Roofing contractors measure and estimate the size of roofs in a unit of measure called a square. A square is equal to 100 square feet or a 10ft x 10ft area.
So for example, if your home was perfect square of 50 feet wide and 50 feet long, that would equal a surface area of 2,500 square feet or 25 squares.
50ft x 50ft = 2,500 sf
2,500 sf / 100 sf per square = 25 squares
Unfortunately, most homes are not a perfect square and have pitched roofs with complex shapes, valleys & dormers, which makes the exact measurement of roofs more difficult.
There are two different ways that you can calculate the size of your roof:
For the most accurate calculation you will need to get on the roof and draw a diagram of the roof shapes and measurements. Once you have the roof shapes measured, calculate the area of each shape and add up all the areas to calculate your total roof area.
While measuring from the ground isn’t as accurate as getting on the roof, it is faster, safer and gives you a rough calculation of the property roof area.
Assuming you have a rectangular shaped house, you can simply multiply the length by the width of the house.
Unfortunately, houses are not typically a perfect square or rectangle. If you have a house that is a complex shape, you will have to breakdown the floor plan into segments to calculate the building footprint area.
In the example on the right the floor, plan has been broken down into 4 different segments. The area of each segment is calculated by multiplying the length by the width.
The 4 segments areas are then added together to calculate the building footprint.
Next, you need to convert the property square footage to squares by dividing by 100.
Building Footprint Calculation
Area 1 = 25 FT x 40 FT = 1,000 SQFT
Area 2 = 15 FT x 15 FT = 225 SQFT
Area 3 = 10 FT x 20 FT = 200 SQFT
Area 4 = 10 FT x 20 FT = 200 SQFT
Total Area = 1,625 SQFT
Once your calculate the roof footprint, you will need to factor in your roof pitch or roof slope. Roof pitch or roof slope is the number of inches the roof rises vertically for every 12 inches it extends horizontally.
The slope of a roof is split up into three categories.
To factor in your roof pitch, you can use the roof pitch multiplication table on the right:
Most roofs are 4/12 up to 7/12, so when in doubt a safe multiplier would be 1.15 (7/12).
In our previous example, we measured the property's footprint to be 1,625 square feet. If we assume that our roof's slope is 6/12, then we need to use the 1.118 multiplier.
1,625 sf * 1.118 = 1,625 / 100 square feet per square = 16.25 squares
16.25 * 1.118 = 18.1675 squares
When your roofing contractor purchases & installs your roofing materials, the roofer will buy an extra 10% of material to cover waste of scrap materials.
So in the above example, the roofer needs to purchase 16.25 squares of roofing, with an additional 10% of material that will be scrapped/wasted.
18.1675 x .10 = 1.81675 extra squares in waste
18.1675 + 1.81675 = 19.98425 squares or 20 squares
A final factor that you may want to consider is an allowance for contingency which will cover any uncalculated square footages such as roofing overhangs, valleys, gables, etc.
Generally, if you have a complicated roof structure with may overhangs, valleys, gables & elements, a 10% contingency would be recommended.
19.98425 x (1+ .10) = 21.982675 squares of roofing or 22 squares
So maybe all of this math is a bit overwhelming, but now you know how professional roofing contractors estimate the size of your roof.
With that said, since we now know how professional roofers estimate costs, we can now simplify this down to a quick rule of thumb.
We know that we first need to calculate the square footage of the roof footprint. Once we find the footprint of the roof, we need to factor in the roof pitch, waste factor & contingency.
Earlier, we discussed the average roof is 4/12, which added about 5% (1.05 multiplier) to the square footage. With the added in amount for waste at 10% & contingency at 10%, we’ve roughly added about 25% to the calculation.
For this reason, as a quick rule of thumb, you reasonably can take the building footprint square footage & multiply it by 1.25 to get the roughly calculated roof area.
Roof Size = Building Footprint x 1.25
Roof Size = 1,625sf x 1.25
Roof Size = 21 Squares
There are several different types of roofing materials that are currently used on residential properties. Below we discuss the different roofing options and their pros, cons, life spans and costs of each:
Asphalt Shingles are the most common residential roofing material in the United States and will likely be your material-of-choice for your flip projects. Asphalt shingles are manufactured in two styles, three-tab or architectural shingles.
3-Tab Asphalt Shingles - Three tab shingles are the most economical and popular type of asphalt shingle
Architectural Shingles - Architectural shingles provide more architectural dimension, more durability & provide a longer warranty, but are generally 20 to 40% more expensive than 3-tab shingles.
