# How to brace from flexing? 46" wide door with plywood and 1x3.



## CanofWorms2 (May 30, 2017)

I'm building a door for a basement room with a 46" wide opening. 
The floor is irregular and the opening a little askew.
Burn those aren't my questions.
I have the door mostly complete but want to cross brace the face of the door so it remains relatively flat.
I took 1/2" finish plywood and wrapped the perimeter with 1×3" pine. I added extra 1×3 blocks to give the gate hinges some meat to hold onto. 
I can see the door flexes and want to put a diagonal plank on each side forming an with one plank on the outside and a second on the inside going the opposite direction. 
The hinge is on the right and it opens out. 
HOW should I place the diagonals?
See these two pics.
The solid line is the plank that will be on the out side and the dotted line is on the other side.
Or is there another way?
Also, now that I look at the door it is a little twisted from the get go. 
I think I need to dampen the door and lay it down flat with weights to dry? 
here are the pics


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## RichT (Oct 14, 2016)

Pics?


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## bandit571 (Jan 20, 2011)

Add a second "skin" to the other side of the door. Can even be just 1/4" stuff. Glue it to the framing.


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## chrisstef (Mar 3, 2010)

You want the weight transferring toward the hinges.


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## CanofWorms2 (May 30, 2017)

> You want the weight transferring toward the hinges.
> 
> 
> 
> ...


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## CanofWorms2 (May 30, 2017)

> Add a second "skin" to the other side of the door. Can even be just 1/4" stuff. Glue it to the framing.
> 
> I'm thinking this would make my door super heavy. It's already heavy…
> 
> - bandit571


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## chrisstef (Mar 3, 2010)

IMO you only need one cross brace. The solid line in your first pic and dashed line in your second pic. Its that top, outside edge (furthest from the hinges) that wants to sag.


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## clin (Sep 3, 2015)

Cross braces prevent sag by holding a rectangular frame square. But the plywood already does that. There's a reason a traditional door is made of relatively think stock with M&T joints.

Bandit571 has the answer. You would have been much better off to have created a frame and put two thin plywood sheets on both sides of this frame. What you have done here is put the plywood in the middle, where it does the least possible good.

The concept is how typical hollow core doors work with very thin skins. These are more generally referred to as torsion boxes. They are VERY stiff and do not bend easily.

At this point the best you could do is add a skin on each side. As bandit said, these can be thin. You could even go much thinner, even 1/8". Any thin sheet goods will work. Plywood or hardboard.

However, you need to add more internal supports for the skin. These DO NOT need to be braces, they DO NOT need to join to each other. They can just be randomly placed. They just need to be the same thickness as the framing you already added. And then need to be well glued to the existing plywood and the new skin.










Again, these are not braces. They function as shear webs. What they do is make sure the skins stay the same distance apart. Think of it like this. If you have two parallel lines, and then try to bend them into a circle, one has to bend more (smaller radius). To do this, it has to slide relative to the other sheet. If it is glued to it, by the shear webs, it cannot do this.

Also, when you attach the skin. make sure the door is flat. Because whatever shape it has when you attach the skins, will be the shape it will have from then on.


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## papadan (Mar 6, 2009)

In a basement with a door that wide, I would have hung it on a track like a barn door.


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## Desert_Woodworker (Jan 28, 2015)

It will bow or flex as shown…


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## CanofWorms2 (May 30, 2017)

great feedback guys. 
Could I do luan on one side with the blocks and then a diagonal brace on the other?

If I had not already glued the 1×3 down I would disassemble.

but then again, would I really be saving that much work… I could just start over.

Could I do luan on one side with the blocks and then a diagonal brace on the other?


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## clin (Sep 3, 2015)

> great feedback guys.
> Could I do luan on one side with the blocks and then a diagonal brace on the other?
> 
> If I had not already glued the 1×3 down I would disassemble.
> ...


