Why are most house roofs slanted instead of flat?

Monday, June 25, 2018

Flat and low-slope roofs were briefly popular during the post-war decade of the 1950s and into the early 1960s, largely due to the influence of the spare, angular style of the modern architecture movement. The futuristic designs of architect-designed homes for wealthy clients during the era, like in the concept rendering shown below, percolated down to more modest versions in model homes of housing developments around Florida.

    Here’s two interpretations of that design concept, scaled down for housing developments in Gainesville, Florida.

    And an even more basic example with a flat roof in Gulfport, Florida.

    There are several reasons, besides the fact that flat and low slope roofs went of out style, that they are rarely used for newer homes. Here’s our list:

  1. Air conditioning and heating ducts are usually installed in the attic, but no space for them in a flat or low-slope roof home. Ducts have to be installed in dropped soffits, usually in hallways, around the home. This severely limits where air conditioning vents (registers) can be located.
  2. The space required for ceiling insulation is limited or non-existent. Energy costs were cheaper back then, and it was not so important, but complying with today’s energy code standards requires plenty of room for insulation below the roof.
  3. Homes with a sloped roof can use shingles and other roofing materials that overlap downhill to make them water-resistant. They depend on the rain water flowing downward to be effective. Flat roofs have to endure standing water, called ponding by roofers, and must be sealed watertight. They are more expensive to install and leak-prone as they age. Here’s an example of ponding on a flat roof below. 


  4. A sloped roof, like in the photo below, makes any house look taller and look more impressive. 

• • • • • • • • • • • • • • • • • • • • • • • • • • • •  

  To learn more about roofs and attics, see these other blog posts:

Why is my roof sheathing sagging between the trusses?

Why is granule loss a problem for an asphalt shingle roof? 

What are the mistakes to avoid when doing attic improvements?

What causes roof shingles to curl up at corners?  

What causes shingles to buckle along a line on the roof?

What causes leaks at a fake roof dormer? 

What causes a sagging roof ridge line?

What causes bubble-like blisters in a built-up and gravel roof?  

Why does it cost so much more to replace a steep roof than a low slope roof? 

What is "ponding" on a flat roof?

Is an attic required to have a light by the building code? 

How can I inspect my roof for hurricane damage?

Why is premature curl of roof shingles a problem?

How can I tell if a roof has more than one layer of shingles? 

What are the common problems with attic insulation? 

What is the life expectancy of an asbestos cement shingle roof? 

What's the average lifespan of a roof?

Why is it a mistake to replace a roof and not replace its flashings? 

Why is there no attic access hatch in the house?

What is the building code requirement for an attic access hatch, scuttle, or door? 

Does a roof with multiple layers of shingles last longer?

What can I do to prevent roof leaks?

Are roof trusses better than roof rafters (stick framing)? 

Why is a popped nail in a shingle roof a problem? How do I fix it?

What are the most common problems with wood roof trusses?

What causes a lump or dip in the roof? 

If my roof is not leaking, why does it need to be replaced?

How can I be sure my roofing contractor got a permit?

How many layers of roofing are allowed on a home? 

What are the dark lines running parallel to shingles on my roof?

Can metal roofing be used on a low slope/pitch roof? 

How can I make my roof last longer?  

What are the warning signs of a dangerous attic pull-down ladder?

How can I find out the age of a roof? 

Should I buy a house that needs a new roof?

Should I buy a house with an old roof? 

What are those metal boxes on the roof?

What does "lack of tab adhesion" in an asphalt shingle roof mean?

Why do roof edges start leaking?

Why do my dormer windows leak? 

Do home inspectors go on the roof? Do they get in the attic?

Should I put gutters on the house? 

How much of a roof truss can I cut out to make a storage platform in the attic? 

What's the difference between an "architectural" and a regular shingle roof? 

What does a home inspector look for when examining a roof? 

Do stains on the ceiling mean the roof is leaking?

How can I tell if the house needs a new roof?  

 Why does my homeowner's insurance want a roof inspection?

What are the hazards to avoid when going into an attic? 

     Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

Architectural renderng at top of page by architect Ralph Rapson of “Greenbelt House,” part of the Case Study House program, initiated by Arts & Architecture magazine, 1945

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