What kind of filler should I use to fill the crack as shown in the picture? This is outside in my balcony where the sliding door frame meets the wood. I have actually seen small mushrooms sprout up in the crack every few weeks so if I should do any thing for them before filing it up (I just use a box cutter to cut the mushroom out of the gap), that would be great to know.Gap in wood

Zoomed out view of the balcony on the third floor. zoom outenter image description here

  • Can you also post a zoomed out photo? Nov 24, 2019 at 22:10
  • Adding a zoomed out picture. This is actually a balcony on the third floor of the townhome.
    – proteus
    Nov 25, 2019 at 21:39

1 Answer 1


The piece under the slider sill appears to be face nailed facia. You try removing this piece of wood, maybe they did water proof properly and only this piece of facia is rotting. If it is only the facia rotting then you could replace it and then to seal the gap, depending on the size, use a piece of foam backer rod and some exterior grade paintable caulking. The backer rod allows the caulking to properly expand and contract.

I'd pull the bottom piece and replace it with PVC facia. Home depot carries it but you could probably find others. After replaced use backer rod to fill the gap and caulking to seal it.

For caulking I'd probably go for something like:

GE M90026-30 Paintable Silicone Supreme Window & Door Sealant Caulk

For backrod size, you generally want something slightly bigger than the gap. Sometimes if the gap varies over the width you'd start with a larger diameter until it couldn't fit and then move to a smaller one.

  • Good point. Lots of folks think it's a good idea to fill up a gap so that three sides are 'stuck' with caulk/silicone/mastic/etc. Not so. This creates too rigid an arrangement. Using a backing foam rod means only two sides are caulked, allowing the usually differing density materials to expand and contract freely.
    – handyman
    Nov 26, 2019 at 19:31
  • There does appear to be a touch of rot in there, but before pulling the whole thing out, poke it with a screwdriver. If it's still pretty solid, you can most likely get away with just caulking it. If the whole piece of wood is soft and squishy, then yes, you'd want to replace it.
    – FreeMan
    Dec 3, 2019 at 19:58
  • Thanks and what is the caulking for exterior called? Not looking for brand names per se but rather what to look for when at the hardware store. And how to determine what width backer rod I need? I am a newbie at DIY so please bear with my questions.
    – proteus
    Dec 3, 2019 at 23:57

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