like the samples in the photograph above.

waterproofing plywood siding ply impregnated with dyed cpes after sealing

The CPES was then allowed to cure once more, and the sections were cut and sanded as above.

The dyed regions are far smaller than before. The wood has far less porosity and will be far more water resistant.

This explains why sound wood treated with Clear Penetrating Epoxy Sealer resists rot so very well. The CPES will also make the final varnish or paint finish last much longer, see  Varnish Outdoor Wood: Priming To Make Varnish or Paint Finishes Last for details.

The Rot Doctor in America recently completed some endurance testing of plywood with and without CPES treatment. You can see the results of this testing here: Exterior plywood exposure testing with CPES.

waterproofing plywood exterior exposure smiths cpes

The Example: A Hand Made Walnut Bath Panel

For this example I wanted to create a bath panel to match the flooring in my new bathroom. The bathroom is tiny, little more than 2 metres cubed, and so it had been relatively cheap to floor it in Walnut. It looked great, the new bath looked great, but I could not face having a huge cheap plastic bath panel to look at.

Walnut Bathroom Flooring

There were plenty of ‘walnut effect’ bath panels available, but I thought that the best way to make one of those look really nasty would be to put it next to real walnut. Decision made, more walnut floorboards would be bought, and a panel fabricated to size.

Making the Bath Panel

Rather than just gluing the floorboards together on their tongues, I decided to glue them on to a substrate of 10mm Plywood. The additional weight would not be massive, and the 10mm ply would be dimensionally stable. This is after all the same reason that engineered floor boards are made of solid wood faced onto plywood, and are required in wet areas and areas where underfloor heating is to be used; the plywood is far more dimensionally stable than regular timber.The floorboards could be lined up one after the other. Individually glued and then screwed from the back through the ply panel with the screws seating into the back of the floorboards. Each board could be done one at a time, and all joints made nice and tight before proceeding with the next.

Do not use PVA Wood Glue

This is my big mistake with this project. I thought that gluing the boards would add significant additional strength to the end result. I happily glued each board down with 3 or 4 lines of PVA adhesive. The next morning my nightmare became true, the construction had bowed so badly from the moisture that I had introduced to it. Perfectly flat plywood and floorboards, held down with weights whilst setting, had warped. Worse it had warped into a structure so strong that I could not easily straighten it. If I do anything like this again I shall use a non water based adhesive between the boards and the ply, probably Oak and Teak Epoxy. I am sure other glues would be fine for this laminating, just ensure that they are not water based, as it was the water that warped the boards. The cure involved two massive steel sections, both glued and screwed to the panel. The steels were first cleaned of rust, and then phosphated by treating with Phosphoric Acid to ensure good adhesion of the adhesive. Smiths Oak and teak Epoxy was used, which bonds very strongly to the phosphated iron and to the panels themselves. Whilst this fix worked, do please save yourself some time, and use a non water based wood glue in the first place. I certainly wish that I had! The glued and screwed steels are shown below.

Bath Panel sealed with CPES with Steels fitted

The phosphating marks on the steel are clearly visible on the steels in this shot:

Phosphaetd steel stuck to wood panel with oak and teak epoxy

Finishing The Bath Panel

The bath panel was made slightly over size, and was then cut to rough size with a skill saw. Once close the final cut to size was performed with a router against a straight edge. This technique leaves a lovely finished edge, and the finished panel is shown below.

Fabricated bath Panel cut and router trimmed to final size

Waterproofing Plywood Composite Bath Panel

We now have a bath panel with massive strength, with a lot of exposed edge of plywood from both the engineered floorboards and the base plywood panel. One of these edges will sit directly on the floor, which is going to get wet, and the other just under the rim of the bath. As I have a young boy for testing the waterproofing of bathroom components, all plywood edges needed sealing badly. Waterproofing plywood edges is imperative. As children love throwing water around I decided to seal with CPES all faces that were not factory finished with 2 part polyurethane, ie the walnut floorboard surface itself. This is what that bottom edge of plywood is subjected to each bath time:

Wet edge of plywood from bath use

Wet edge of plywood from bath use 2

Smiths Clear Penetrating Epoxy Sealer was used to seal the edges and faces of the plywood. Particular attention is given to the edges, as they offer exposed end grain which will naturally wick up water. Fortunately the edges also wick up the CPES very readily. Apply coats until they stop taking any more, allowing the solvents to evaporate (the smell to dissipate) between coats. Typically 3 coats are enough. No further treatment is required as the CPES will not be exposed to sunlight, so no UV protection is required.

Smiths CPES sealed edge of Plywood bath panel

Waterproofing Plywood Results

The bath panel above was constructed in January 2015 and has been in daily use with our tester since fabrication. It shows no sign of water damage or ingress after well over 300 ‘rinse cycles’. The following picture was taken in early summer 2016, and slight discolouration is visible from the dirt that resides at floor level, but no damage to the wood, which is still as firm as when it was first treated.

sealed bottom plywood edge after 2 years water exposure

The Rot Doctor in America has just published his results on the preservative effects of CPES on plywood under exposure to the most extreme elements. The results are astonishing, and demonstrate how well CPES acts as a waterproofing preservative on plywood. Click here to view the results.

We anticipate a beautiful bath panel that will outlast the bathroom, and we will update the photos above every so often as befits any good durability story.

Want to have a go:

These are the products I did use, and should have used. Oak and Teak to actually laminate the floorboards to the plywood backing will avoid warping which may ruin your day if you use a water based adhesive.