Multitasking with oven ( roast and potatoes)

This is one of the reasons fan ovens were invented - the oven elements in standard ovens are in the top and bottom of the compartment, so they don't work well for multi-level cooking, as items placed on the top shelf prevent heat from reaching the top of items on the bottom shelf, and vice-versa.

In your case I believe you have two options:

1. Tip out most of the liquid from the meat roasting pan and roast the potatoes with the meat. A layer of fat a few mm deep is desirable. This might, however, cause your roast to dry out, so you would have to be diligent in basting the meat.

2. Take the meat out to rest, and then cook the potatoes. If you turn your oven up to around 200°C, you can roast the potatoes (assuming you pre-boil them, and you should) in about 45 minutes. The meat will cool a little, but it will be well rested, and a bit of hot gravy will compensate for the heat loss anyway.

I always roast my meat on a rack placed over a roasting tin - the tin beneath then has the roast potatoes cooking inside. If the meat I'm roasting is fatty, as in duck, then I'll cook it on its own for 45 minutes or so, empty out the fat in the tin, add the potatoes and return to the oven. It may or may not be necessary to empty out more fat as cooking proceeds because you're right, too much fat in the tin means soggy potatoes. The trick is simply to keep an eye on it and empty out any excess as cooking proceeds. Not usually a problem with roast chicken, but lamb sometimes can produce a little too much liquid fat. I usually remove the meat when its done and let it rest, and in that time, the potatoes will be on their own in the oven, turned up a bit higher if they're not as crispy as I'd like, but usually, I don't have to do that. As for cooking the potatoes separately but directly beneath the meat tin, my experience is the same as yours, despite having a fan oven - they don't crisp and they take longer to cook. My belief is that the tray above blocks heat access because its solid, unlike an oven rack.

To make really crispy roast potatoes you need very high heat, which is bad for your meat, therefore you will need to take the meat out and roast the potatoes while the meat rests. You should be resting your meat anyway, for beef and lamb at least 45 minutes to an hour under a foil tent or wrapped in foil. This gives plenty of time to blast the potatoes.

In the UK it's traditional to par-boil the potatoes until they start to get tender, then drain them and rough them up before roasting them in oil or fat for about 45 minutes. This gives you potatoes that are tender on the inside and crispy on the outside. For best results use a medium moisture potato, and an oil or fat with a high smoke temperature. Maris piper or king edward potatoes are popular in the UK, in the US I've found Yukon Golds to work, or any white potato variety. Here's a decent guide.

Here's what I'd suggest:

1. Reverse your food positions. Roast the meat toward the bottom of the oven and the potatoes toward the top.
2. Place the potatoes on a jelly-roll sheet, not in a roasting pan.
3. Stagger the pan positions. The less that they are exactly on top of one another the better. Yet, at the same time, no pan can be so close to any oven wall that it cuts off circulation. So, choose pans sizes that will allow for this. During baking, rotate each pan 180 degrees.
4. It takes more energy to heat more items. When you put more items into the oven than called for by the recipe, you must increase either the heat, or the time, or both.

For a long time I had this exact problem, and I asked many people for their technique, but nothing really worked well. Then I kind of stumbled onto the answer recently. The key is that roast potatoes crisp up really well in groundnut oil (peanut oil) even at a low temperature.

Here's what I did this Christmas to make the best roast chicken and roast potatoes I have ever made:

Dry brine the chicken, and put it in an oven at 160ºC. While many recipes suggest 180ºC, you'll find that the white meat ends up much more tender at this lower temperature.

For the potatoes:

Boil the potatoes in very salty water, with a dash of vinegar. The salt helps to reduce the amount of water absorbed by the potatoes (the damper they are, the less crispy they'll be). The vinegar (which you won't be able to taste) prevents the potatoes falling apart in the next stage.

When the potatoes are fairly well cooked, drain them, and leave them to sit for a few minutes to let off steam and cool down, then give them a good shake to fluff them up. Add some salt to them as you do this. Then pour some groundnut oil over them, and keep shaking, until they're covered in a thin layer of oily mash.

Pour them into a hot metal tray, oiled with some more groundnut oil. Now they can go into the same oven as the chicken. Turn them every 15 minutes.

Amazingly, the potatoes will start to turn golden and will crisp up.

Take the chicken out of the oven and leave it to sit, turn the temperature up to 210ºC to give the potatoes a final crisp, and cook the Yorkshire Puddings.