palabravampiress agrees: Umm... "make much of a problem?" Lol! Is there something you aren't telling us? Are you at some scary increased risk for combustion?
Think of it as this
You have a Pier with small, centimetre wide gaps between the planks. Imagine the lines below are the Platform on the Pier ignoring the structure underneath the pier for the time being.
Fig a) Sun Rays coming from the The East
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When the suns rays hit the cm gap, light hast to follow a straight line. This means the effective span would probably be captured in the gap itself as the gap between planks is generally smaller than the thickness of said plank for safety reasons.
The Vampire would be safe.
Fig b) Sun Coming from the West
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Same conclusion as Fig a
Fig c) Sun directly overhead
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Now these rays would go straight down through the small gaps ONLY if the Sun was at 0 degrees based on the horizon being 90 degrees.
These would pose a threat to the Vamps underneath IF there wasn't a super structure of beams and metal holdings forming the base of the Pier.
Even at 1+/- degrees it wouldn't be impossible to expect the suns rays to get through.
Now, from the episode we get the impression that the sun is not at its greatest above the pier SO Fig c) is not an issue AND the fact there is a shadow under the pier allows us to deduce that the Sun isn't at 30/90 degrees as it too would find a way directly underneath as the pier is only a maximum of 10 metres across.