Bending plywood for roof

DrewE wrote:

Gdetrailer wrote:

You do realize "EXTERIOR GRADE" plywood simply means it is another name for BC grade plywood? BC grade is very rough texture on both sides with one side being lightly sanded.

Exterior grade plywood glue is not moisture resistant, get it wet or damp and it WILL eventually come apart, to get moisture resistant glue you have to MARINE GRADE and that is not sold in big box stores.

Everything I've read has said that exterior grade plywood means it uses waterproof glues, while interior plywood uses merely moisture-resistant glues. That's the main differentiator. Exterior plywood can come in many different face grades (as can interior plywood), though BCX is the most readily available one. Plain BC plywood (as opposed to BCX) is not exterior grade; that's what the X indicates.

Marine plywood is a higher grade for sure, and costs correspondingly more.

Drew I agree with you it's all about the resin or glue.
BC refers to the grade of the sides BC is basically one good side one rougher side. Sheathing grade plywood is often CDX. This is two sides that are relatively rough generally used where a smooth side is not required like a roof. Most construction grade plywood is pine or a softer wood however Fir and other woods are used for construction when more strength is desired.
Luan is general made with interior use in mind. The glue between the plies is interior grade. If the plywood gets wet the glues will dissolve and the plies will separate. This is classic delamination.
On the other hand Luan is desirable because it is a hardwood which gives it added strength. Luan is also very smooth. More importantly it is cheap which is why the RV industry uses it. Cheap is the magic word in the slap together RV world.
If it stays dry Luan works well. If it gets wet it delaminates easily however roof leaks generally come later after the warranty has expired.
There are products like advantech that is made with high grade exterior resin that hold up much better to moisture.
Pick your poison and your price point.

Gdetrailer wrote:

You do realize "EXTERIOR GRADE" plywood simply means it is another name for BC grade plywood? BC grade is very rough texture on both sides with one side being lightly sanded.

Exterior grade plywood glue is not moisture resistant, get it wet or damp and it WILL eventually come apart, to get moisture resistant glue you have to MARINE GRADE and that is not sold in big box stores.

Everything I've read has said that exterior grade plywood means it uses waterproof glues, while interior plywood uses merely moisture-resistant glues. That's the main differentiator. Exterior plywood can come in many different face grades (as can interior plywood), though BCX is the most readily available one. Plain BC plywood (as opposed to BCX) is not exterior grade; that's what the X indicates.

Marine plywood is a higher grade for sure, and costs correspondingly more.

Lantley wrote:
I have never seen exterior grade luan.I'm not saying it doesn't exist. But what I have seen in the big box stores is interior grade.

Do you THINK RV manufacturers use "moisture resistant" anything on RVs?

NOPE.

You do realize "EXTERIOR GRADE" plywood simply means it is another name for BC grade plywood? BC grade is very rough texture on both sides with one side being lightly sanded.

Exterior grade plywood glue is not moisture resistant, get it wet or damp and it WILL eventually come apart, to get moisture resistant glue you have to MARINE GRADE and that is not sold in big box stores.

The ONLY exception I have found is TREATED plywood (green color) which they must use water resistant glues since they must fully saturate the wood with WATER BASED PRESERVATIVES. That is not recommended for inside use or use where it may be exposed to inside or to fire and it is extremely heavy when new.

Exterior grade is meant to be COVERED and not seen, Luan on the other hand can be used exposed or covered. I have even substituted Luan for inside paneling in my trailer rebuild, just stained and hung it.. Much cheaper than paying twice the money for MDF paneling.

Luan on the other hand uses the SAME glue as exterior grade plywood but has one very smooth finegrain side and the sub layers have a finer wood grain than any plywood I have ever seen and with a bonus that they tend to use much higher quality sub plies which do not have voids.

1/4 luan is a lot nicer to work with than 1/4 plywood since it has finer wood grain and the sub layers do not have hidden voids and will be stronger because of that.

For a roof application Luan can be used, that is what I have done, works great and as long as you keep up with normal roof maintenance and keep the leaks at bay it will last as long as any other plywood.

Although if you are the scared person, then simply give the Luan a skim coat of polyester resin and boat cloth to seal it.. Many boat builders use Luan with fiberglass and resin and those last many years.

Bottom line we don't know what the OP's roof looks like. Never heard of 7/16" plywood, there is 1/2" that is in reality 15/32. OSB comes in 7/16" but I am NOT a fan of that for his application.

1/2" 3 ply cdx will bend as I mentioned on a gradual curve at a MUCH lower expense as the bender board.

I have never seen exterior grade luan.I'm not saying it doesn't exist. But what I have seen in the big box stores is interior grade.

theoldwizard1 wrote:

Gdetrailer wrote:
forgo the 7/16 single sheet of plywood and instead use two thinner sheets of plywood laminated in place. Thinner sheets will bend and conform much easier than one thick piece and will be far stronger, faster and safer than sawing a lot of relief cuts to the underside of the plywood.

Not sure why I didn't think of this, but it is much easier. Might cost a few $$$ more, but it will save a lot of grief.

Laminating is how it is done for this kind of bending application.

Thicker wood unless steamed (not good or recommended for plywood due to water based glues used) must have relief cuts which compromises strength for small radius bends.

I would skip the 1/4 plywood, 1/4 plywood typically has a lot of voids and coarse grain, use Luan, has much finer grain and virtually no hidden voids than 1/4 plywood, which also means more strength and less chance of breaking or snapping in the wrong place.

Overall Luan a better product to work with.

theoldwizard1 wrote:
For that thickness of plywood, I would kerf the back side with a circular saw about 1 ply deep. It will take several kerfs about 2"-4" apart. Fasten the flat end down and SLOWLY apply pressure to bend it.

I will join the chorus. This is how you do it. Not sure if 1 ply or more without seeing the wood. Might need 2 depending on the degree of curve.

Gdetrailer wrote:
forgo the 7/16 single sheet of plywood and instead use two thinner sheets of plywood laminated in place. Thinner sheets will bend and conform much easier than one thick piece and will be far stronger, faster and safer than sawing a lot of relief cuts to the underside of the plywood.

Not sure why I didn't think of this, but it is much easier. Might cost a few $$$ more, but it will save a lot of grief.

1/4 inch Luan or Plywood is all you need. 7/16 (OK- 1/2 inch) is way too heavy and too thick and is NOT needed. Why do you feel you need 7/16? Doug

forgo the 7/16 single sheet of plywood and instead use two thinner sheets of plywood laminated in place. Thinner sheets will bend and conform much easier than one thick piece and will be far stronger, faster and safer than sawing a lot of relief cuts to the underside of the plywood..

7/16 is .435" or about 10.1 mm (I don't speak mm but in this case it makes it easier to understand).

You could us two layers of "1/4" Luan which now days and is sold in mm thickness of 5.2 mm..

By the way, as long as you are not planning a lot of foot traffic on the roof, most manufacturers only use 1/4" for the roof and only add additional reinforcement at the roof vent where the roof air will be located.

Glue and screw the first layer down then glue and screw the second layer to the first layer..

Then cover with your choice of roofing material.

Laminating thinner materials in place is how a lot of boat builders make those sharp bends without breaking material or adding a lot of stress in the wrong places (that wood will always want to return to it's natural unbent state).