Life Span: 20 to 25 years
3-Tab Asphalt Cost Range: $225 to $450 / SQ
Architectural Asphalt Cost Range: $250 to $500 / SQ
Wood shingles were the go-to roofing choice for hundreds of years, but due to their cost & flammability, wood shingles have lost popularity.
Generally, wood shingle roofs on a flip project would only be selected if you were seeking a specific architectural look. Be sure to check with the local HOA & your local building/fire codes to determine if wood roofs are allowed in your neighborhood.
Life Span: 15 to 25 years
Cost Range: $500 to $800 / Square
Metal roofs are gaining popularity, but would likely only be used on a flip project if you were seeking a certain architectural-look. Again, check with your local HOA covenants to see if metal roofs are allowed in your neighborhood.
Life Span: 40 to 50 years
Cost Range: $500 to $800 / SQ
Clay, Concrete or Slate Tile Roofing are some of the most expensive materials used for roofs. These types of roofs are typical on certain home styles such as Mediterranean, Mission, Southwestern or Spanish style. You would likely only install these types of roofs on a property to match the property architectural design, and meet neighborhood HOA requirements.
Life Span: 40 to 50 years
Cost Range: $1,000 to $2,000 / Square
Note: If you are changing from a lightweight roof system (asphalt or wood) to a tile roof, there will likely be additional expenses for structurally reinforcing the roof framing.
A roofing crew can typically re-roof an asphalt shingle roof on smaller to average size properties (1,000 to 2,000 sf) in 1 to 3 days.
On small properties an efficient roofing crew can completely tear-off and re-roof a property in a single working day.
On a typical 1,500 sf property that needs 16 to 20 squares of roofing you should expect the project to take 2 days at most.
Note: More labor-intensive wood or tile shingles installations will likely add several days to the duration.
The installation costs mentioned above are for a complete re-roof with new roofing shingles, however those costs may not includes these other costs listed below:
The most obvious factor that influences roof pricing is the type of roofing material being installed. Asphalt shingles are the cheapest and easiest to install roofing material available, with tile roofing being the most expensive and labor intensive.
In construction, there is a term called economies-of-scale. The larger the project, the more efficient the contractor can be installing the roofing. So in other words, on smaller projects you should expect to pay more per square of roofing. If your project only requires a small roof patch, the price per square will be significantly more expensive. As the size of the roofing project increases the price per square should decrease due to efficiencies and economies of scale.
Previous homeowners (or even house flippers)may have tried to cut costs on re-roofs by installing a new layer of roofing over the old layer of roofing. Some insurance companies won't insure homes with multiple layers of roofing and in many cases multiple layers can invalidate the roof warranty. If your house has multiple layers of roofing your roofing contractor will likely charge a premium for the additional labor required to tear off extra layers of roofing.
Roof decking on homes built before the 1960s were often built with 1x8 wood shiplap because exterior grade grade plywood was not available. Shiplap boards are also commonly used under wood shingle roofs. The problem with shiplap boards is that they shrink over time leaving large gaps between the boards which leaves more opportunities for new nails to miss the wood completely which causes eventual leaks. If you have an older home that used 1x8 wood shiplap or a home with wood shingles you may have to install new plywood sheathing over the existing shiplap decking in order to provide a solid substrate for your new roof.
On most moderately pitched roofs the roofing crew can easily install the roofing materials directly on the roof. As your roof gets steeper the roofing crew may be required to work from ladders or install scaffolding which can be significantly less productive than direct installation. If you have a steep roof pitch on your house you should expect your roofing contractor to charge a premium to account for this lost productivity.
If your roof has odd shapes, roof elevation changes, & features it will take more time for the roofing crew to measure, cut and to install the roofing materials. Each of thease shapese and transitions will also require roof flashing materials.
The location of your home and project will have a significant impact on the cost as well. If you are installing the roof in downtown San Francisco the roof will obviously cost much more than the same roof being installed in Suburbia.
Spring and Summer is peak roofing season and roofing contractors are generally swamped with work, so if you are getting a roof installed during this time you will likely pay a premium.
Roofing contractors will generally provide an all inclusive bid to 'furnish and install' all labor and materials on the project.
Here is an example of how to write a Scope of Work that insures your Roofing Contractor provides an all-inclusive bid.
All Roofing work is to be Contractor Purchased, Contractor Installed.