how to buy flat MDF

[Paulh Tremblay]

I just had 4 sheets of MDF delivered to my house. The box store picked out the 4 sheets themselves.

As I moved the last sheet to my basement, I noticed a very bad cup in the sheet (about 1 inch over 8 feet.) I called the box store, and they handled it very well by having the driver bring me a new sheet.

However, in between the time I called and the new sheet came, I cut another sheet to 4 X 6. When I put a straight edge on it, it had a 3/8'' cup. I can't use this for the top of my torsion box, which will function as an assembly table.

The replacement sheet was dead flat. I checked one of the other sheets, and it seems dead flat, too.

My question is how to make sure other sheets I buy are flat. If I go to the box store alone, I can't pull a sheet off the big pile and test it. Even if I could pull the sheet off the pile, you can really only test it by placing it on a relatively flat surface. (Maybe I could put it on the floor of the store and test it?)

I choose this box store because it delivers for very good price. I don't have a pick up truck. I could rent a truck and buy from a real lumber yard, but the problem then is unloading it. My wife is not always available.

[Matt Day]

Couple questions.

Where is your shop? How far do you have to go once you have the sheet In your driveway?

Can you breakdown the sheets at the box store before bringing home?

Could you have a cutting station (track saw) in the garage to break down to size?

I rented a truck from HD when I bought 15 or so sheets of ply and it worked great. The truck bed is designed to haul things like sheet goods. I used a simple stanley plastic panel carrier (google it) and I could carry sheets of 3/4 ply solo - mdf would be harder.

[Tony Haukap]

I would bet what you're seeing is a 'set' the MDF has taken from storage. If you're storing it by leaning it against a wall, even with 3/4" material, it can happen in hours. Heat and humidity also plays a big part. If you want it to stay flat you need to store it by laying the sheets fully supported on a flat smooth surface.

[Phil Thien]

I would bet what you're seeing is a 'set' the MDF has taken from storage. If you're storing it by leaning it against a wall, even with 3/4" material, it can happen in hours. Heat and humidity also plays a big part. If you want it to stay flat you need to store it by laying the sheets fully supported on a flat smooth surface.

^^^ This. MDF sags, and needs to be supported if you want it perfectly flat.

[Tom M King]

We hand pick everything we use. The junk on top of the stack, whether board, plywood, or any other sheet, gets laid to the side, and we pick what we want. The junk gets put back on top of the stack. This goes for anything from framing lumber, on up. When the store gets a call for a delivery, it's the perfect opportunity to just grab the first pieces the help gets to off the top of the stack. I discovered this when I built my first house in 1973, so it's nothing unusual these days. Here, all sheet goods are stored flat until use.

[pat warner]

Top 2 or 3 sheets always cuppped. Get your first one in 4th place.
Must be delivered in a truck that can support them on edge out of the sun.
If the stock hits the sun during delivery: Pretzel city.
Store flat and cover with an expendable cover sheet.

[scott vroom]

I would think that minor cupping would be easily corrected when fastening the MDF to the dead flat torsion box. Did I miss something?

[Phil Thien]

I would think that minor cupping would be easily corrected when fastening the MDF to the dead flat torsion box. Did I miss something?

I agree. It sags easily. It flattens equally easily.

[pat warner]

But at the expense of bending something else up.
The stress of mishapen stock, whence pinched down to something else, is shared x all of the components it's fastened too.
3/4" MDF has substantial modulus, skinny stuff less so.
Best to start out with flat stock.

[Art Mann]

I'm quite certain all MDF starts its life as a flat sheet. The material stored anywhere, home or Home Depot, will warp if not correctly stored or supported. If your structure requires a sheet that the design itself will not hold flat, then you probably need to choose a different material. MDF is a cheap material and isn't appropriate for a lot of applications where it is commonly used.

[Phil Thien]

But at the expense of bending something else up.
The stress of mishapen stock, whence pinched down to something else, is shared x all of the components it's fastened too.
3/4" MDF has substantial modulus, skinny stuff less so.
Best to start out with flat stock.

Disagree that MDF has substantial modulus [of elasticity]. That is the entire reason it wants to sag to begin with.

Just build the frame for the torsion box, leave the MDF on the top over night, and it will be resting flat by the next morning.

[Myk Rian]

Here's how you carry sheet goods without a truck.

[glenn bradley]

I would think that minor cupping would be easily corrected when fastening the MDF to the dead flat torsion box. Did I miss something?

I agree. MDF is very pliable. We prize it for its flat parallel faces. It should easily conform to your gridwork. Even laminated 3/4" slabs used for router tables will sag if not properly supported.

[pat warner]

"Disagree that MDF has substantial modulus [of elasticity]."
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You are correct. But given large surfaces you have long levers and stuff can be pulled flat fairly easily.
Try flattening a 12" x 12" x 3/4" square with say a 1/16" cup in it.

[Andrew Hughes]

Well at least he used the twine they offer.