Roof Terminology
“Square” = 100 square feet of roofing surface area (10 ft x 10 ft). When a roof is said to be about 35 Squares, it usually refers to all of the roofing materials required to do the job including main field shingles, starter shingles, hip and ridge shingles, and waste. A rambler with a floor square footage of 2700 sq might require 35 squares depending upon the overhangs and the pitch of the roof. Garages can also add roof square footage.
“Bundle” = the number of packages of roofing material it takes to cover a square (100 square feet). 3-tab is 3 bundles per square, Dimensional is from 4–5, and Presidential shingles are 5–6 bundles per square. All cedar shakes and shingles are bundled at 5 bundles per square at the standard exposure — 10″ for shakes, 5″ for cedar shingles.
“Pitch” = The steepness of a roof is called the pitch and refers to the ratio of a roof’s vertical rise in inches to each foot (12 inches) of horizontal run. A roof with a pitch of 4/12 will rise 4″ vertically for every 12″ of horizontal distance.
The standard minimum pitch for composition shingles is a 4/12 pitch. Below 4/12 the manufacturers usually require a layer of Ice and Water Shield such as Certainteed Winterguard. Roofs with less than a 2/12 slope need to be re-roofed with a low slope roofing material such as torch down, hot-tar, PVC, or TPO membrane.
“Courses” — On pitched roofs, materials are installed horizontally. Each line of roofing material is called a course.
“Exposure” — The amount of roofing material exposed to the weather. Typically, composition shingles have about a 5 5/8″ exposure; 24″ cedar shakes have 10″ of the shake exposed to the weather.
Components of the Roof Structure
Ridge — The highest horizontal portion of the roof.
Gables — The end of the house where the edge of the roof goes straight up from the gutter to the ridge.
Rafters — Rafters can be on 24″ or 16″ centers. They support the roof sheathing.
Valleys — Where two slopes meet.
Hips — The roofline which slopes up toward the ridge at an angle, from an outside corner.
Fascia — The trim wood behind the gutters (usually a 1×6 or 1×8, either whitewood or tight-knot cedar).
Eaves — The area of the roof which extends out beyond the walls.
Soffit — The area beneath the overhang, behind the gutters. Usually vented.
Built-in gutters — Gutters enclosed in the eaves and lined with torch down, PVC, or hot tar. These often have scupper drains 2″ or 3″ in diameter that drain into the downspouts.
The Roof Decking
Decking or sheathing comes in several forms: solid planks, shiplap, skip sheathing with 1×4s, plywood, or OSB.
Shiplap — In some very old homes in Seattle, the roof deck consisted of planks previously used as forms for concrete foundations. Older roofs were also framed with 1×6 shiplap decking.
Skip or Spaced Sheathing — Usually 1×4 boards spaced every 4″. Common on most cedar shake and shingle roofs. Since cedar was abundant in the Northwest in the early 1900s, almost all roofs were made of shingles or shakes over skip sheathing.
CDX Plywood — CDX refers to exterior grade plywood made specifically for roofing. It comes in two common thicknesses: 1/2″ and 3/8″. Today, 1/2″ CDX is the norm and is required by most shingle manufacturers for their heavier shingles such as CertainTeed Presidential Shake or GAF Grand Sequoia.
OSB — 7/16″ Oriented Strand Board is less expensive than CDX and is very common on new construction homes. Structurally, it is just as strong as plywood; however, we are still biased toward CDX plywood. We offer the less expensive OSB as an option to save money when the situation is appropriate or at the customer’s request.
Underlayments
The purpose of the shingle underlayment is to provide an additional moisture-resistant layer to back up your roof shingles. For many years, “felt paper or tar paper” served as the leading type of shingle underlayment. As continual improvements in the building materials industry came about, synthetic underlayments emerged as the preeminent underlayment material used today. Though pricier than felt, synthetic underlayments are lighter in weight, better at lying flat (which prevents “wavy” shingle installations), and offer superior water resistance.
