Flat Roof Repair — Leaking Garage, Porch and Addition Roofs
Water on the garage floor after every rain, a stain on the porch ceiling, a puddle that sits on the roof for days — flat roof leaks found and fixed, patch or new membrane.
JMB Builders does flat roof repair on garages, porches, additions and shed dormers across the Twin Cities metro: patching split rubber (EPDM) membrane, resealing lifted seams and drip edge, fixing the low spot where water ponds, and replacing the whole membrane on a small roof when patches no longer make sense. Priced per project from your photos within one business day, with the membrane and flashing details listed on the quote.
- Priced per project, patch or full membrane
- EPDM rubber, rolled roofing and low-slope shingles
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
Flat roofs in Minnesota do not fail in the middle. They fail at the edges and the joints: the seam where two sheets of rubber overlap, the drip edge that was glued and let go, the corner where the membrane was folded instead of patched, the spot where a branch landed. Snow sits on a flat roof for months and then melts from underneath, so a seam that is open a quarter inch takes on water for weeks before it shows up inside. The fix is usually small once you find it, and finding it is the job.
Most of the flat roofs we see are on detached garages, three-season porches, additions built onto the back of a story-and-a-half, and shed dormers. Many have rolled asphalt roofing that has gone brittle, or a rubber (EPDM) roof from the nineties with the seams lifting. Some have a low-slope shingle roof that never had enough pitch for shingles in the first place. We patch the ones worth patching with the right primer and cover tape, rebuild the edge metal, and replace the membrane on small roofs where the patches would outnumber the good spots.
Ponding gets its own line. A low area that holds water for more than a day or two after rain will find the seam eventually. On a garage that can mean sistering a rafter or adding tapered strips under the new membrane so the water goes to the edge.
What you supply
- Photos of the whole roof from a ladder or an upstairs window, plus close-ups of the seams, corners and edge metal
- A photo of the stain or drip inside and where it is relative to the walls
- For a porch or addition: where the flat roof meets the house wall, from outside
What JMB supplies
- Leak tracing: seams, edges, penetrations and the roof-to-wall joint checked, and the finding written up
- EPDM patches with primer and cover tape, or full-sheet membrane on a small roof
- New drip edge and termination bar at the perimeter, sealed and fastened on a layout
- Pipe boots and flashing at any vent or duct through the flat roof
- Counter flashing or Z-flashing where the flat roof meets the house wall
- Tapered strips or rafter sistering to remove a ponding low spot
- Sheathing replacement where the deck has gone soft
- Photos of the repair before it is covered, in your job folder
How it is priced: per project
Quoted in writing from your photos within one business day. What moves the number:
- A patch at one seam or corner vs. a new membrane over the whole roof
- Roof area and how many edges, walls and penetrations it has
- Soft or rotted sheathing under the leak, replaced before the membrane goes back
- Fixing a ponding low spot with tapered strips or framing
- Whether the old roofing has to come off or the new membrane can go over a clean, dry layer
- A torch-applied or built-up roof, which we assess and may refer to a flat roofing specialist
- Height and access; a second-story addition roof takes longer than a garage
Photos of the seams and the edge metal from above are what we need most. A photo of the inside stain tells us where to look first.
How long it takes
A patch or edge reseal: a few hours. A new rubber roof on a detached garage: one to two days, weather permitting; the deck has to be dry before the membrane goes down.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized quote within one business day. We only come out to measure first when the job needs a tape measure — no pressure, no games.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
Situations and options
EPDM seam and corner patches
Lifted seams cleaned, primed and covered with cover tape; splits and punctures patched with a round of membrane. Corners at walls and curbs get preformed corner pieces, not folded rubber. The common fix on a rubber roof that is otherwise sound.
Drip edge and termination bar
The perimeter is where wind and ice lift a flat roof first. We pull the loose edge metal, reseal or replace the drip edge, and add a termination bar where the membrane turns up a wall so it cannot peel again.
New rubber roof on a garage or porch
Old rolled roofing or a patched-out rubber roof comes off, the deck is repaired and dried, and a single sheet of EPDM goes down fully adhered with new edge metal and boots. On a small roof there is one seam or none.
Ponding water fix
Where water sits for days, we build slope with tapered strips under the new membrane or correct a sagging rafter from below, so the roof drains to the edge or to a scupper instead of soaking the same seam every rain.
Low-slope shingle roofs
A porch or dormer roof with too little pitch for shingles gets ice and water membrane over the whole deck under new shingles, or a rubber membrane when the pitch is really too low for shingles at all. We tell you which after seeing the pitch.
Flat Roof Repair — Leaking Garage, Porch and Addition Roofs: questions, answered
Can a flat rubber roof be patched instead of replaced?
Yes, when the membrane is still flexible and the leak is at a seam, a corner or a puncture. The area is cleaned and primed, then covered with EPDM cover tape or a patch that bonds to the old rubber. If the rubber has gone hard and chalky, or there are more patches than open field, a new sheet on a small roof is the better use of the money and we say so on the quote.
Why is my garage flat roof leaking?
On a garage the usual causes are an open seam in the rubber, edge metal that has lifted at the drip edge, a puncture from a branch or a ladder, or old rolled roofing that has cracked at the laps. Water that ponds in a low spot makes any of those worse. We check the edges and seams first, then the field, and send you a photo of what we found.
How do you fix water ponding on a flat roof?
First we find why it ponds: a rafter that sagged, a deck that was never sloped, or a scupper or edge that is higher than the roof around it. Then we build slope with tapered insulation strips under a new membrane or correct the framing from below, and lower or add a drain point at the edge. A puddle that dries in a day is normal; one that sits for several days is what we fix.
Can you put a rubber roof over old rolled roofing?
Sometimes, if the old roofing is flat, dry and well attached and there is only one layer. A separation sheet goes between them and the new EPDM is adhered on top. When the old roofing is bubbled, layered or holding moisture, it comes off so the new membrane bonds to a clean deck. We check by lifting a corner during the visit.
Do you repair torch-down or tar and gravel flat roofs?
We patch small areas on modified bitumen with cold-applied materials and fix the edge metal and flashing. Torch-applied work and built-up tar and gravel roofs on a whole house are a specialty trade; we will tell you if your roof is one of those and refer you to a flat roofing company rather than patch something that will not hold.
How long does a flat roof repair take?
A seam patch or edge reseal is a few hours in dry weather. A new rubber membrane on a detached garage or porch is one to two days, including tearing off the old roofing and repairing the deck. Rubber has to go down on a dry deck above a minimum temperature, so late fall jobs sometimes wait for the right day.
Get a written quote for this job
Three photos and a sentence is enough. You hear back within one business day. Or text the photos to 763-204-2099.
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Read morePage reviewed September 24, 2026 · JMB Builders LLC, MN license BC809005 · All roofing pages
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