I had replaced the microwave oven that came with the house 15-18 years ago, and late last year the replacement failed. It was the over-the-range type and had installed fairly easily, but we discovered over the years that the microwave was too close to the range top, making it difficult to stir tall pots sitting on the back burners.
This view shows an 18 quart pot and lid on the big back burner under the old microwave. It is not a pot we use very frequently, but even stirring pasta in an eight quart pot on the same burner proved somewhat problematic. To make matters worse, the replacement microwave oven we wanted was even taller and wouldn’t fit in the space between the shelf and the top of the tiles of the backsplash. The solution was to install the new microwave higher, which involved tearing into the shelf above the range and installing a new shelf.
After removing the old microwave and trim piece from the cabinet shelf, here is what it looked like:
Over-the-range microwave ovens are supported by a bracket mounted on the wall behind it, but much of the oven’s weight is borne by bolts through the shelf above the microwave. It isn’t clear in the pictures, but the old shelf shown here is supported very well in rabbet joints or slots in the vertical part of the cabinets on either side of it. The solid wood trim piece (not shown) adds rigidity to the plywood shelf, helping to minimize sagging of the shelf when the microwave is bolted to it.
I used an electric jig saw to cut through the shelf and tore it out in two pieces.
From a piece of oak plywood purchased at a home center store, I cut out a shelf using a circular saw such that it would fit snugly in the space between the two cabinets. The plan was to mount the new shelf 4.0 inches higher than the original shelf. Cutting the shelf piece involved careful measuring, marking, and sawing. Having a good quality circular saw really helped.
I used the template that came with the new microwave oven to modify the shelf before installing it. I stained and finished the wood with leftover supplies from when we had the kitchen cabinets refinished.
The holes in the shelf didn’t seem to quite line up with the holes in the new oven, so I filed one of them into a slot or slightly elongated hole. I used a rat-tail file to do this.
Prior to tearing out the original shelf, I used a scribe to mark a line 4.0 inches above the old shelf. I extended this along the thin plywood back of the cabinet and cut out the excess part of the back of the cabinet with a sharp knife.
I slid the new shelf in and aligned it with the line I had scratched with the scribe, and supported the shelf with temporary metal supports that were originally manufactured to hold screens into window frames. I also marked where the shelf would go with pencil and masking tape. In the photos you can make out where later I drew the center line of the shelf and marked about every inch where holes for supports would go.
I drilled nine pilot holes from where the shelf would be into the cabinets on either side. This way when I would back-drill the pilot holes from inside the cabinets, they would be centered in the shelf even if I didn’t drill these pilot holes particularly level.
Here are several photos showing how the pilot holes looked before sliding the shelf back into place. They show the nine pilot holes on each side, from both sides of the cabinet walls:
Next I slid the shelf into place and lined it up as perfectly as I could.
Then, I back-drilled four pilot holes into the shelf for nails that would hold the shelf in place for the next step: attaching it between the two cabinets.
With the shelf pinned by nails, all that remained was to fasten it more permanently. I had thought I would used wood screws, but realized screws would act as wedges that could split the plywood shelf. Some experimentation with scrap material bore this out, and wood dowels seemed more like the way to go. I got some 5/16″ wood dowels and following advice I read on the internet, also employed a clamp to hold the plywood together the best that I could while drilling the shelf and installing the dowels. I didn’t have a proper wood clamp, so I did the best that I could. The dowels that were to support the most weight were installed where the clamp was most effective, close to the front of the shelf.
I employed a makeshift depth gauge to back-drill the pilot holes to the correct depth for the dowels …
… and did the same thing for the 5/16″ drilling of holes for the dowels. Here is what it looked like before installing the dowels on the left hand side.
I made a nozzle for my glue bottle using pieces from old caulking tubes. This helped greatly in loading up the drill holes with wood glue.
After installing seven dowels on one side, I could pull out the two nails and install two more dowels.
After removing the clamp, I found a bit of a glue mess, but that was easy to clean up.
The dowels on the right hand side went in quickly and easily. I used a blend/fill pencil to cover up the imperfections.
I followed the directions that came with the new microwave oven to install the wall bracket. I added a support under the shelf at the back, but didn’t figure that really helped much.
You can see the trim piece that I glued and nailed to the front of the shelf. As mentioned earlier, this adds rigidity to minimize the shelf sagging under the weight of the microwave. With help, I installed the new oven. It was quite heavy, but went in easily.
I had set up the microwave oven’s vent fan to exhaust up so that exhaust air could vent through a hood boot to the existing vent on the roof. The vent pipe was not centered, and the spacing was such that standard parts would not allow connecting the oven’s vent to the vent pipe.
I designed a hood boot and a flat piece to finish off the hole through the top of the cabinet. The rectangular dimensions of the hood boot are per the instructions that came with the microwave, but I came up with the offset and height through careful measuring. Here are the designs of the flat ring and the microwave hood boot. A local custom sheet metal company fabricated the two parts.
Fortunately the vent pipe would move up about an inch, making it possible to set the hood boot without too much trouble. I used metal tape to connect the boot to the stack, and screws to install the flat ring. Here is how it turned out.
To finish the project, something has to be done about cabinet doors since the old ones are too tall now. Also something has to be done at the backsplash because now there is a gap between the tile and the bottom of the new microwave oven. (The bottom of the new microwave is a little over three inches higher than the old microwave.) We are still trying to decide what to do on these last two items, but in the meantime, we have a microwave oven that works, and a hood vent that works.