Flat Roofing

Modified Bitumen Flat Roof Solid State

The surface of our roof takes a lot of abuse through the year. It is expected to survive for nearly a quarter of a century while the sun, winds, rains, ice, and trees each attack the roof surface with their own cruel punishments. 

Roof systems can be some of the most expensive repairs that home owners (even condo owners) will have to do on their homes. Having a licensed home inspector review your roof can give you a good idea when you need to budget for this expense. 

Are Flat Roof’s Really Flat?

A flat roof is defined as any roof with a slope less than ‘2-in-12’ because it needs to be water tight (like a swimming pool) to keep the structure dry. A 2-in-12 slope means the roof rises 2’ for every 12’ of length (or 2” per foot if you prefer). With a slope greater than 2-in-12, water is able to flow off the roof sufficiently with gravity that the material does not need to be water tight.

Even roofs that appear to be totally flat like large commercial roofs or condos still have slight slopes to direct water towards internal drains. On large commercial buildings or condominiums, there could be many built in water drains with segments of the roof directing water to each.

What is the Difference in Material Between a Flat Roof and a Sloped Roof?

Sloped roofs allow water to flow off the surface. This flowing water means material does not have to be 100% water tight, instead, it can be like a series of umbrellas directing water away. On a typical asphalt tile roof, each tile is water resistant, and the water flows with gravity from one tile to another till it leaves the roof.

On a flat roof, water in high winds can blow back up and under a product like an asphalt shingle and into the house structure. Flat roof systems need to be water tight to prevent structural damage.

What are Common Flat Roof Materials?

The most common materials for flat roofing in our climate are Modified Bitumen, built-up tar and gravel, metal, and single ply EPDM or PVC.

  • Modified Bitumen - This product is a rubberized asphalt membrane that is installed off of long rolls about 40”. The product is laid typically in multiple layers with each layer heated with a propane blow torch to bond to the roof surface and to the rows of material on each side and below creating a water proof system (the picture at the top of the blog shows modified bitumen being installed on a flat roof). The final top layer of material typically has granuals to help protect the asphalt from the suns harmful UV rays. Well installed and maintained modified bitumen roofs can last 15-20 years.
  • Built-Up Tar and Gravel - This system is commonly identified by large loose gravel on the roof surface. The gravel is used to protect the layers of asphalt soaked roofing felts that create the water tight roof surface below. This roof system lasts anywhere between 10 and 20 years depending on the number of layers of material.
  • Metal Roofing - Typically installed in large sheets, the main area of the metal roof is water tight with weak spots at joints in material and the fasteners. This material is more likely to be found on low-slopes than on very flat roofs.
  • Single-Ply - Single-ply roofing products like EPDM (synthetic rubber) and PVC systems are like upside down swimming pool material. These products come in very large sheets and require that the material be 100% water tight in a single, durable layer.
  • Alternative Flat-Roof Products - There are alternative systems for flat roofs such as ‘green’ materials and gardens, concrete, or recycled tire material. These roof systems still require a 100% water tight membrane system to be effective in a flat area.

Is ‘Ponding’ Water a Problem on a Flat Roof?

Ponding water is defined as water remaining on a flat roof more than 48 hours after a rainfall. Ponding water often occurs as building settlement causes slopes and drains to no longer match properly for complete drainage. A common example in wood buildings is the wood shrinks an inch or two but the metal drain pipes in the flat roof do not and end up standing above the roof surface.

Some roofing experts feel that as flat roof systems are water tight like a swimming pool, that ponding water should not pose a problem for flat roofs. In fact, some water on the roof may even help with keeping roof temperatures down in summer prolonging material life and keeping the building cooler (the weight of the water is typically inconsequential to the roof structure if less than 2”). Other roofing experts say ponding water (perhaps amplified by growth of organic materials) can shorten the roof system life by up to 50%. Often, the only way to correct for ponding water on a flat roof is to repair and replace some or all of the roof system which is why ponding water areas are usually corrected at re-roofing periods.

What do Home Inspectors Look for on a Flat Roof?

Tricky Flat Roof

Wood shingles on the main roof pitches was suitable but not on the low sloped shed style dormer. Wind driven rains could push water inside the structure. As seen by Solid State Inspections in an Anmore Home Inspection.

Home inspectors are looking at roofing slope to determine correct use of materials. In some cases, materials not suited for flat roofs are used on low slopes (see the photo beside). Home Inspectors also look at all roof penetrations (e.g. pluming stacks), chimneys, walls intersecting the roof, flashing details, and the material itself for any visual concerns and installation quality. 

A good home inspector will be able to tell you if the roof is reaching the end of its service life so you can budget for replacement. In some cases, maintenance may be all that is required to extend the life of your roof system and your home inspector will advise you to contact a licensed roof installer for more details and recommendations. Ideally, your home inspector will be able to confirm appropriate roofing materials that are well installed and have many years of life left to them.

Final Thoughts

There is no disadvantage to buying a flat roof home versus a conventional sloped roof home. The cost to replace the roof systems is similar and the materials have a similar life span. Flat roof systems do require higher skilled labour than more forgiving conventional sloped roofs so it is best to leave repairs and replacements to the professionals.

If you are unsure of the use of material, length of service life, or any other details in the roofing system of your home, call your local home inspector or roofing contractor to give you an expert opinion.

By James Bell - Owner/Operator of Solid State Inspections Inc.