Part H · the register

The Roof Over Your Addition May Need a Different Material

How to tell whether a low slope Kirkwood addition needs membrane roofing instead of another layer of shingles.

A ranch addition, enclosed porch, kitchen extension, or converted sunroom can look like part of the main house from the ground. Its roof may not perform like the main roof at all. The deciding fact is its slope. If that smaller section is too flat, ordinary shingles may let wind driven rain, melting snow, or backed up ice reach the layers below. Replacing worn shingles with new shingles does not correct that basic mismatch.

This comes up often on Kirkwood houses that have changed over several decades. A steep original roof may drain quickly, while a later addition has only enough height for a shallow roof between the exterior wall and the new eave. The two sections meet at a transition that carries water from the larger roof onto the smaller one. Before accepting a roof proposal, find out how each plane was measured and what material is specified for the shallow section.

Town roofing

Roof slope is written as rise over run. A 4 in 12 roof rises four inches for every twelve inches of horizontal distance. A 2 in 12 roof rises only two inches over that same distance. The difference is visible from a ladder, but it should be measured rather than guessed from the yard or an aerial image.

Asphalt shingles shed water. They are not intended to form a continuously sealed surface. Each course overlaps the course below it, and gravity moves water toward the gutter. The lower the slope, the longer water remains on the roof and the more easily wind can drive it sideways or uphill beneath a shingle edge.

Manufacturer instructions control where a particular shingle can be installed. Many asphalt shingle systems require special underlayment procedures between 2 in 12 and 4 in 12, and do not permit shingle installation below 2 in 12. The exact requirement belongs in the installation instructions for the named product. Keys installs Atlas and Malarkey shingles, so the proposal should identify the product and the low slope instructions that apply to it.

A section below the shingle limit generally needs a roof covering designed for low slope service. On a house, that may be a self adhered modified bitumen system or another residential membrane assembly. The material should be named in the proposal. The words roof replacement are not specific enough when one building has both shingles and membrane.

The transition deserves as much attention as the field of the low slope roof. Water running off a large upper plane becomes concentrated where it reaches the addition. The connection may require membrane extending up the steeper roof, properly lapped underlayment, and flashing arranged so water cannot enter behind the upper material. Smearing roof cement across the joint is not a substitute for a designed transition.

Kirkwood weather tests this detail in several ways. Spring hail can damage both surfaces even though the marks look different. Summer storms can push rain beneath a poorly detailed shingle course. During repeated freeze thaw cycles, water caught at a seam expands, loosens material, and returns through the same opening. Snow and ice can also slow drainage at the eave. If heat escaping from the house melts snow above a cold overhang, ice damming can send water back toward laps that normally face downhill.

The roof deck matters too. A low section that feels soft may have deteriorated decking, but it may also have been framed with a slight sag or built without enough drainage. Covering a low spot does not create positive slope. Once the old material is removed, the contractor should check whether the deck carries water toward the edge or leaves it sitting behind a transition, curb, skylight, or gutter.

What it costs

A small low slope roof is not always priced like a small patch of shingles. Its square footage may be modest, but the crew still has to remove material, inspect the deck, prepare the surface, install edge details, and connect the system to the wall or upper roof. Those details can take more labor per square foot than an open shingle plane.

Ask for the low slope section as its own line in the proposal. The line should give its approximate area, removal scope, deck assumptions, insulation or cover board if included, membrane type, edge metal, wall flashing, and transition method. It should also state how damaged decking will be priced if deterioration is found after tear off.

Access changes the number. A rear addition behind a narrow side yard may require material to be carried farther and debris to be moved by hand. A roof tucked beneath second story windows may need careful flashing removal and replacement. A skylight, chimney, plumbing vent, or exhaust outlet adds penetrations that have to be integrated into the membrane rather than sealed only at the surface.

Drainage at the bottom edge can add work. A shallow roof may empty into a short gutter that also receives water from a steep upper roof. During a St. Louis downpour, that concentrated flow can overshoot a small trough or overwhelm a restricted downspout. The proposal should separate necessary roof work from any gutter changes, so you can see what each decision costs.

Do not compare two totals until both bidders have classified this area the same way. One number may include shingles across the entire house. Another may include a separate membrane system, new transition flashing, and deck preparation. The second proposal can cost more because it describes a different assembly, not because the contractor assigned a higher price to the same work.

Repair economics also depend on the main roof. A localized repair can make sense when the surrounding shingles remain flexible and compatible material is available. Once an older three tab roof has become brittle, repeated lifting can break adjacent tabs and turn a small repair into a widening area of disturbed shingles. That does not automatically require full replacement, but it changes what a responsible repair can accomplish.

What to ask

Start with one direct question: What is the measured slope of the addition? Ask to see the measurement. A digital level, pitch gauge, or careful rise and run measurement can answer it. The result should then be checked against the installation requirements for the proposed roof covering.

Next, ask the bidder to point out where the low slope assembly begins and ends. On some houses, the transition is obvious. On others, a shallow plane continues beneath shingles that visually match the main house. Photographs taken from the roof can show the change in pitch, the condition of the seams, and any evidence of ponding or repeated surface repairs.

Useful questions include:

• What is the pitch of every roof plane over the addition?
• Does the proposed shingle manufacturer permit installation at that pitch?
• What underlayment method is required at this slope?
• If shingles are not permitted, what membrane system is being proposed?
• How will the membrane connect to the steeper roof above it?
• Will siding, counterflashing, or window trim need to be removed and reinstalled?
• Where does water leave the section, and is there any place it can pond?
• How will deteriorated or poorly sloped decking be documented and priced?
• Will the gutter and downspout accept water from both roof sections?
• What workmanship coverage applies specifically to the transition and penetrations?

Also ask whether the proposed work requires a Kirkwood permit and who will obtain it. Municipal requirements vary across St. Louis County, and an address that people describe as Kirkwood is not a reason to assume which permit desk has jurisdiction. The contractor should confirm the actual municipality before work begins and put permit responsibility in writing.

If there is existing interior damage, keep the roof question separate from the ceiling repair. A stain below an addition can originate at the wall connection, a window above the roof, a plumbing vent, a skylight, or a low point several feet away. Water often travels along decking or framing before it appears indoors. Ask for photographs of the suspected entry point and the route water may have taken.

A written scope from Keys Roofing can identify the measured slopes, materials, transition details, photographs, and decking rates before work begins. To arrange an inspection, call (314) 220-2333. Have the contractor inspect both the addition and the upper roof feeding water onto it, because treating either section alone can leave the same vulnerable joint in place.