Back-building & bump-out roofs
Rear kitchen roof repair in Philadelphia
Behind most Philly rowhomes is a narrower back building with its own low roof over the kitchen. It's small, it's easy to forget, and it takes the runoff from the roof above it. Call and a local roofer can check it along with the main roof.
The roof behind the roof
How a back-building roof works
A typical rowhome is two buildings joined together: the full-width main house at the front and a narrower back building stretching toward the yard. The back building is often a story lower, so its roof sits just under the rear bedroom windows of the main house. On some blocks it's a one-story bump-out added later for a bigger kitchen or a back porch enclosure.
Back buildings on neighboring houses are usually built in pairs, sharing one wall and leaving a narrow side yard on the other side. So a back-building roof has edges on three sides: the rear wall of the main house, the shared wall with the neighbor's back building, and the open side above the side yard.
That shape explains most of its leaks. It has a lot of wall joints for its size, it sits under the main roof's runoff, and it's crowded with kitchen exhaust vents, plumbing stacks and sometimes a skylight.
When to call
Signs the back-building roof needs attention:
- A stain on the kitchen ceiling, especially near the back wall of the main house
- Water around the kitchen exhaust fan, light fixture or skylight
- Damp plaster under a second-floor rear window
- A worn, shiny or cracked patch right under a downspout
- Water still standing on the low roof a day or two after rain
- A gutter that overflows onto the side yard in heavy rain
Common problems
Where back-building roofs let water in
The main-house wall joint
Where the low roof meets the taller rear wall, the membrane turns up the brick and is covered by counterflashing set into the mortar. When that counterflashing is only caulked to the face of the wall, water gets behind it with every hard rain.
A downspout dumping on it
On many houses the main roof's downspout simply ends above the back-building roof. That concentrated stream wears through the surface in one spot and floods the low roof in a downpour.
Kitchen vents & stacks
Exhaust fans, plumbing vents and old flues all pass through this small roof. Grease from a kitchen fan softens some roofing materials around the vent, and older vent boots dry out and crack.
Rear window sills
Second-floor rear windows often sit only inches above the low roof. When snow piles up or water backs up, it can reach the sill and run into the wall, which looks like a roof leak from inside.
The shared side wall
Where your back building meets your neighbor's, two roofs of different ages meet at one wall. That seam needs care from both sides, and is often where a patch on one roof fails.
Too little slope
Small roofs are often built nearly dead flat, draining to one short gutter. A little sag or a clogged outlet is enough to leave water standing across most of the surface.
The work
What a back-building roof repair involves
Check both roofs
The roofer looks at the kitchen roof, the main roof above it and the downspout between them, since the source is often one level up.
Fix the wall joints
Old caulked flashing comes off. New flashing turns up the wall, and counterflashing is set into a cut in the mortar joint so it sheds water.
Repair or re-roof
Worn spots and failed vent boots are repaired. If the surface is worn out everywhere, the small roof is replaced, often with better slope to the gutter.
Redirect the water
The main roof's downspout is extended down to the yard or into the lower gutter, so it stops pouring onto the kitchen roof.
Planning ahead
Doing it with the main roof
If the main roof is also near the end of its life, replacing both at once usually makes sense. The ladders, the dumpster and the setup only happen once, and the same crew handles the joint where the two roofs meet, which is the detail most likely to cause trouble later.
If the main roof still has years left, it's fine to do the back building on its own. Just ask the roofer to look at the main roof's rear edge and downspout while they're there, so the new kitchen roof isn't ruined by water coming from above.
Side-yard tip: the narrow yard beside a back building is often the only way to reach the roof with a ladder. Clear it before the roofer comes, and let the neighbor know if the side yard is shared.
Planning your job
What affects the price
A back-building roof is small, but it's mostly edges and openings. These drive the scope:
- Length of wall flashing. The rear wall of the main house and the shared side wall each need their own detail.
- Brick condition. Crumbling mortar where the flashing ties in may need repointing first.
- Openings in the roof. Kitchen exhaust fans, plumbing vents, flues and skylights each add work.
- Repair or replace. A worn spot is a much smaller job than a new roof across the whole back building.
- Access. Reaching the roof from a narrow side yard or through a rear window takes more time.
- Downspout changes. Extending or rerouting the main roof's downspout.
- Combining jobs. Doing it with the main roof shares the setup.
Get a quick read on a kitchen-ceiling leak with the Roof Problem Checker.
Related pages
- South Philly flat roof repair: back buildings on almost every block
- North Philly flat roof repair: older rowhomes and layered roofs
- Northeast Philly flat roof repair: rear additions and gutters
- Roof leak repair: water coming in right now
- Flat roof replacement: replacing the main roof at the same time
Kitchen roof questions
How can I tell which roof is leaking into my kitchen?
Notice where the stain is and when it appears. A stain right at the back wall of the main house that shows up in wind-driven rain points to the wall joint. One in the middle of the kitchen ceiling points to the low roof itself. One that only appears in heavy storms may be the downspout overflowing above. A roofer confirms it from the roof.
The main roof's downspout pours onto my kitchen roof. What's the fix?
The usual fix is to extend the downspout down past the low roof to the ground or into the back building's own gutter. If that's not possible, a protective walkway pad or splash pad under the outlet spreads the water so it doesn't wear through one spot.
Water comes in under my second-floor back window. Is that the roof?
It can be. If the window sits just above the kitchen roof, water that ponds or snow that piles against the wall can reach the sill. The fix may involve the roof's flashing, the sill, or both, so it's worth having a roofer check the area from outside.
My back building shares a wall with my neighbor's. Do I need their permission?
Work on your own roof generally doesn't, but flashing along a shared wall touches both sides. It's courteous, and often practical, to let your neighbor know, especially if a ladder needs to stand in a shared side yard.
Have a different question about your kitchen roof? Call and ask a local roofer.
(215) 702-6130