Best Boat Floor Replacement Plywood Guide

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Mike Lillyman

Mike Lillyman is a writer with a deep love of the great outdoors and a passion for boating. As an avid outdoorsman and resident of the open water, he brings a wealth of knowledge and experience to the world of boating, offering practical advice and insider tips for anyone looking to get the most out of their vessel. Whether he's fishing, exploring new waterways, or simply enjoying the peace and tranquility of the open water, Mike's writing will inspire and motivate readers to take to the seas and embark on their own adventures.

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The DownTrodden Boat Floor

The boat floor takes a real hammering. It has to deal with heavy loads carelessly dumped on it. Water slopping around in dark corners and stagnant air in musky hollows are all part of the floor’s experience. In addition, a boat floor is a perfect place for plywood to rot. Stale air and moisture are the elements that encourage spores to feed. If these conditions are rife around your boat floor, then that is where the spores will be licking their lips in anticipation of a great meal. It is your boat floor that is on the menu.

The sagging sections in your floor can no longer be ignored. Sadly rot has set in, and now you need to replace the floor. The best floor replacement plywood is typically Marine Plywood. That is until you see the price and then glances focus on the alternatives. Here we are going to have a look at what makes Marine Plywood the first choice. But perhaps some other options will ride a little more gently over the bank balance, so let’s consider some other options as well.

What Causes Rot

Trees fall and decay. Fungi spores work away on the forest floor, breaking up the wood to ensure that it re-enters the forest’s life cycle. 

This same fungus will happily set up a demolition site in the plywood layers of the boat floor when the conditions are right. For ‘Brown’ or ‘Dry Rot’ to develop, water and warm temperatures are required. The fungus attacks the cellulose in the wood and results in a significant reduction in the strength of the wood. This results in the wood becoming brittle and eventually breaking up.

The presence of water also causes wet rot. It often changes the color of the wood to a white or pale appearance due to the fungus feeding on the lignin in the wood. Wet rot is limited to areas where the wood is wet and does not spread like dry rot.

Some form of protection needs to be introduced to prevent this from happening to your boat floor. A barrier will keep moisture out of the wood and keep it dry.

What types of plywood should you use for the boat floor?

Plywood is graded. Some would say from ‘expensive to very expensive.’ But, seriously, when choosing plywood, the following gradings have definite characteristics:

Marine plywood is designed to be used in boat building. The wood used, the glue, and the construction method, are all aimed to withstand the demanding conditions found in boat building. The American National Standards Institute determines the criteria for grading marine plywood.

Top of the list for quality are boards marked A-A. What makes this plywood so good is the use of hardwoods throughout the board.  There are no voids in the layers and no defects in the back and front plies. The faces are sanded to a good finish. While side joints are permitted in the core plies, end joints are not allowed. This has important implications for voids in the sheet, as we will see a little later.

The use of hardwood throughout the board does not mean that the wood will not rot. On the contrary, given the right conditions, it will deteriorate. Therefore, the only benefit your hard-earned money will get you is a slower rate of rotting.

Marine-grade plywood does not undergo any treatment to protect it from moisture. The glue used in Marine Plywood is guaranteed to withstand continuous immersion in water. However, the individual layers of plywood enjoy no such protection.

Frequently, the glue used is described as WBP, which indicates a Weather and Boil Proof glue. What this means is that the plywood will not delaminate even if immersed in water. The glue also prevents moisture from penetrating through the individual plies.

So Marine Plywood will have superior strength primarily due to the greater number of layers found in the plywood and the fact that all the layers are of excellent quality hardwood with no end joints.

However, Marine Grade Plywood is expensive and, it needs protection. So why can’t cheaper exterior quality plywood be used instead of the costly marine plywood if both plywood grades need protection? Let’s have a quick look at the differences between the two types of plywood.

What are the differences between Marine and Exterior Plywood?

Apart from the price, there are some technical differences between the two types of plywood. Both plywoods are intended for external use, but marine plywood is recommended for the more stringent boat building requirements. ‘But what about for floors?’ I hear you say.

One of the significant differences is that end joints are permitted in Exterior Ply but not in Marine Ply. This is significant because the likelihood of void spaces in the layers is reduced significantly with no end joints. Void spaces affect the strength and integrity of the sheet. They also are ideal venues for spores to settle in and begin their trail of destruction. In Exterior plywood, voids are permitted, as are knots of up to one and a half inches wide.

As Marine Plywood consists of specially selected hardwood layers, the board’s appearance is superior to that of the External plywood. Therefore, you need to ask just how vital the visible layer on your floor is? If you are going to paint the floor, then the external layer’s appearance is of relatively minor significance.

What factors determine the replacement Plywood?

A Marine Plywood floor will cost a fair amount more than the closest appropriate replacement plywood, so what would validate the additional expense?

The most important determinant of choice is going to be the aesthetics of the floor. A beautifully crafted varnished boat is going to cry out for a floor of charming wood grain. External plywood will be at a disadvantage in appearance and the amount of sanding required to produce a fair enough surface for varnishing. Don’t forget knots are permitted in exterior ply as well as slight imperfections. Both of these issues are going to impact the appearance of the finished product.

No doubt someone will come along and suggest the myriad of synthetic decking materials available today. For example, synthetic sheets that can cover the plywood base and, to be sure, there are some absolutely beautiful samples around.

The trouble is that once you have bought the external plywood and then laid out some more money for the synthetic sheeting (these synthetic sheets are not cheap), the comparable cost of marine plywood may be less.

In my experience, varnished floors are great just so long as all the crew and guests take their shoes off before boarding. So if you are lucky enough to have such well-behaved guests and crew, by all means, stay with a varnished marine plywood floor.

Conclusion

Cost is always a factor but let’s go through the actual process of fitting the new floor to see if there is genuinely a benefit in using Marine Plywood.

The most susceptible areas for plywood to absorb water are the end grain areas and, it doesn’t matter too much if it is marine or external plywood. They will both absorb water through the end grain. So it is vital for the longevity of the plywood to seal the end grain, and my suggested way of doing this is to fix a solid hardwood frame around the plywood. Using the appropriate waterproof glue to fit the hardwood will be an enormous advantage.

Regardless of the type of plywood used, the underside of the flooring will require careful sealing, and this would typically be painting. Unfortunately, this plywood side gets the lesser time devoted to it, yet it is the side more likely to be getting wet.

So up to this stage, both plywoods have had the same treatment, and I really cannot say there has been an advantage to warrant the more expensive plywood. Now we have to deal with the visible side.

If the flooring is going to be covered, such as flooring inside the cabin, I would save the additional cost and use external plywood. Flooring inside cabins has an easier time unless there is permanent water in the bilges and ventilation is less than adequate. Then, if appropriate measures are in place, such as effective bilge pumps and good ventilation, external ply  (suitably sealed ) will be completely satisfactory.

As beautiful as the highly glossed varnished grain of marine plywood is, my preference is always for flooring that is nonslip and easy to maintain. Keeping the brightwork in top condition is work enough, and I don’t want my boating time eroded by the need to extend my varnishing chores to include the flooring.

As soon as you decide to paint the plywood flooring, the advantage of marine plywood is diminished, and as long as the external plywood is appropriately sealed and painted, it will be up to the task.

There is one area where external plywood is not going to compete with marine plywood, and that is in conditions where weight is of high importance. The fact that marine plywood has more plies makes it inherently stiffer and stronger and means that a thinner ply can be used. So if weight is critical, you may need to spend that extra amount and buy the A grade Marine Plywood.

For me, I want a floor that will provide secure footing even when wet and external plywood properly treated has served me well over the years.

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