The Roof Assembly for Trades Who Penetrate It
Why this matters
Solar installers, HVAC crews, plumbers, antenna and satellite installers and electricians all end up cutting holes in roofs that somebody else built. A shingle roof is not waterproof in the way a bathtub is. It is a set of overlapping layers that shed water downhill, and every layer depends on the one above it lapping over the one below. A hole, a fastener or a flashing installed against that order leaks, usually slowly, often far from where the water went in, and often months later when the roofer gets the call and the homeowner remembers the satellite guy.
Safety first
- Fall protection is set by the kind of work, not the roof. Installing new equipment through a roof is construction work, and OSHA's residential construction rule requires fall protection at 6 feet or more (29 CFR 1926.501(b)(13)). Service and maintenance work falls under general industry, where the walking-working surfaces rule triggers at 4 feet (29 CFR 1910.28). Either way, the first person up needs a plan for how they are protected before an anchor exists.
- Ladders: set on firm ground, extended at least 3 feet above the eave, tied off at the top, never leaned on a gutter.
- Power lines: look up before raising a ladder or carrying pipe or panels near the service drop; keep at least 10 feet from overhead lines unless the utility says otherwise.
- Surface condition: frost, dew, moss, loose granules and steep pitch all turn a walkable roof into a slide. If you would not walk it without holding something, do not walk it without being tied off.
How water moves through the layers
Read a roof from the deck up:
- Roof deck (sheathing). Plywood or OSB nailed to the rafters or trusses. It is the structure the roofing is fastened to, not a water barrier.
- Underlayment. Felt or synthetic sheet over the deck, lapped so each upper sheet overlaps the one below. It is the backup layer that catches water that gets past the shingles.
- Ice barrier. In cold climates, a self-adhering membrane along the eaves where ice dams back water up under the shingles (IRC R905.1.2 in substance, as adopted where ice damming is a known problem).
- Shingle courses. Each row, or course, overlaps the row below so that the nails of the lower course are hidden and covered. The part of each shingle you can see is its exposure, commonly 5 to 5-5/8 inches depending on the product. Water runs over the shingle surfaces and off the bottom edge of each course onto the next one down.
- Flashing. Metal or rubber pieces where the roof meets something that is not roof: walls, chimneys, pipes, skylights. Flashing works by the same lapping rule as the shingles.
THE LAP ORDER, UPSLOPE AT TOP OF THE LIST
WALL OR CHIMNEY SIDE FIELD OF THE ROOF
counterflashing, set into
the wall, bent down OVER shingle course 4
the step flashing laps OVER
step flashing, course 4 shingle course 3, cut
laps OVER around the pipe, laps
step flashing, course 3 OVER the boot's upper
each piece under its own flange
course, nailed to the boot's upper flange slides
deck only, high on the UNDER courses 3 and 4;
roof leg its only nails go here
course 2 runs UNDER the
boot's lower flange
all of it on top of underlayment, upper sheet
over lower
roof deck (sheathing) on rafters or trusses
The rule the whole roof obeys: anything upslope laps over anything downslope. Water that finds its way under a layer has to come out on top of the next layer down. Every leak at a penetration is a place where that order was reversed: a flange sitting on top of the shingle above it, a fastener through a surface that water runs across, or a seal of caulk standing in for a lap.
Pipe boots and other penetrations
A pipe boot is a flashing with a flat flange and a rubber or metal collar that fits the pipe. The upper part of the flange slides under the shingle courses above the pipe, and the nails that hold it go there, hidden. The lower part lies on top of the course below, exposed, with no nails. Water that runs down the pipe and the collar lands on top of the lower flange and runs off onto shingles.
Three ways other trades get it backwards:
- The whole boot laid on top of the shingles, so water running down the roof above slides under the upper flange and straight to the hole.
- Nails or screws through the exposed lower flange. Every fastener there sits in the path of running water.
- Sealant used as the flashing. A bead of roofing cement around a flange or a collar will hold for a season or two; ultraviolet light and daily heating and cooling crack it, and then it holds water against the hole instead of shedding it.
Equipment mounts, solar standoffs and dish brackets follow the same rule: they need a flashing whose upper edge goes under the shingles and whose fastener is covered by it. A mount screwed through the face of a shingle and caulked is the same mistake as the nailed lower flange. See related: How to Seal a Roof Penetration That Will Not Leak.
Step flashing and counterflashing at walls and chimneys
Where a roof slope runs along a wall or a chimney side, the joint is covered by step flashing: one L-shaped piece per shingle course, one leg on the roof under that course and one leg up the wall. Each piece laps over the one below it. It is nailed to the deck only, high on its roof leg, so the wall and the roof can move separately.
