What would be the typical cause for this Dracaena to get brown tips? It it grows in soil consisting of normal soil in a bag, lots of bark chips and some perlite.
It dries out before every watering. I can see now the roots a coming of the bottom, but it has always been like this, with the brown tips.
It doesn't grow in burning sunlight, but it is situated under a window in the roof. Should it be placed in half shadow, instead?
Common problem; maybe you let it get too dry between waterings, that can cause this. Calcium deficiency is another possibility, also low humidity, temperature fluctuations or fluoride in the water. Try to keep it more evenly moist without over-watering it, and mist it regularly. Use calcium supplements - chelated calcium or even gypsum. You can trim the brown tips off with scissors, cut the leaf to a point and it will hardly be noticeable.
Filtered sun is recommended, but sunburn wouldn't be on the tips hanging down, it would affect leaves most exposed to the sun. If the leaves closest to the skylight don't look sunburned, the location is probably ok.
Hmmmm - It's almost certainly not a Ca deficiency, so please don't treat your plant thinking there is one. If you're using a fertilizer w/o Ca AND a commercially prepared soil, the soil was undoubtedly pH adjusted with dolomite, which contains Ca and Mg, which serves to pick up the slack left by most soluble synthetic fertilizers that usually lack both Ca & Mg. Since Mg is about 125X more soluble than the Ca in dolomitic lime, a Mg deficiency would occur long before a Ca deficiency. Also, since most drinking water is pH adjusted with CaOH, there is no reason to suspect a Ca deficiency, which would normally manifest itself in this plant in the form of deformed emerging leaves, or any nutritional deficiency for that matter - at least not in view of the info provided.
Necrotic leaf tips & margins ARE a common problem with dracaena, the underlying cause is almost always the result of compromised root function. If we chase the cause just a little further, we'll see that over-watering/inappropriate soil or a high level of soluble salts in the soil from dissolved solids in tap water and fertilizer solutions are to blame, or a combination of both because both are regularly associated with a soil that retains too much water.
Fluoride is a possibility, but there isn't enough fluoride in your drinking water to cause the burned tips/margins unless it has help from a high level of soluble salts, as just discussed. Fluoride is very often forced to shoulder the blame that rightly belongs attached to a high level of solubles in the soil.
Raising the humidity in the room can help to some degree, but misting is ineffectual as a tool for combating the illustrated problem. If you mist, it marginally raises the humidity surrounding the plant for maybe 5-10 minutes. Even if you religiously mist twice daily, that leaves about 23-3/4 hours of each day where the humidity level is what it is. Simply raising the humidity w/o addressing the underlying cause would be much like trying to cure a disease by treating the symptoms.
Sunburn is also very unlikely in the diffused light it's in. About the only way it could be sunburn is if the plant was recently moved from a dim location to direct sun; even then, it may not burn because of the shading coefficient of the glass in your window, which only allows something around 70% (or less) of visible light to pass. If it's in direct sun indoors, it should tolerate the light load just fine, but the heat stress on leaves associated with passive solar gain could be a problem, which could be easily fixed with a little air movement.
To grow this plant well & keep it attractive, you simply need a soil that allows you to water thoroughly at will; that is, you need to be able to water to beyond the saturation point so you're regularly flushing accumulating dissolved solids (salts) from the soil as you water; and you need to be able to do this w/o concern that your soil will remain wet/soggy so long that root rot becomes an issue. Also helpful would be frequent low doses of an appropriate fertilizer. I use Foliage Pro 9-3-6 liquid synthetic fertilizer for practically everything I grow. It's not easy to find in stores, but the reasons for using it are many. If you're interested, I'll go into more detail.
I think the answer to your issue is in your soil; and if it's not, it's in your watering habits, which is a good thing because that's the easiest of the fixes. Should we talk about your soil?
Dracaenas are super-sensitive to fluoride ( do you have fluoride in your municipal water in Denmark? ), and it certainly is a cause of tip browning in the U. S., but it can be limited by using distilled water, or rain water.
As Tapla said, the soil is suspect also. A mix that is too acid will give the same symptom.
Another common cause is a plant that is sitting in water, Be sure that the plant's base is elevated out of the bottom of the container so the base of the pot does not stand in the run-off water. I use small blocks of plastic or styrofoam to lift the pot out of the saucer.
On the whole, it is my experience that people water Dracaena marginata too much; they are really quite drought-tolerant. Your light level where the plant is placed seems very good to my eyes.