How to Install a Back Pan on the Upslope Side of a Narrow Chimney
A small sloped pan that catches water behind the chimney and sends it to the sides. The simple alternative to a cricket.
- Advanced
- 1 to 3 hours
- Updated
The short answer
A back pan is a small sloped metal saddle on the upslope face of a chimney up to 750 mm (about 30 inches) wide. Fold the corners, slope the pan to each side, turn it up the chimney 4 to 6 inches, and run the upper flange under the shingles and underlayment far enough up the slope to be at least as high as the wall leg, never less than 1.5 times the shingle exposure. Lap its ends over the top step flashing, then counter flash.
On the upslope face of a chimney, water running down the roof hits the masonry and has to go around. On a narrow chimney it splits and runs down each side by itself, but it still needs a flashing to do it cleanly: something that turns up the chimney face, slopes gently to each side and sends the water out over the step flashing. That is the job of the back pan.
It is a simpler detail than a cricket, and suits chimneys up to 750 mm (about 30 inches) across the slope. Above that width the BC Building Code calls for a cricket, which it calls a chimney saddle. It has to be carefully made, because the corners and the laps are where back pans leak. See chimney flashing parts explained for where it fits.
Tools
- Sheet metal brake or hand seamer
- Tin snips (straight cut and offset)
- Tape measure
- Roofing hammer
- Caulking gun
- Soldering iron or rivet tool, if the corners are soldered or riveted
- Cut resistant gloves
Materials
- Galvanized, prefinished steel or copper sheet, 26 gauge or heavier
- Ice and water shield
- Roofing nails, 1-1/4 inch
- Polyurethane or hybrid polymer sealant
- Shingles
Safety first
- Work tied off. The back of a chimney is on the ridge side, often the steepest and highest part of the roof. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Soldering or riveting metal in a confined spot behind a chimney puts your hands near sharp edges. Wear gloves and keep a clear path to your anchor.
- A large flat sheet catches the wind. Hold it with both hands and set it down flat.
Step by Step
Step 1: Decide between a back pan and a cricket
Narrow chimneys do not block enough water to need a full cricket. The BC Building Code requires a cricket, which it calls a chimney saddle, once the upslope side is more than 750 mm wide, unless the flashing is run much higher up the chimney and the roof. Consider a cricket even on narrower chimneys on steep roofs, in heavy snow or where debris collects behind the chimney, and check with your local building department for any added requirement. See how to build a chimney cricket.
Step 2: Turn the membrane up the chimney face
Tie the membrane into the side membrane at the corners so there is no gap. The membrane goes under the back pan and is the backup layer. The field underlayment above it should lap over the membrane so water flows from the roof onto it. See how to install ice and water shield.
Step 3: Measure and cut the back pan
The pan has three parts: a wall leg turned up the chimney 4 to 6 inches, a sloping floor, and an upper flange that runs up under the roofing above. The ends extend past the corners so they can lap over the top step flashing pieces. Use 26 gauge or heavier steel, prefinished steel or copper.
Size the upper flange from the code rule, not a habit. Where the roof rises from the back of the masonry, the BC Building Code has the flashing run up the slope to a point as high as the top of the flashing on the chimney, and never less than 1.5 times the shingle exposure. On a shallow roof that is a long way: with a 6 inch wall leg, it is about 13-1/2 inches up the slope on a 6/12 roof and about 19 inches on a 4/12. The table in the section below has the common cases.
Step 4: Form the pan with a slope to each side
The pan should have a gentle peak at the centre, a little higher than the sides, so water runs toward both corners. The slope does not have to be large, but it must be real, because a flat pan holds water and debris. Fold the end walls up so the corners are closed. Fold corners rather than cutting and gluing them. Soldered or riveted and sealed corners last longest. See how to use a sheet metal brake.
Tip: Dry fit the pan against the chimney before fastening. A pan that does not sit tight to the masonry will not be fixed by sealant.
Step 5: Tuck the upper flange under the roofing above
Lift the shingles above the chimney, slide the flange under, and fasten it near the top edge so the nails are covered by the shingles. The pan sits over the membrane and under the roofing above, so water from the roof drops onto it and never gets under. Keep fasteners away from the lower part where water flows.
Step 6: Lap the ends over the step flashing
The ends of the pan must sit on top of the last step flashing pieces, not under them, because water flows off the pan onto the step flashing and then down the side. Check that the lap is at least 3 inches, the same head lap the step flashing needs, and that nothing tucks under the wrong way. See how to step flash a chimney.
