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  • Writer's pictureJohn@LiteHouse

A Bit About Low Slope Roofs

Updated: Aug 22, 2023

Low-pitched roofs were a late invention in construction. This was mostly because steeply pitched roofs exhibited excellent water drainage properties. Also, before industrialization, much of the production of goods was done in people's homes by hand or using basic small machinery, so there was no need for larger buildings.

But as the Industrial Revolution progressed, it became clear that larger commercial buildings were needed to house the heavy machinery that would enable factory-style production. However, larger buildings meant increased construction costs and required more space to continue implementing the steep-slope design. Thus, a roof with a low pitch was developed, which significantly reduced construction costs and the amount of space required.

In the 1920s, low pitched roofs were found on factories in large cities and quickly became a symbol of modernity. In fact, "false facades" were added to the buildings to disguise the steeply pitched roofs. The style became popular especially in the West, but also spread internationally and created a more urban atmosphere.

Steep-sloped roofs have remained the foundation of most residential buildings to this day, while low-sloped roofs can be found on commercial, agricultural, industrial, and single-family structures, as well as structures such as aircraft hangars that do not fall neatly into any of these categories.

Installation of the roof

Since low-slope roofs are not able to drain rainwater or snow quickly, waterproofing through various installations is necessary to prevent moisture from entering the roof assembly. The first low-slope roofing membranes were coal tar, which was a waste product from coal gas production at the time and is now a by-product of steel production. In the late 19th century, asphalt was introduced as a low-slope roofing membrane, a byproduct of the oil refining process. Both installations were layered and glued with roofing felt over the wooden deck. Thus, the built-up roofing (BUR) was invented. Over time, this installation ranged from organic materials (paper or fabric) to asbestos to finally fiberglass after World War II. The 1960s gave rise to other roofing membranes that exhibited superior waterproofing properties and durability and are collectively known as modified bitumen or single ply roofing membrane.

The first roof decks were built from thick wooden decks and supported wooden beams. The wooden decks significantly affected the stress level of the roofing. Wood decks have a naturally limited capacity for thermally induced expansion and contraction and are prone to cycles of expansion and contraction due to moist air rising from the interior of the living space to the underside of the deck. This was offset by a dark colored roof that allowed heat to penetrate the deck, dry out the moisture and re-shrink.

Roof insulation did not appear until the 1960s when air conditioning was introduced and energy costs rose. Isolation creates a unique complication. Before insulation is applied, heat is easily transferred through the roof sheathing and membrane. Conversely, insulation hinders the rate of heat transfer, which can be problematic. If heat remains trapped in the roof system, it can negatively affect the durability and longevity of the roof. Proper installation and maintenance are necessary to limit these effects.


Low-pitched roofs were originally invented to accommodate the changes introduced by the Industrial Revolution to manufacturing processes and commercial enterprises. Previously, all roofs were made with a steep slope. But because of the low pitched roof's inability to shed water as quickly as steep pitched roofs, unique roofing materials have been developed. Nowadays, a steep-sloped roof is mainly used on residential buildings, while a low-sloped roof is used on commercial and industrial buildings.

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