How to Trim a Hollow-Core Door to Height
A clean, efficient way to measure, mark, cut, and fill a hollow-core door
Occasionally, especially on re-cuts, you have to cut a door above where there is solid material, and you can’t just leave the bottom of the door open. In this video, I’ll demonstrate how to cut a door and then block in the bottom so it doesn’t warp over time and it has a nice solid bottom.
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First, I’m going to wrap the bottom of the shoe of my circular saw with masking tape to protect the door from getting scratched and dirty. On this door, I’m going to measure up 2-1/8″, which is not unheard of in houses where the flooring and framing aren’t planned well. I’ll make a few marks, and then, using a straightedge and a utility knife, score a line all the way across. That helps break the paint and the fibers in the surface of the door so you get a clean cut. I’ll then place my saw with the blade just below the line I just made, and then I’ll make a mark next to the edge of the base of the saw in order to locate a straightedge to guide my cut. I’ll measure the distance up to this mark and then make a mark the same distance up on the other side of the door. Now I can clamp the straightedge (in this case, a scrap of trim material) to the door to keep my saw running straight when I make the cut.
Now I will cover the knife cut I made earlier with masking tape in order to keep the circular-saw blade from chipping the edge of the wood as it rotates up and out of the face of the door. It’s not entirely necessary to wrap the tape all the way around the other side of the door, because the blade plunging into the underside is much less likely to cause chipping, but I go ahead and do it anyway to be on the safe side. Then, I run the circular saw across the door, alongside the straightedge I’ve secured, to make my cut.
After making the cut, I’ll remove some of the foam filler material in the bottom of the door to make room for the blocking. This can simply be done with a wide chisel. I can gently pry against the inside edge of the door if the foam is in there tight.
When I’m ready to cut my blocking, I measure the width of the opening in the bottom of the door at the edges. I don’t measure in the middle because the door is flexible enough that warping could give me an inaccurate measurement. After ripping a piece of wood on the tablesaw, I slip it into the door to check for fit, and I mark it and cut it to length. Before I put the piece of wood in, I run a bead of glue on the inside edge of each face of the door. I use a hammer to tap the filler piece into position, being careful not to push it in too far. Once one end is flush, I’ll tack it in place with my brad nailer, then work my way down the bottom of the door, tapping and nailing as I go. I use brads because they leave very small holes that will fill easily when I paint the door.
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