Yes to the luan, no to the diagonal. A diagonal won't help anything other than to add a bit of stiffness. Adding a skin with the blocks to support it will make it much stiffer. Diagonals help hold a rectangular frame square. You have an entire sheet of plywood doing that.

Is there 1×3 on both sides, or is the other side just plywood?

Given what you have, I wouldn't be surprised if adding the blocks and a luan skin doesn't make it good enough. It will be much stiffer than it is now.

As it is now, the frame does almost nothing for the overall support and stiffness of the door. You pretty much have a 1/2" door. If you add a luan skin, the door will act as if it were about 1 5/8" thick. This comes from 1/2" plywood + 7/8" frame and blocks (assumed actual thickness of 1×3), and the 1/4" of luan.

And the nature of thickness, is things get stiffer by the cube of thickness, if it is solid, but it is almost as good for a torsion box. . So 1 5/8" will be about 30X stiffer than the 1/2" you have.

If you only add the blocks and skin to one side, it will not be symmetric. Essentially 1/2" ply one side and 1/4" on the other. Being different you may experience some warping with moisture changes. Fortunately plywood is pretty stable this way. But a coat of paint or other sealer would help. Anything that slows how quickly moisture moves in and out will be a help. But again, it's plywood not solid wood, so I wouldn't expect it to be too much of a problem.


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## CanofWorms2 (May 30, 2017)

1×3 on both sides.

I just need to add luan on both sides.

It's just a paint to try to figure out how to weight this beast down while gluing.

I need to just suck it up and luan both sides one at a time.


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## jonah (May 15, 2009)

You are going to end up with a hell of a heavy door if you do blocks and 1/4" plywood on both sides. You'd probably be better off starting over and making a torsion box-ish design with two sheets of 1/4 or 3/8" plywood separated by blocks of 2×3 spaced out around the area. Then use a solid 2×3 on each side for something to drill hinges into.


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## clin (Sep 3, 2015)

> You are going to end up with a hell of a heavy door if you do blocks and 1/4" plywood on both sides. You d probably be better off starting over and making a torsion box-ish design with two sheets of 1/4 or 3/8" plywood separated by blocks of 2×3 spaced out around the area. Then use a solid 2×3 on each side for something to drill hinges into.
> 
> - jonah


I agree. If you skin both sides, you'd have a fine door, but the 1/2" sheet in the middle is doing almost nothing but adding weight.

If you don't mind the weight, nothing wrong with plowing ahead, add the blocks and skinning both sides with luan.

A 4×8 sheet of luan is going to be about 20 lbs. Your door is almost that big. So you'd add about 40 lbs. Which is also about the weight of your 1/2" plywood. So if you just skin what you have, you'll have a 90-100 lb door.

If you start over you can shave 40 lbs off that. Does that matter? It is a very large door.

As an example, a solid core door weighs in the range of 4.5 lbs/sq ft. Assuming your door is about 7 ft tall, you have about 28 sq ft. So an equivalent solid core door would weigh 126 lbs. So if you just continue to work with what you have, it's not like you will have some crazy heavy door for it's size.

Given the plywood in the center, I understand why you need to stick with strap (gate) hinges. But I'd add blocks higher up, lower down, and in the middle (three hinges). The higher up the hinge, the less force on it for a given weight. Though if moving the hinges is a problem on the door jamb, I'd leave them. But still add the blocks, just in case. If you do, be sure to take a photo and note the location of the blocks, because you won't be able to see them after you skin it.

Of course a typical door has a solid frame on the sides so it makes no difference where you put the hinges.

Something else to look into before you spend any more $$$, is to get a quote form a door shop, if you have one in your area. I'm not sure if 46" width is possible for most shops, but you can get a custom sized hollow core door for well under $50. Of course, LJ is all about DIY, so I'm just pointing this out as a possibility.