Counterflashing covers the top of the step flashing's wall leg. On siding it is the siding and the wall's water-resistive barrier lapping down over it; on masonry it is a separate metal piece set into a cut mortar joint and bent down over the step flashing. Pulling the counterflashing out to run a conduit or a gas line, or drilling through the step flashing to anchor a strap, opens the joint. See related: Fireplaces and Chimneys for Roofers and HVAC Techs and The Exterior Wall Assembly for Trades Who Mount Things on It.
Where not to walk, and what not to cut
- Hips, ridges and the caps on them crack and lift under a boot. Walk the field.
- Hot shingles in summer scuff and lose granules underfoot; brittle shingles in cold weather crack when bent back to slide a flange under them. Many crews schedule penetrations for mild mornings and break the seal strip with a flat bar rather than by lifting the tab.
- Soft spots. A deck that gives underfoot may be rotted sheathing. Step off it and tell the roofer.
- Tile, slate and metal need the roofer's methods and often their crew: tile and slate break underfoot, and metal panels are slick and have their own flashings.
- The framing. Before cutting a hole through the deck, find the rafter or truss from inside the attic. A truss web or chord is never cut; a rafter is cut only by design. See related: Roof Trusses and Why You Never Cut a Web.
Worked example: a ceiling stain under a new furnace vent
A homeowner calls a roofer about a stain on the bedroom ceiling. A month earlier an HVAC crew replaced the furnace with a condensing unit and ran a new 3 inch PVC vent through the roof. The roof is 6/12 pitch (6 inches of rise for every 12 inches of horizontal run).
Where the stain is, and where the water went in. In the attic, the roofer measures the stain's center at 4 feet horizontally downslope of the new pipe, measured on the floor plan. Along a 6/12 slope, each foot of horizontal run is the square root of (1 + 0.5 x 0.5) = 1.118 feet of roof surface, so the water traveled about 4 x 1.118 = 4.47 feet down the deck before it dripped onto the ceiling. Check from the other direction: 4.47 / 1.118 = 4.0 feet horizontal. The leak is at the pipe, not above the stain, which is why "patch the shingles over the stain" fixes nothing.
What the roofer finds at the pipe.
- The boot's flange is sitting entirely on top of shingle courses; its upper edge is not under anything.
- Two roofing nails go through the exposed lower flange, each with a dab of sealant.
- A ring of sealant around the collar, already cracked.
Count: three faults, each a reversal of the lap rule. Water from the roof above runs under the upper flange (fault 1), down to the hole, and also seeps past the two exposed nails (fault 2). The sealant (fault 3) was the crew's attempt to stop what faults 1 and 2 caused.
The repair. The roofer breaks the seal strips on the two courses above the pipe, removes the old boot, slides a new boot's upper flange under those courses, nails its upper corners where the shingles cover them, lets the lower flange lie on top of the course below with no fasteners, and reseals the shingles. The collar fits the pipe snugly without sealant. The furnace manufacturer's termination instructions set the vent's height above the roof and its distance from other openings, so the roofer leaves the pipe itself alone and confirms with the HVAC company that the boot type suits PVC vent pipe.
The wet deck. The roofer probes the sheathing around the hole from the attic. It is stained but firm, so it stays. Had it been soft, the decking would have been cut out and replaced before the new boot went on, because a flange nailed into rotted sheathing pulls loose.
What changes the answer
A roof under warranty. The shingle manufacturer's warranty and the roofer's workmanship warranty may exclude leaks at penetrations made by others. Ask the homeowner who installed the roof and when, and whether they want that roofer to flash your penetration; whether a given warranty is affected is between the homeowner and the warranty's terms.
Low-slope and flat roofs. Membrane roofs (modified bitumen, single-ply) do not shed water by laps the same way; their penetrations are sealed into the membrane with the membrane maker's flashing details. Leave those to the roofer.
Solar arrays. See related: How to Work on a Roof That Carries Solar Panels.
The words to use when you call
"I'm the HVAC contractor who added a 3 inch PVC furnace vent through your roof at [address] last month. We set a pipe boot on top of the shingles and nailed the lower flange. I'd like you to reflash it properly, upper flange under the courses above, and look at the deck around it. The vent height and location can't change; I can send the furnace's termination instructions."
References
- OSHA 29 CFR 1926.501(b)(13), residential construction fall protection, and 29 CFR 1910.28, general industry fall protection.
- International Residential Code Section R905, as adopted by your jurisdiction, and the shingle manufacturer's application instructions.
- See related: How to Seal a Roof Penetration That Will Not Leak
- See related: Roof Trusses and Why You Never Cut a Web
- See related: How to Work on a Roof That Carries Solar Panels