Step 7: Seal, cover with counter flashing and test
Use sealant at folded corners and laps as a backup only. Then cover the wall leg with counter flashing set into a reglet. Test with a hose for 10 to 15 minutes from above the chimney while a helper watches the attic.
Back Pan vs Cricket
- Back pan: simple metal saddle for chimneys up to 750 mm (about 30 inches) wide. Less material, fewer joints.
- Cricket: framed and sheathed peaked roof for wider chimneys. Splits snow and water properly, costs more.
- Either: must lap over the step flashing and under the roofing above, and be covered by counter flashing.
Why Corners Matter
A back pan has four corners where water can find a hole. Fold rather than cut them. A soldered or riveted and sealed fold holds for decades, while a cut corner filled with sealant opens within a few seasons of sun and cold. Avoid butt joints along the pan floor. Use a single piece.
How Far the Upper Flange Runs Up the Roof
The flange has to climb the roof until it is as high as the top of the wall leg on the chimney, so water that backs up behind the chimney still meets metal. Measured along the slope, with a 6 inch wall leg, that works out to roughly:
- 4/12: about 19 inches up the slope.
- 6/12: about 13-1/2 inches.
- 8/12: about 11 inches.
- 12/12: about 8-1/2 inches, which is also the 1.5 times exposure minimum for laminated shingles at 5-5/8 inches.
- 4 inch wall leg: multiply the figures above by two thirds, but never go below 1.5 times the shingle exposure.
Fraser Valley Notes
The back of a chimney is the wettest spot on most valley roofs. Needles from firs and cedars, moss and wet leaves collect against the upslope face and hold water on the pan through the whole wet season. Give the pan a real slope to both sides, keep the corners folded rather than cut, and clear the debris every fall. See how to clean debris off a roof. In Hope and up the canyon, where wet snow sits behind the chimney for weeks, build a cricket rather than a back pan on anything close to the width limit.
Common Mistakes
- A flat pan. Without a slope to the sides, water and debris sit behind the chimney.
- Cut, not folded, corners. A corner that depends on sealant opens within a few seasons.
- Flange over the roofing above. The upper flange goes under the shingles and underlayment, not over.
- Ends under the step flashing. The pan must lap over the top step flashing pieces.
- Nails through the pan floor. Fasten only at the upper flange where the shingles cover the nails.
- Using a back pan on a wide chimney. A wide chimney needs a cricket to split the water and snow.
When to Call a Professional
A back pan sits at the ridge side of a chimney with the highest fall risk on the roof, and a mistake at its corners or laps is hard to see and easy to miss. If the chimney is tall, the roof is steep or you cannot work tied off, have a roofer do it.
If you are not sure whether your chimney needs a pan or a cricket, a roofer can tell you from the width and the snow load.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Install a Chimney Back Pan: Questions
What is a back pan on a chimney?
A back pan is a small sloped piece of metal flashing on the upslope face of a chimney. It turns up the masonry, slopes slightly to each side, runs under the roofing above and laps over the top of the step flashing at each end. It catches water behind the chimney and sends it around the sides.
What is the difference between a back pan and a cricket?
A back pan is a simple metal saddle used on narrow chimneys, up to 750 mm (about 30 inches) wide across the slope, which is where the BC Building Code starts to require a cricket. A cricket is a framed and sheathed peaked roof used behind wider chimneys. Both divert water around the chimney and both must lap over the step flashing, but a cricket handles snow and heavier flow.
Does a chimney need a back pan?
Every chimney needs something on its upslope face to divert water. Narrow chimneys use a back pan and wide ones use a cricket. Without either, water pools behind the masonry and finds gaps in the flashing. The back pan must be lapped correctly with the roofing above and the step flashing on each side.
Should back pan corners be soldered?
Soldered or riveted and sealed corners last longest, but a well folded corner sealed with a quality sealant is acceptable on many installations. The key point is to fold rather than cut the corners, because a cut corner depends on sealant alone and opens within a few seasons. Copper and steel can be soldered, and aluminum is usually riveted.
How far up the roof should a chimney back pan go?
Far enough that the top of the upper flange is at least as high as the top of the flashing on the chimney face, and never less than 1.5 times the shingle exposure, which is the rule in the BC Building Code for flashing where the roof rises from masonry. With a 6 inch wall leg that is about 13-1/2 inches up the slope on a 6/12 roof and about 19 inches on a 4/12. The flange goes under the underlayment and shingles above, never over them.
Related Guides
All Chimneys guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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