How to Lay a Felt Roof for Flat Roofs
Laying a felt roof offers a durable, cost effective solution for flat or low slope structures. This guide covers proper preparation, material selection, installation steps, and common pitfalls to help homeowners achieve a lasting, weatherproof seal. Attention to surface condition, correct seam laps, and proper flashing are essential for long term performance and minimizing leaks.
Materials And Tools
Before starting, assemble required materials and tools. A well prepared list helps maintain steady progress and ensures quality workmanship.
| Item | Purpose |
|---|---|
| Asphalt roofing felt (40–90 mil) | Core waterproof layer; choose weight based on climate and roof size |
| Roofing cement or hot asphalt | Adhesive for laps and flashing |
| Roofing nails or staples | Secure felt at edges and penetrations |
| Primer | Prepares substrate for better adhesion |
| Flashing metal (aluminum or lead) | Seal joints around chimneys, vents, and edges |
| Angular trowel and roller | Even distribution of cement and smoothing laps |
| Broom and clean rags | Surface preparation and cleanup |
| Utility knife | Cutting felt to size |
| Safety gear | Gloves, eye protection, fall protection as needed |
Surface Preparation
Effective preparation ensures maximum adhesion and longevity. Remove debris, loose coatings, and protrusions from the roof deck. Repair any minor damage to the substrate, and ensure the surface is dry before applying roofing materials. If the deck is wood, fill any soft spots and allow to dry fully. For concrete or metal substrates, clean, dry, and free of oils or dust.
Underlayment And Primer
In most installations, a primer improves adhesion of the felt to the deck. Apply primer as directed by the product manufacturer and allow it to dry completely. Some flat roof systems use a base layer of self adhesive or cementitious underlayment. Follow project specifications for the exact sequence, ensuring a clean, dry surface before laying the first felt sheet.
Installing The Felt
The goal is to create a continuous, watertight membrane with secure laps and closed seams. Start at the low edge and work upward, avoiding wrinkles or air pockets. Fold over edges neatly to prevent lifting in windy conditions, and maintain even tension while rolling and smoothing.
Lap And Seam Details
Typical laps are 2–4 inches (5–10 cm) for horizontal seams and 4–6 inches (10–15 cm) for vertical laps, depending on climate and manufacturer guidance. Apply cement to both surfaces or use self adhesive felt where appropriate. Press firmly to remove air bubbles and ensure a tight bond along the lap.
Thermal Considerations
On hot days, felt becomes more pliable; on cold days, it can crack. Plan installations for moderate temperatures if possible. Use a hand roller to smooth out laps and expel trapped air. Avoid walking on newly laid felt during installation; use planks or roofing boards when moving across the surface.
Flashing And Penetrations
Flashing protects joints around chimneys, vents, skylights, and edges. Cut and bend metal flashing to fit the contours, then secure with fasteners and seal with adhesive cement. Seal all penetrations with appropriate mastic or sealant. For larger skylights or pipes, ensure a generous lap and proper counter flashing where feasible.
Edge Treatment And Roof Perimeter
Install a continuous edge strip or metal flashing along gutters and parapets to direct water away from the deck. Ensure that the edge gets a tight seal and no gaps are left where wind-driven water can seep underneath the felt. Pay special attention to corners and transitions, reinforcing with extra laps and cement if needed.
Weather Windows And Curing
Follow manufacturer guidelines for curing times between layers and sealing periods. Do not install in rain or when rain is forecast within 24 hours. Humidity and temperature impact adhesion; plan accordingly and use weather protection measures if interrupted.
Inspection And Maintenance
After installation, inspect for loose edges, lifted laps, or bulges. A light white wash can indicate dampness under the membrane; redress any issues promptly. Regular maintenance includes clearing debris from gutters, inspecting flashings, and reapplying cement to worn seams every few years, depending on weather exposure and roof traffic.
Common Problems And Solutions
- Lifted Edges: Refasten with appropriate nails and cement; re-seal with fresh laps.
- Wrinkles Or Air Pockets: Carefully lift and reposition; reapply cement and roll to remove air.
- Water Infiltration At Joints: Add extra laps and ensure proper flashing; check for punctures and patch.
- Edge Deterioration: Install new edge metal and reseal to restore waterproofing.
Safety And Best Practices
Working on rooftops requires caution. Use sturdy ladders and fall protection where necessary. Wear slip-resistant footwear and gloves. Keep tools organized to minimize trips and trip hazards. If the roof shows signs of structural damage, consult a professional before proceeding.