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

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Forum topic by CanofWorms2 posted 05-30-2017 06:12 PM 2550 views 0 times favorited 28 replies Add to Favorites Watch
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CanofWorms2

10 posts in 197 days


05-30-2017 06:12 PM

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


28 replies so far

View Rich's profile

Rich

1977 posts in 425 days


#1 posted 05-30-2017 06:42 PM

Pics?

-- No matter how much you push the envelope, it'll still be stationery.

View bandit571's profile

bandit571

18614 posts in 2519 days


#2 posted 05-30-2017 06:52 PM

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

-- A Planer? I'M the planer, this is what I use

View chrisstef's profile

chrisstef

17022 posts in 2842 days


#3 posted 05-30-2017 07:43 PM

You want the weight transferring toward the hinges.

-- Its not a crack, its a casting imperfection.

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CanofWorms2

10 posts in 197 days


#4 posted 05-30-2017 08:29 PM



You want the weight transferring toward the hinges.


++ so I would do from the upper outside to the lower hinge on both sides?

- chrisstef


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CanofWorms2

10 posts in 197 days


#5 posted 05-30-2017 08:29 PM



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

17022 posts in 2842 days


#6 posted 05-30-2017 08:45 PM

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.

-- Its not a crack, its a casting imperfection.

View clin's profile

clin

751 posts in 831 days


#7 posted 05-31-2017 03:28 AM

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.

-- Clin

View papadan's profile

papadan

3584 posts in 3204 days


#8 posted 05-31-2017 03:53 AM

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

View Desert_Woodworker's profile

Desert_Woodworker

1281 posts in 1050 days


#9 posted 05-31-2017 04:37 AM

It will bow or flex as shown…

-- Desert_Woodworker

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CanofWorms2

10 posts in 197 days


#10 posted 05-31-2017 06:20 PM

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?

View clin's profile

clin

751 posts in 831 days


#11 posted 05-31-2017 09:44 PM



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?

- CanofWorms2

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.

-- Clin

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CanofWorms2

10 posts in 197 days


#12 posted 05-31-2017 10:51 PM

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.

View jonah's profile

jonah

1448 posts in 3134 days


#13 posted 05-31-2017 10:58 PM

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.

View clin's profile

clin

751 posts in 831 days


#14 posted 05-31-2017 11:46 PM



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.

-- Clin

View JJohnston's profile

JJohnston

1622 posts in 3127 days


#15 posted 06-01-2017 12:04 AM

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.

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

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