I'm a fan of Ultra Gel or Clear Jel for thickening pie. Flour makes a cloudier filling than cornstarch, and I would choose tapioca flour over tapioca, and cornstarch over flour. Flour is my least favorite.

Because apples are approximately 86% water, as the fruit cooks the moisture becomes steam and liquid in the bottom of the pie, and the fruit shrinks leaving the gap between the fruit and crust, so consider some of these possible fixes. These methods should also help prevent "boil-over" and watery fillings.

-Choose apples that can hold their shape to begin with. Avoid using Golden and Red Delicious and Macintosh apples for pies because these apples tend to turn to mush.

-There are theories about the size of the slice. If you use a 1/4-inch slice, layer the slices in the bottom (tart-style), and build it up towards the center, rather than dumping the apple slices in a heap. This will prevent gaps and voids between the apples.

-In a bowl, mix the apple slices (whatever size of slices you use) with the sugar and spices. Place the sugar/spice coated apple slices in a colander over a bowl for anywhere between 1-1/2 to 3-hours (3 hours is best if you can spare the time - but there's NOTHING quick about making a pie, and the slices can sit while you make the pastry and during the 2-hours the pastry needs to chill before rolling out). This will partially sweeten the fruit, the sugar helps preserve the shape of the fruit, and it draws out the moisture.

After the resting time, pour the accumulated liquid in the bowl over the apple slices once the slices are placed in the pie. Another option with the liquid, cook it and reduce the liquid, and this will intensify the flavors. Let it cool before adding to the pie.

-Be generous with the size and number of vent holes in the top crust to allow the steam to escape.

-Grainlady

My apple pies would never make it in a judged show.
And that's okay with me,
I have no intention of entering them in a show!
But my family, and every friend I have ever served any to, love them!

I thicken them with tapioca,
In fact, I thicken any and all fruit pies with tapioca.
I use a mixture of apple varieties,
And almost always include one or two each
Of Macintosh and Golden Delicious.

And I almost always add just a bit of apple cider.

As I said, my pies would not stand up to a judges scrutiny,
But that's okay with me,
As long as my family is happy with them.

We also like a LOT of filling in them.
My crusts don't stand up in a dome after baking,
But I think I will try your colander method, Grainlady.
Along with the heaping in the center.
Thanks for sharing those tricks.

Rusty

I like tapioca because of its neutral non-flavor. I would never submit an entry for judging.
My only few fruit things are rustic galettes/crostatas. Garden fruits in season. Letting the lightly smashed fruit mixed with juice of a lemon or orange with the tapioca for a few hours or overnight softens the tapioca just enough.
Is clear gel modified corn starch? Thickens at room temp? I use it at work for a paint thickener. And is used often medically for patients that have trouble swallowing liquids...
(am i making this up, lol). I do buy it in bulk from a medical supply. It does not need heat so regular corn starch would be fine? Pies are baked with heat. Modified corn starch is more expensive and not necessary for pie?
Not sure. But tapioca does work for me. Flour is not an option...i learned years ago for any fruit desserts that rely on the fresh fruit flavor. A good pastry crust will add a bit of 'gravy' while cooking.
Sweating your fruit a bit like grainlady suggests will help. And do you cap your pie? A little foil hat might keep the top crust a bit soft early in baking and let it fall with your filling, then remove near the end of baking to crisp. Or place lower in your oven so it does not cook so far ahead of the rest of your pie?
Obviously i'm not a pie person, oh well. lol.

I'll have to try to remember all these pointers when I make my apple pie and see what happens. I've always used golden delicious apples for my pies so that may have contributed more to the apples sinking. Shall I use Granny Smith? or Fuji? So many things to think about now am getting a little worried but am very thankful for this forum.

I thicken my pies with flour but my mother always used tapioca. And I always try to use Spy apples but they are only available in October at the orchard.

I also found recipes for "Apple Pie Baked in a Brown Paper Bag." While I couldn't quite make myself put my pie in a paper bag in a 425 degree oven, I did opt for the alternative method of parchment paper (encasing it completely, stapling it all around). That is now the only way for me. No need for those annoying pie crust covers to keep it from over browning on the edges, and if anything overflows, it is all in the parchment paper, so no messy cookie sheet or oven. And the pie comes out beautifully brown in one hour.

I also thicken my pies with flour, that's how my family likes them, and how Grandma taught me. She'd have the basics, but nothing "exotic", LOL.

I do not care for the Clear Jel, it makes that gelatinous goo that you find in canned pie fillings, I just don't like the texture.

Beachlily kindly sent me some tapioca starch, all I could find here were the round "bubbles" of tapioca, and I used that in fresh strawberry pie last summer but have not used it in a baked pie.

I have submitted pies for judging, as has my daughter and granddaughter. I only won blue ribbons, but Amanda took a Grand Champion ribbon for rhubarb pie and Makayla got a Grand Champion ribbon last year for sweet cherry "hand" pies. She'd gotten a blue the year before that for pie too. And they were all made with flour as a thickener and used the old fashioned pastry recipe from Farm Journal's Pie Book. They were prettier than mine too, LOL, they are both more patient than I am.

Annie

To prevent both of your problems; Try this

Place your cut up apple slices in a large skillet, in portions if you are making a deep dish 9" , and quickly cook them for a short period with a little butter, your sugar and cinnamon. As the apples cook the liquid will develop and you can thicken them with a bit of flour.

By browning the flour, you get rid of the flour taste and can adjust the thickening to your desire, and by precooking your apples, they shrink slightly before you place them in your crust, so your crust does not cave in. It will also reduce your cooking time.

You didn't say if your bottom crust gets soggy, but you can blind bake it before your place your apples in it to prevent that.
Just make sure to cool your apples before putting them in your pie crust!

Good luck!