Time to trot out my pet theory on Reed Richards' power: he doesn't "stretch".
It looks like stretching to the casual observer, but Reed's superpowers, like those of the other members of the FF, aren't what they appear, they're just described in terms the layperson can understand: "stretch", "invisible", "flame"... In actuality, all of their powers represent control over fundamental properties of physics.
Understanding the true nature of Reed's power comes from noting one undeniable fact: he apparently defies gravity. He routinely assumes positions which would clearly result in his body tipping over under ordinary circumstances, like when his feet are on one side of the room and most of the rest of him is "stretched" over to the other side.
Reed's not stretching, he's controlling gravity, specifically, he's in complete control of how gravity affects his physical form.
See, in physics' best descriptions of gravity, based on Einstein's General Theory of Relativity, gravity is not just the force that is generated between two masses, such as the force between the mass of the Earth and the mass of our body that gives us weight and keeps us on the ground.
Gravity is a distortion of spacetime, dictating how the atoms that make up our bodies (and everything else with mass) move through space over any period of time in the presence of any and all other matter. With his complete control over gravity, Reed can choose to occupy any desired space at any moment in time. If he wants to have his feet on one side of the room while the rest of him is on the other, with his control of gravity he distorts his position in space relative to the ordinary world around him, with the constraint that his body remains contiguous: his parts stay connected, because from an internal perspective, he's retaining a "normal" shape. His heart is still the same distance from his brain, from the local frame of reference, and blood is still pumping at the same velocity, although it wouldn't appear to be from our external perspective.
If that's so, though, wouldn't Reed be able to "fly"? Couldn't he just as easily choose to move all of his mass to some chosen volume of space that just happened to be 10 feet over our head, and then keep moving it further and further away? Yes, that should be possible, but he doesn't do that, does he? So we have to hypothesize something that prevents him from doing so.
My hypothesis is that he must keep at least some portion of his body compliant with the physics of the "normal" gravitational field. This serves as his reference frame that allows him to return fully to the gravitational frame as experienced by the rest of us. Losing connection with your reference frame, as I can tell you from my experience developing control systems for space vehicles, is a very bad thing. Reed may know, either instinctively or through intellectual reasoning about the statistics, that if he completely disengages from "our" gravitational field, that he may never find his way back to it. He might find himself drifting gradually away unless he can find a way to return precisely to our frame of reference. He may find himself permanently distorted as viewed from our perspective, perhaps never able to get his face back into its original relationship with conventional spacetime, looking forevermore like a man with a grotesquely stretched-out head.
And so he keeps "one foot in the real world", so to speak. If he wants to "stretch" up to the rooftop, he keeps the soles of his feet under the influence of "our" gravity, distorting the rest of his body until his fingers reach the edge of the roof. Since he's still "in touch" with "our" gravity, he can return those fingertips to the original gravitational reference frame, using that as his anchor to the real world as he now releases his feet from conventional gravitational forces and distorts his body to occupy space closer to his finger tips, thus appearing to us to return to a normal human shape, but now clinging to a rooftop several stories above where he started!
But what about those times when we see him drop from a height, spreading out like a parachute to slow his descent? Wouldn't he "lose connection" that way? No, he's still allowing his center of mass (CM) to be in sync with "our" gravity, and occuping a space around it, in the shape of a parachute, so that air resistance slows the descent of his CM in full compliance with the external gravitational physics, still keeping some portion of his body complying with the same rules of gravity everyone else is.
So can he teleport? Can he decide he wants to be entirely on the other side of the room instantaneously, without passing through the space between? No, he's controlling how gravity affects him, not negating it entirely. General Relativity mandates that nothing can travel faster than the speed of light, so teleportation would violate that. It also dictates that speeds approaching the speed of light would result in changes to mass that would prevent Reed from attempting to push his luck and perform rapid spacetime distortions to approach speeds that would look to us like teleportation, lest the changes to masses wreak disastrous consequence to himself or his surroundings.
So that's my theory: Reed controls gravity, he doesn't "stretch".