Posted by: andy idsinga | August 11, 2010

Kitchen DIY : patching a furnace vent hole

I’ve been working on my DIY kitchen remodel project for a while now. This past weekend’s task was to get the floor prepared for the new in floor heating and nice new tile.

Here is a picture of me removing the old flooring. I used a pry bar to slide under and pry up the old flooring pieces.

Once I had all the mess cleaned up I started in on the patch work. The goal was to patch up the floor where an old furnace vent had been.
The patch must be:

  • strong enough for me to stand on (200+ lbs) and take walking abuse for many many years.
  • stiff enough to not flex as we would be putting tile on it – and if tile flexes it may crack.
  • flush with the surface of the floor to avoid a bumpy or sagging appearance in that area. (which could also cause the tile to crack).

Here is a picture of the hole in the floor after I removed the old furnace vent and duct-work. I used my circular saw set at 3/4″ depth to cut a wider hole out of the original floor boards – the top most boards. The 3/4″ depth was chosen so that I would NOT cut into the sub-floor boards!!

These three pics show how I progressively built up the patch from a 2×4 board that I screwed into two floor joists just below the sub-floor boards with three layers of wood that brought the patch flush with the original floor top.

Here is a diagram (made in google sketchup) to show all the of pieces involved. The floor joists in in the diagram are not visible in the pictures above – because they are hidden by the floor and sub-floor boards. Interestingly, the top floor boards are about 1/8 larger than my 3/4 material – this is why I added a spacer made out of masonite.

Here are couple pics of the final product : a nicely patched floor.



  1. […] (note: in my previous post I covered demolition of the old floor and patching a furnace vent hole.) […]

  2. Nice work on that patch!

  3. I’m getting ready to do a dozen of those, our furnace & ducting needed replacing, it was far cheaper to just remove it all and put in baseboard heating.
    Only thing I’m going to do different is instead of using a 2×4 horizontally between joists as the bottom layer, I’m going to use two vertical 2×8’s (same size as joists) to brace the bottom, should make for zero flex. Then just lay the same thickness of subfloor and plywood. (I can’t make the edge around holes bigger in my case because we aren’t replacing the carpet around the holes).

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