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ARTICLES - General - Making a Sheet Metal Block-Off plate for a Masonry Fireplace

How to make a metal block-off plate for a masonry fireplace opening.
Here is the answer as to why you might need this!

Hint #1 The plate does not fit it up at the damper area, but lower than that - about 2” above the fireplace opening…this has many advantages….


Materials needed
Sheet metal - 24 or 26 gauge galvanized metal is ideal for this application - available at many plumbing and heating supply houses
Masonry or cut nails - approx 1.5”
Silicone sealant
Furnace Cement


First, remove the fireplace damper. This usually involves removing a cotter pin and lifting the damper out. Store damper in a safe place in case you ever want to restore the fireplace to original.

Now, study the picture showing the side view of a fireplace (cutaway).
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The plate which you are going to make is shown in red. The front edge of it will fit against the back of the steel lintel (#4 in diagram). For an example, we will use a level area approx. 2” higher than fireplace opening (#1 in diagram).

Now, you take only three measurements…as shown in the measuring diagram

#1 The front (at a level plane about 2” above fireplace opening)
#2 The rear (at same height approx.)
#3 The front to back in the center…..
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Hint #2 - Take these measurements SHORT. In other words, if the actual measurement of the front is 36 3/4”, use 36” for your measurements.

Note: for this example, we will pretend that our “short” measurements are 36 along the front, 28 along the rear and 15 front to rear (back of angle iron lintel to read wall of fireplace) - AND we will be bringing a 6” pipe through it.

With these three measurements, the side angles will work out correctly. The key is this - Work from the center!
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If your piece of sheet metal is square or rectangular, here are the steps.

1. Measure back 1” from the straight edge and draw a line.
2. Then measure from this new line using measurement #3 and draw a line parallel to the first
3. Then measure another 1” (for rear flange) and make an additional line.

So assuming that your measurement #2 from diagram (front to back) was 15”, you would have a line at 1”, 16” and 17” - or in other words the plate would be 2” larger than the measurement front to back.

Next: 

1. Mark the center of measurement #1 in the center of this line, so there is enough room on both sides for total plate plus 1” extra on all side.
(Example: 36” measurement, we would need a piece of metal at least 38” long and we would mark the center at 18” on the front line 1” in from the edge).
Using a T -square, draw a line front to back using the front of the sheet metal as a guide and the center point which you marked. This line is 90 degrees offset from the front of the sheet metal.

2. Measuring from this center line and on the line 1” in from the front, mark at 1/2 the measurement #1 in both directions (in the example, we would use 18” from each side of the line. Then make another mark at 1” past that line (or 19” from the center line).

3. Repeat this step along the line representing measurement #2 (fireplace rear, in the example, 16” from front of sheet metal and then 14” and 15” on each side.)

Connect all the marks so that your layout will look very much line the top inside of your fireplace with a 1” extra lip around it.

Cut the metal on the outside line and then notch the corners.
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You now have to fold the 1” down..less than 90 degrees so it can make up for the difference since you shorted on your measurements….. There are numerous ways to do this:
A. A sheet metal brake (bender) if you have one or access to one.
B. A hand brake (usually 3” or so long) , which can be purchased at many plumbing supply shops or online. Vise Grips also makes one that will do the job - in this case you bend down a little at a time.
C. Use a soft hammer over the edge of a metal table or piece of wood.

Test fit and bend edges further up or down as needed. Note that many times you will have to use the rear handle of your hammer or other blunt tool to push it up into place. To remove it, grab the lip with a pair of pliers or a vise-grip and pull down hard.
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Now calculate how and where pipe will go through - the best way it to have the pipe already positioned in the fireplace throat and then measure back from the rear wall to where the pipe starts. In our case, the 6” pipe is hanging 3” from the fireplace rear, so we remove the plate and hold a spare piece of pipe centered on the plate side to side and 3” from the rear and scribe around the pipe.
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Cut the pipe hole round at first, and cut it at least 1/4” larger than the line - you’ll need the extra room. Cutting this hole can be accomplished in numerous ways:

A. You can purchase a hole cutting attachment to a drill (Malco) for about $40.
B. You can bust through the metal somewhere within the hole with an old screwdriver or chisel, and then use aviation snips
C. If you are really lucky, you have a set of power nibblers! You can also get drill attachments that do this.

Getting the hole right is often one of the most difficult parts of this job. Often, you will have to ovalize the hole in a front to back fashion…this is because the pipe often passes through the plate at an angle. You can use a hammer to tap around the inside of the hole to make it slightly larger as needed.


When the plate is inserted, it will hold itself up BUT, use a could masonry nails into mortar joints on both sides to make certain. No need to drive the nails all the way in, in fact just pinning is OK, cause they are easy to remove if needed.

Silicone the joint between plate and wall and furnace cement between pipe and hole…

Simple Instructions (assuming you see the diagrams above and are a visual and mechanical person).

1. Measure a level plane 2” above fireplace opening - 3 measurements, front, read and front to back (in approx. center) - use “short” measurements - in other words, not exact by on the short side.
2. Lay out on piece of sheet metal - layout from center and leave enough room for a 1” extra flange around actual measurements.
3. Cut on the larger line (including 1” flange), notch all four corners and fold down the 1” flange at about a 60 degree angle.
4. Test fit - once it fits determine where the pipe will come through and cut pipe hole.
5. Insert plate up around pipe and fasten using masonry nails into mortar joints on side and rear - seal against fireplace with silicone and against pipe (if needed) with furnace cement.

that’s all for now… Here is a pic of a painted block off plate fastened into place.
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