Flat roofs were once exclusively the province of professionals, but new materials enable do-it-yourselfers to install flat roofs. If your flat roof is covered with gravel, you will likely need to hire a professional to reroof it. If the surface is smooth you may be able to reroof using sheets of modified bitumen or EPDM (rubber) roofing.
Consult with your inspector to see whether you need to tear off the old roofing or if you can reroof over the existing roofing. Use flashings approved by your building department. Modified-bitumen roofing may have a granular surface as shown here or it may be smooth. If you install the smooth product, cover it with aluminum coating.
These steps show a "torch-down" method, which looks a bit scary but actually is not difficult. Renting a large propane torch is inexpensive. You can also set the sheets in adhesive; this sounds easier but is actually messier and more time-consuming. Expect to spend one or two days, working with a helper, to lay insulation and modified-bitumen roofing for a 500-square-foot roof. Before you begin, measure and make a drawing of your roof and consult with a dealer.
What You Need
- Tape measure
- Chalk line
- Large propane torch
- Utility knife with hook blade
- Spark igniter
- Roof insulation
- Heavy roofing felt
- Modified-bitumen roofing
- Drip-edge and other flashings
- Spray primer
- Cap nails
- Roofing nails
- Roofing cement
Caution: How to Work with a Propane Torch
Place the propane tank in a central location so that you can work across the roof without having to move the tank during the installation of a sheet.
The tank may ice up (propane acts like a refrigerant), which will cause the flame to diminish. If this happens lift the tank and lightly heat it with the torch.
If you set the nozzle onto the felt while it is still hot, it may melt the felt or set it on fire. Protect the roof with a rag, set it on a leather work glove, or overhang the nozzle beyond the roof.
At all times pay attention to the nozzle when it is flaming or hot. It is possible to start a fire on the roof if you leave a hot or flaming nozzle unattended.
Step 1: Clean and Insulate the Roof
Remove all obstructions and sweep the roof clean of debris. You may need to remove old flashings. If your roof is not already insulated, it is usually worth the time and expense to lay sheets of rigid insulation made for a flat roof. Cut them to fit fairly tightly and make sure that you will support the flashing pieces at the ends. Attach the insulation with fasteners as recommended by the manufacturer—in this case hold-down plates and specially-coated screws long enough to penetrate the decking.
Step 2: Apply Roofing Felt
Apply extra-heavy (typically 43-pound) roofing felt over the sheathing, old roof, or the insulation. Drive cap nails 6 to 8 inches apart in rows along the top and bottom of the sheets, as well as in a row in the middle.
Step 3: Attach Drip-Edge Flashings
Attach drip-edge flashings as needed. You will install rake-end flashings later. Use wide flashings approved for use with a flat roof and attach them with roofing nails every 8 to 12 inches.
Step 4: Apply Primer
To ensure that the roofing will stick firmly to the flashing, spray the flange of the flashing with a special primer.
Step 5: Roll Out Roofing
Roll out a sheet of roofing. Allow it to overhang the rake or side of the roof by a couple of inches; you will cut it precisely to length later. Take care to avoid bubbles or creases.
Step 6: Cut Roofing to Length
Cut the roofing roughly to length, again allowing it to overhang by a couple of inches.
Step 7: Adjust Roofing
Adjust the position of the sheet so that it overhangs the drip-edge flashing by 1/2 inch or so all along its length. Check that the roofing lies perfectly flat.
Step 8: Partially Reroll Roofing
Taking care not to move the roofing out of position as you work, reroll the roofing halfway. Keep the roll fairly tight so that it will roll back neatly.
Step 9: Light the Torch
Turn on the gas and light the torch, using a spark igniter. Wear heavy gloves and long sleeves to protect yourself from the heat. Adjust the torch so it produces a concentrated flame that is mostly blue.
Step 10: Roll Roofing
Run the tip of the flame along the intersection of the roofing roll and the sheathing; the idea is to lightly melt both the roofing and the felt. Once you see heat bubbles along the roll's width, use your foot to gently roll the roofing forward.
Step 11: Roll and Heat
Take your time rolling and heating so that the roofing is heat-adhered at all points. Take special care to produce tight seals at the ends. Often you can use the spark igniter as a tool to gently lift up an end while you apply heat.
Step 12: Seal at the End
Along the flashing and at the rake end, press down with your foot to ensure a tight seal.
Step 13: Align Next Sheets
Align the next sheets so they cover the unfinished top edges. Because the sheets are wide, you usually do not need to worry about precise alignment of each sheet in relation to the ridge.
Step 14: Continue to Torch and Roll
Torch and roll succeeding sheets the way you did the first sheet. Take special care to fully heat the seam between the sheets.
Step 15: Tighten the Seal
Press with your feet, then a gloved hand, to tighten the seal at the seam between the sheets. A small amount of melted black sealant should ooze out all along the joint.
Step 16: Fully Seal at Butt Joints
At a vertical seam (or butt joint), fully seal the first piece onto the underlayment. Cut the next piece so it overlaps by 4 to 6 inches. Align the second sheet and torch and roll it toward the first.
Step 17: Seam Vertically
Complete a vertical seam by pressing with gloved hands, making sure that a small amount of melted black sealant oozes out all along the joint.
Step 18: Position Sheets Along Ridge
If you have a ridge, first install sheets on each side. Position a sheet along a ridge and cut it lengthwise so it will just overlap the seam seal.
Step 19: Torch Ridge
Torch one side of the ridge, then the other. Take special care to make tight, fully adhered seams.
Step 20: Cut Rake Ends
Cut rake ends flush with the outside edge of the sheathing. You can use a chalkline for this, but it is usually not necessary because the rake will be covered with flashing.
Step 21: Attach More Flashings
Install drip-edge flashing along the rakes and spray with primer.
Step 22: Cover Flashing
Cut a number of strips, about 8 inches wide, to cover the rake flashing. Use a chalkline and tape measure to make precise cuts only if the roof will be highly visible.
Step 23: Torch Flashings
Starting at the bottom, position the strips so they overhang the flashing by 1/2 inch or so. Torch the pieces onto the flashing and the roofing.
Step 24: Press and Install Strips
Press the strips firmly into place. When you reach the ridge, cut and install a strip that is the same length as the width of the ridge sheet.
Bonus: How to Tie Into an Adjoining Roof
If a flat roof meets a sloped roof, be sure to configure the flashings and roofing pieces so water has no opportunity to infiltrate as it flows downward. In some cases this means that flashings will direct water onto the sloped roof. If the old roof worked well, copy its flashing and roofing arrangement. Visualize how water will flow and avoid places where water can puddle or flow under an overlapping roofing piece.
If you have a short parapet wall, you can usually roll the roofing sheets up onto it. The sheets cannot make a tight turn, so there should be flashing or built-up roofing in the corners so the roofing can make a gentle, sweeping turn upward. Seal the top of the roofing completely.