Replace a Flat Roof

When a roof leaks, you need to fix it quickly. Water leakage can cause severe damage to the structure and ruin the contents inside. Follow these tips and you'll be able to replace a flat (low pitch) roof.

Steps

  1. Decide what type of low pitch, flat roof you want to install. There are five basic types. All require periodic inspection and can need maintenance to fix any punctures, tears, cracks or cuts. They are all in rolls of 1 yard (91.44 cm), or in 1 meter width (39.4 inches) except for Single-ply Membrane which come folded:
    • Built-up Roofing: This method, known simply as BUR, is done by overlapping rolls of treated felt using hot (molten) tar or asphalt. It becomes brittle and cracks up over the years.
    • Single-Ply Membrane Roofs: This type is excellent but requires propane torch heat to install the seams. The membrane may be injured, cut, etc. and with only the one layer that can be problematic.
    • Sprayed Polyurethane Foam: This kind of roofing material is sprayed, or squeegeed flowed onto the surface and may be protected by a special second coating.
    • Modified Bitumen (Asphalt) Roll Roofing: This is a method in which a single layer of asphalt-treated, granule coated felt is cold cement sealed or hot sealed to the primed decking or to a base sheet.
    • "SA" Self-adhesive (SBS, APP or modified bitumen) rolls: These often include (1) both an SA base sheet and the SA cap sheet, both come in rolls -- or (2) only a single cap layer is applied to a primed decking. (The material has a split backing where after a length of it is rolled-out and aligned, then half the backing is pealed off and that half smoothed down; next the other half of backing is peeled and smoothed down as well.)
  2. Determine what materials you will need to complete the job. Different roof types require different materials. The amount of each material depends on the surface area of the roof.
  3. Remove the old roofing. Use a special scraper, square nose spade or shovel, or such to loosen the old roofing material.
  4. Replace any sections of the plywood decking that have been damaged by water or "dry-rot" (fungus) before you install a new roof. Check for and repair low areas because they will not drain well. The roof rafters are like joists; there is not much pitch, or incline, and they may warp and sag. Repair any sagging rafters and decking to prevent ponding/standing water. Some roofs have drains, such as having [parapet] walls around the roof and have drains to be keep cleared and intact.
  5. Install wooden or synthetic can't strips. Can't strips should have 2 45 degree cuts and be fitted into inside corners of the roof surface. The felt used in BUR cannot bend at right angles. The can't strips help ameliorate the problem.
  6. Lay down the felt (paper) base sheet (if required for your process). It is made of paper, polyester OR fiberglass saturated with asphalt. It is set down with a 2 to 4 inch (5 to 10 cm) seam, or overlap, between sheets.
    • Fasten the sheets with special (large, thin-headed) roofing nails (in appropriate length for your process and thickness of deck or sheathing), spaced about 8 inches (20.32 cm) apart along the overlapped edges and staggered 12 inches (30.48 cm) apart elsewhere ("in the field"). The edge of roof nailing of roofing (if required) is staggered (to not split deck) close, 2 inch (~ 5 cm) spacing.
    • Cut the base sheet to fit around vents in the roof and lay it over the can't strips.
  7. Add flashing to the roof. Flashes are material you apply around vents and other areas to prevent water damage. You can further protect the roof from water damage by applying plastic roofing cement at these sites.

BUR: Built-up Roofing, Hot Asphalt Method

  1. If BUR is legal as some cities do not allow BUR, especially on residences, then "build up roofing" layers held in place by mopping molten, hot tar or asphalt -- coal tar is a known carcinogen.[1] Installing asphalt BUR roofing typically uses LP gas as heat, contributing to air pollution, and toxic, green-house gases are released during installation). There are several types of felt you can use to replace your roof. Most are either coal-tar or asphalt saturated paper or fiberglass fibers (all may be called fabric or felt). The felt material comes in rolls that are 36 inches (91.44 cm) or 1M (39.4 inches) wide.
    • Read labels. Consider the type of roof and climate before choosing the felt and the asphalt, tar or other roof cement so that you select the correct types for your roofing process.
  2. Cut a 12 inch (30.48 cm) starter strip to mop it down as the first row of the starting edge, often the lowest edge and working up to the higher elevated part of the roof.
  3. To apply the first coat of hot asphalt over the nailed felt base, spread the hot asphalt with a large mop -- as the felt is rolled out into the mopped hot "cement", one row at a time. Asphalt works to seal the asphalt-saturated felt to make the roof waterproof.
    • Mop and lay another strip 12 inches (30.48 cm) wide, over the first such strip, forming a double thick, starter edge-seam.
    • Mop full width felt over that double thick 12 inch starter across the roof.
  4. Continue laying each row with a 24 inch (61 cm) overlap of the one before it, using full width felt, over the entire roof for the first complete layer.
    • Keep the asphalt at optimum temperature as the roll is unrolled onto the mopped, hot, liquid asphalt little by little. It is important to immediately roll felt paper into it to ensure effectiveness, as that hot liquid is the adhesive, and becomes less liquid and finally solidifies as it cools.
  5. Hot mop each felt as the next layer is rolled on. You can repeat the entire process of laying the asphalt saturated felt and mopping each layer with hot asphalt 3, 4 or up to 5 layers over the entire roof.
  6. Optional: Finish it by spreading crushed granite aggregate (called ballast/gravel) after you've installed all of the desired layers. To do this, give the roof a heavy final coating of hot asphalt and then cover the entire surface with crushed-granite aggregate. The covering protects the roof from the effects of the sun, and helps protect the roof from objects such as fallen limbs.
    • Alternative covering: Sometimes the BUR roof is topped by spraying or rolling on a silver colored, metallic aluminum, asphaltic "paint".
    • Another alternative: White elastomeric (rubbery) plastic coating may be used to cover the BUR.

Warnings

  • Make sure your local landfill will accept the material you remove from your roof as some of it is "hazardous material" not accepted except at special landfills to protect water.
  • Asphalt degrades, as UV-rays increase oxidation of the surface of the asphalt and produces a chalky residue. As plasticizers, liquid in the materials, leach out of the asphalt, becoming brittle.[2]
    • Cracking and buckling-up into irregular ripples follows, allowing water to penetrate the system causing blisters, more cracks and leaks.

Things You'll Need

  • Scraper
  • Base sheet
  • Roofing nails
  • Hammer
  • Roofing felt, coal-tar- or asphalt- treated felt of: paper, polyester or fiberglass (as needed)
  • Self-adhesive: SA base, and/or only SA cap rolled-sheet

BUR, Hot mopped process

  • Bitumen, usually asphalt (25 lb./100 square feet or 11.3 kg/9.29 square m)
  • Asphalt kettle
  • Buckets (to carry asphalt)
  • Heavy mop, if BUR roof
  • Can't strips* Crushed granite aggregate (450 lb./100 square feet or 204.1 kg/9.29 square m)

Related Articles

Sources and Citations

  1. ↑ https://en.wikipedia.org/wiki/Flat_roof#Coal-Tar_Pitch_Built_Up_Roof
  2. ↑ https://en.wikipedia.org/wiki/Flat_roof#Asphalt
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