This estimating guide is a continuation of our two-part series for contractors, on how to measure and estimate roofing prices. See part one of this guide to learn how to measure a roof.
Calculating Roofing Materials and Estimating Prices
For the purposes of this guide, I will use asphalt shingles as a default material, but the same principles can be applied to metal, slate, etc. Cedar shingles/shake will be slightly different, as the waste factor is higher.
Calculating roofing materials First – you need to know how many sq. ft. or squares of shingles you will need. You should take your total roof square footage and divide it by a 100 – this will give you the number of roof squares. Round the number up to the nearest square. Most shingle roofs will have 5-10% waste factor, and if you use most pieces during the installation, waste can be reduced to as little as 2-3%. If you have valleys / dormers and side-wall flashing, use 15% waste factor. See our roofing square guide, to learn how roofers and builders measure roofs.
Besides buying just shingles, you will also need to calculate felt / tar paper (or any other approved underlayment), as well as Ice & Water Shield, which is a code requirement in most northern states and in Canada. For Ice & Water shield, you will need to apply at least one row (3 feet wide roll) along the eave, or drip edge overhangs. Usually, I&W shield comes in 66 feet long rolls, so if you have 75 feet of eaves, you will need 3 rolls for two lengths of roof eaves. I&W shield should also be installed in all valleys, so measure the length of your valleys and add that to the total length of I&W shield needed for the job.
Underlayment should overlap felt by a minimum of 6″, but it is better to buy a foot. In Massachusetts, the code requires double coverage of 15 lb felt, so contractors first use an 18″ roll, then completely cover it by the next full roll of felt – 36″ or 3′, and then use 50 percent coverage for each next roll, so only the bottom half of the roll is exposed. For synthetic underlayment you will not need more than 6″ overlaps – thus an average 10 square roll of synthetic underlayment will yield about 8-9 sq. coverage.
You also need to calculate the amount of drip-edge or starter to go along your roof eaves, and depending on desired aesthetics, you may also install drip-edge along the gable ends of your roof. Bear in mind that drip edge comes in 10 foot sections and you should not use single pieces of less than 2 feet long.
Calculate the plastic cap nails for felt/underlayment and roofing nails for shingles. Consult with the manufacturer’s nailing requirements for both. Finally add roof vents or ridge vent, pipe flashings, and new step flashing for chimney / skylights. You can use our roofing shingles calculator to estimate roofing materials needed to install a simple gable roof.
Estimating Installation Cost
Now that you have the total amount of materials and accessories needed to install a roof, and the cost of these materials, you can estimate or approximate what a contractor would charge for installation.
Most roofers calculate roofing prices in squares – either total roof area or the total number of squares to be installed. You can also use a Materials, Labor, Overhead Costs and Profit (MLOP) method to estimate the total price, but the overheads and profit varies greatly between different contractors, so until you know your costs for sure, it’s better to use the price per square method.
In most areas of US and Canada, price per square ranges between $300-450 per square, for a simple, one story ranch home, for one layer tear-off, and installation of a 30 years architectural shingles ( now known as “lifetime shingles” 🙂 ). While this is a huge range for arguably the same amount of work and materials, local economic conditions come into play, and affect your local average price per square.
For more complicated roofs, there is typically a percentage or dollar amount up-charge for each item or per square. For example:
- A second layer tear-off, usually is $50-75 more, per square. The same upcharge goes for 2-3 story homes.
- Plywood / sheeting replacement would be about $80-100 per square
- Chimney and skylight flashing is usually per piece. If it’s a chimney, the flashing might cost anywhere between $100 to $300, depending on what’s involved (e.g. new step flashing or counter-flashing). On skylights, existing flashing is typically reused, and it should not cost more $75-125
- Valley flashing is ether per foot, or per valley up-charge, depending on the length and type of valley (woven, open or other type of valley flashing.
- If the roof is really cut-up, you may add either a certain amount per square (say $100-150) or charge for every type of additional work separately. If there are 6 dormers on the roof, and each requires cutting back cedar siding, you may charge for example $500 per dormer – this will include roof to wall flashing work, valley flashing above the dormer, and the general installation time lost on working around the dormer.
Above are just hypothetical examples of extra charges that may come into play and will depend on the actual roof, as well as the exact amount of extra work.
How This Roof Measuring Guide Can Be Helpful To Homeowners
Now that you know how to calculate your roof size, you are able to have more knowledge and insight into your installation cost. I realize that most people reading this article will not be installing their own roof, but with this knowledge you will know how much your materials cost and what goes toward labor and overhead / profit for the contractor. If you see that materials take up a larger portion of your total proposed roof price – i.e. 50 percent or more – then the price is probably TOO low, and you should think twice before hiring such a contractor. If your roofer will not be making profit, then they are either going to just scam you and run with your money, or they won’t be around for a long time, and who’s going to uphold your warranty?
How Can Contractors Use This Information?
This guide will be especially useful for “new” roofing contractors, since they usually face questions like “how much do I charge for this roof?”. But even seasoned professionals might find this useful, as many roofers out there have not taken a good look at their prices, and may adjust their pricing to better reflect the rising costs of materials and labor.
Additionally, if you are a roofing contractor, try our Roofing Calculator application for iPhone or Android platforms:
Roofing Calculator does essentially what we walk you through in this guide, but uses pre-set prices for materials, labor, and extra work. It provides you detailed results on all the prices and roof sizes. With this app, you can instantly estimate any roof, and give your client the price on the spot.
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