Also, as far as skinning it. You need to lay it on a flat surface. I'm sure your floor is what you'd need. Don't assume the floor is flat. Shim as needed. Glue the blocks to the 1/2" plywood first. Perhaps use a brad nailer (if you have one) to tack these down to the 1/2" ply (still use glue). Then put glue on these and teh frame, lay the luan down and add all the weights you can find. Since the door will be flat, and the luan should want to lay flat, you shouldn't need a lot of weight.

If you have some bar and dumbbell weights, those work well. I have a set of steel weights that have got a lot more use in my shop than they ever did as exercise weights.

If you don't care so much about looks or would paint anyway, shoot some brads through the luan into the frame around the edges to help hold that tight until the glue dries.

One thing is for sure, if you skin both sides (with the blocks) your door will be plenty stiff.


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## JJohnston (May 22, 2009)

One option for keeping weight down is to harvest your shear blocks from the plywood panel itself. Cut evenly-spaced rectangular holes in it, then use the cutouts as the shear blocks. Glue them to the spots between the holes. No net weight gain, except for the glue.


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## Gilley23 (May 9, 2017)

This is a great idea. 


> One option for keeping weight down is to harvest your shear blocks from the plywood panel itself. Cut evenly-spaced rectangular holes in it, then use the cutouts as the shear blocks. Glue them to the spots between the holes. No net weight gain, except for the glue.
> 
> - JJohnston


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## 000 (Dec 9, 2015)

You could go buy 2 - 24" hollow core slab doors. (approx. 25.00 each) 
Cut them to size and glue them together. 
Cut out the top and bottom rails on each one, then and add a new full width top and bottom piece.


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## CanofWorms2 (May 30, 2017)

> One option for keeping weight down is to harvest your shear blocks from the plywood panel itself. Cut evenly-spaced rectangular holes in it, then use the cutouts as the shear blocks. Glue them to the spots between the holes. No net weight gain, except for the glue.
> 
> - JJohnston


plywood is too thin… problem is that the plywood is 1/4 or 3/8 and the 1×3s are whatever that depth is. Though, I could grab a hole saw and cut a ton of holes in the plywood to lighten it up…. But what would that do? 5 pounds at most?


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## clin (Sep 3, 2015)

> One option for keeping weight down is to harvest your shear blocks from the plywood panel itself. Cut evenly-spaced rectangular holes in it, then use the cutouts as the shear blocks. Glue them to the spots between the holes. No net weight gain, except for the glue.
> 
> - JJohnston
> 
> ...


I agree, this would result in minimal weight savings and the plywood blocks (1/2" I thought) would be thinner than the frame. If weight is an issue, I'd start over. But as I said before, it's not going to be an excessively heavy door if it comes out at 100 lbs. Though you'd likely want some help installing it.

If you did start over, you could go with a simple 2×4 frame and blocks of 2×4 inside to act as shear webs. Another option is to use foam board as the interior. The trick is you need it the same thickness as your frame. If the 2×4s are a bit thicker than the foam, and you have a thickness planner, you could bring those down to exactly match the foam thickness.

An advantage to starting over with a solid frame is you you can then use a typical door hinge. Though I would use heavy duty for a door this size.

The trick to this frame is having nice straight wood that will stay straight. Construction lumber is probably not the thing to use.


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## runswithscissors (Nov 8, 2012)

You could saw all around inside the frame and simply remove the plywood panel, since you have already glued on the frame. Then put the skins on. It would save a fair amount of weight. One advantage is it would give future archeologists something to puzzle over when they unearth it in a thousand years or so.


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## CanofWorms2 (May 30, 2017)

So I am revisiting this now.
And I realize that I don'[t have to worry about sagging.

I need to worry about warping.

How should I address this concern?


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## Snipes (Apr 3, 2012)

Is this the desired look you are after? If so leave it as is, they sell ready rod with turn buckle for taking out twist, could be used on inside if not visible. otherwise pull the twist opposite and glue another 1x on inside and it will generally end up straight.