In a modern roof system, a professional contractor will install two different underlayments to maximize protection from the elements. The first is a waterproof ice and water membrane such as CertainTeed Winterguard, installed in areas where roof planes meet to form valleys and around other roof penetrations such as skylights, plumbing vents, chimneys, and roof-to-wall intersections. The second is a tear- and water-resistant material such as Diamond Deck, Tri-Built, or Safeguard 30 that covers the remaining areas of the roof deck and also provides a high-traction walking surface for safety during installation.
Ice and Snow Shield
Ice and Snow Shield underlayments feel like thin rubber or vinyl and are applied directly to the plywood with a “peel and stick” backing. They are waterproof and the material seals the nails and fasteners used to install the roofing. This is usually required on roof slopes from 2/12–4/12 pitch in order to maintain the shingle warranty.
On certain roofs we are now using it in the valleys, around pipes, chimneys, and skylights, and sometimes along the eaves to prevent problems with ice dams. In the Northwest, ice dams have been pretty rare due to the minimal snow we get in the winter — however, the last few winters have brought some significant snowfall. Assuming this may continue in the future, it seems like a reasonable added expense on a lifetime roof investment. Ask us about that option.
Roof Flashings
Step Flashing — Goes up the sides of chimneys, skylights, and dormers. Metal flashing goes up under the siding or counter flashing about 4″ and under each course of shingles or shakes. It should always be replaced during a re-roof whenever possible.
Drip Edge — Along the gable ends, this metal covers the exposed edge of the plywood or OSB.
1×3 Starter Metal — Used along the eave of the roof where the sheathing hangs into the gutter. Prevents wind-driven rain from hitting the edge of the wood and trims the edge out nicely. The bottom edge is hemmed so anyone cleaning the gutters won’t cut their hands.
Saddle or Pan Metal — Usually 5″×13″ metal which sits behind the chimney or skylights. This extends out a few inches beyond the corner of the chimney or skylight to ensure water does not get close to the corner. We snip the bottom of the flashing at the outside edge to prevent water from beading back under the metal, and seal the joint with high-end caulking for additional insurance.
Roof-to-Wall Flashing — Metal which goes up under the siding and out over the shingles where the roof meets a wall (like at the bottom of a dormer or the bottom of chimneys or skylights). Sometimes we cannot replace it without damaging the siding — if so, we wire brush it and paint it, or reside.
Counter Flashings — The metal flashing on a brick chimney that is mortared in between the bricks and folds down over all the other flashings at the base of the chimney. It will usually outlast the roofing shingles and, if painted regularly, can last many decades. If rusted and in poor condition, it needs to be replaced and we will include it in our estimate.
Plumbing Pipe Flashings — The flashings which slide down over the plumbing pipes on the roof. These pipes equalize the pressure in the plumbing system. Flashings come in three versions: all rubber, galvanized metal with a rubber boot, and all lead. We use lead flashings and lead caps on all our roofs as they last essentially forever — or at least as long as the highest end shingles. Flashings with a rubber sleeve tend to crack and split at about 20–23 years. We have had callbacks on these and we don’t like callbacks.
Valleys — For composition shingles, there are 3 types of valleys: woven, closed, and metal W valley. We use all-metal W-valley (14″ wide on each side, 26 gauge, brown or black baked enamel) on all roofs from 30-year 3-tab to Presidential and cedar shakes. It looks great and we never have valley problems.
Ventilation
Proper airflow in the attic space will prevent condensation from developing and rotting out the roof sheathing or plywood. It will help cool the home in the summer and prevent ice dams from building up in the winter.
The basic idea is to ensure that the attic temperature is close to the outside temperature. It is the difference in temperatures that causes condensation in the attic and snow melting above the eaves — then icing up at the gutters in winter.
Traditionally, gable vents at the ends of the house provided ventilation. Roof vents also supplied ventilation by convection when placed at the top of the roof just below the ridge, usually on the backside where they would be less visible. Soffit vents help improve this airflow and are now code. Ridge vents were initially developed to vent cathedral ceiling areas where it was not reasonable to install a roof vent every 2 feet. As they became more common and better looking, they are now the norm and are used whenever possible. On hip roofs which sometimes have limited ridge sections, roof vents must still be installed to provide adequate outflow.
