Built-up roofing (BUR) systems are composed of multiple layers of asphalt or coal tar alternated with reinforcing fabrics, topped with a protective surface like gravel or a reflective coating. Because BUR systems rely on gravity to shed water, the minimum slope requirement is generally around 1/4 inch per foot (about 2%). This slight slope helps prevent water pooling, which can lead to leaks or accelerate material deterioration. If your roof is flatter than that, water may stay on the surface longer, increasing the risk of damage like blistering or premature wear. When inspecting a BUR roof, look for signs of standing water after rainfall or any visible cracks or bubbles in the layers, which could indicate moisture trapped beneath the surface.
You can do some safe checks yourself by observing water drainage during rain or after a heavy watering with a garden hose. If water isn’t draining well and seems to pool, that’s a sign the slope might be insufficient. However, avoid walking on BUR roofs without proper footwear and caution, as the layers can be delicate and slippery. If you notice persistent standing water or visible damage, it’s best to call a professional roofer. When consulting a contractor, ask, “Does my existing slope meet proper drainage requirements for a BUR system?” and “How will you address any low spots or drainage issues during installation or repair?” Knowing these helps ensure your roof’s long-term performance and keeps your warranty intact, as many manufacturers require proper slope for coverage.