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## CanofWorms2 (May 30, 2017)

> Is this the desired look you are after? If so leave it as is, they sell ready rod with turn buckle for taking out twist, could be used on inside if not visible. otherwise pull the twist opposite and glue another 1x on inside and it will generally end up straight.
> 
> - Snipes


I'm not too concerned with the look. Once it is sealed, mounted and has a door knob it will do the job. 
The door looks like the above pic with no cross braces. Should I do that solid diagonal 1x ?


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## CanofWorms2 (May 30, 2017)

I know the rod you are talking about.


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## DS (Oct 10, 2011)

A couple of things;

1) adding a skin to one side with spacers as noted will, in effect, create the torsion box which is fairly stiff. No need to do anything to the other side, IMHO.

2) It is important that your door, or any project, is on a true flat surface when you assemble it. If not, it will adopt whatever irregular surface twist there may be in the table and you will fight the twist constantly.

I suspect, if what you have already made were assembled flat, it would likely not have much of a twist now. I have an assembly bench about 24" above the floor that I maintain relatively true and flat for just this purpose. (Not a workbench)

Even with flimsy materials that flex at times, it would "want" to return to its "quiescent state" of being flat. (Disclaimer: Warping rails or stiles can influence this)

As Mel Gibson said in The Patriot, "Aim small, miss small."-works for woodworking too.


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## CanofWorms2 (May 30, 2017)

> A couple of things;
> 
> 1) adding a skin to one side with spacers as noted will, in effect, create the torsion box which is fairly stiff. No need to do anything to the other side, IMHO.
> 
> ...


Uhm… I think I did it on a flat surface, but not so sure. 
When I mounted it temporarily it seemed true, but was a easily twisted. 
So I really only want to stiffen it.


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## MrRon (Jul 9, 2009)

Clin, I followed this thread out of curiosity and you revealed some ideas I had never thought of. Gluing blocks of wood between panels is pure genius. I have a garage door 10' wide x 8' tall that was sheathed both sides with 3/8" plywood over a 2×6 frame. It has been up for about 10 years, but has warped so badly, that I can't use it. It is on an overhead track and is impossibly heavy to move as it binds. Had I known about your way of building a large door, I would have done it that way. Too late now, but a good idea to keep in mind for the future. I do have another 8×8 door that is a little warped and of the same construction as the other. Both doors are on my workshop, so I can do with one door. I had planned on getting rid of the larger door and walling up the opening. At 82+, I don't know if I will have enough time to do that; but I am happy to learn something new. Thank you Clin.


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## clin (Sep 3, 2015)

> Clin, I followed this thread out of curiosity and you revealed some ideas I had never thought of. Gluing blocks of wood between panels is pure genius. I have a garage door 10 wide x 8 tall that was sheathed both sides with 3/8" plywood over a 2×6 frame. It has been up for about 10 years, but has warped so badly, that I can t use it. It is on an overhead track and is impossibly heavy to move as it binds. Had I known about your way of building a large door, I would have done it that way. Too late now, but a good idea to keep in mind for the future. I do have another 8×8 door that is a little warped and of the same construction as the other. Both doors are on my workshop, so I can do with one door. I had planned on getting rid of the larger door and walling up the opening. At 82+, I don t know if I will have enough time to do that; but I am happy to learn something new. Thank you Clin.
> 
> - MrRon


Believe it or not, doors are sometime made with pretty much the equivalent of corrugated cardboard, on edge just zig zagging back and forth trapped between two skins.



> So I am revisiting this now.
> And I realize that I don [t have to worry about sagging.
> 
> I need to worry about warping.
> ...


I'm starting to sound like a broken record here. Make it into a torsion box, as I described earlier. It will be extremely stiff and will NOT warp. Torsion boxes are magic things. You get nearly the stiffness of a solid piece of material, but can be skinned with plywood which is much more stable than any solid wood would be. And it will be lighter than the equivalent solid slab.

What's not to love. Lighter, more stable, and almost as stiff a as solid chunk of material the same thickness.


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