The Missing Membrane Can Make the Lower Roof Bid Cost More

Two roofing bids can name the same shingle and still describe different roofs. One difference may be buried in a short line labeled ice and water membrane, leak barrier or self-adhering underlayment. The important question is not whether that line appears. It is where the membrane will be installed and how much of the roof it will cover.

This membrane is not ordinary synthetic underlayment. It has an adhesive backing and seals more tightly around many fasteners. Roofers use it in places where water is more likely to move beneath the shingles, including eaves, valleys, roof-to-wall intersections and penetrations. It adds protection, but it is not a substitute for properly formed flashing, sound decking or correctly installed shingles.

A proposal that says only "ice and water shield included" leaves the comparison unfinished. It could mean one narrow strip at the eaves. It could mean the eaves plus every valley. It could include the chimney, skylights and walls. Those are materially different scopes.

Roofer comparison

Start by marking every place on your roof where water concentrates or where one surface meets another. On a simple gable roof, the list may be short. On a St. Louis house with additions, dormers, a chimney and several valleys, the membrane plan can affect hundreds of square feet.

Ask each bidder to identify coverage in these locations:

• The lower eaves, especially above heated rooms.
• Valleys where two roof slopes collect water.
• Roof-to-wall intersections beside dormers and additions.
• The uphill side of a chimney.
• Around skylights, plumbing penetrations and other openings.
• Low-slope transitions where water drains more slowly.

Do not assume a bidder includes every location because the product appears in the material list. Ask for the locations in writing. A quantity can help too. If one proposal allows two rolls and another allows six, the difference deserves an explanation. Roll dimensions vary, so compare square footage or stated coverage rather than the roll count alone.

Eave coverage matters in this climate because snow can melt over a warm part of the roof and refreeze near a colder edge. That can contribute to ice damming at the eave. Water can then move backward beneath shingles. A membrane provides secondary protection in that area, although attic air sealing, insulation and ventilation are still part of correcting the conditions that form ice.

The freeze thaw cycle also exposes weak details. Water enters a small opening, freezes, expands and opens the path further. A membrane beneath the shingles can reduce the chance that water reaching the deck enters the house. It cannot repair loose step flashing, a poorly built chimney cricket or a wall detail that directs water behind the roofing.

Valleys deserve separate attention. They carry more water than the surrounding field of shingles during a hard St. Louis rain. Leaves and seed pods can slow that flow. Hail can also damage shingles along with the rest of the roof during spring and summer storms. A written bid should tell you whether the valley receives membrane beneath the finished valley system and whether old valley materials will be removed before the new assembly is installed.

Watch for proposals that use vague phrases such as "as needed" or "to code." Different municipal permit desks operate across St. Louis County, and the applicable requirements depend on the address and adopted code. Ask which permit office governs your property and what that office requires. Then ask whether the contractor's proposed coverage stops at the minimum requirement or extends to additional vulnerable details on your particular roof.

More membrane is not automatically a better design. Covering large areas without considering ventilation and drying can change how the roof assembly handles moisture. The goal is a deliberate plan based on the roof shape, interior conditions and manufacturer's installation instructions. A bidder should be able to point to each proposed location and explain the reason for it.

What it costs

The price difference includes more than the membrane itself. Installing it requires surface preparation, careful alignment and additional labor around corners and penetrations. Old adhesive material may be difficult to remove. If the decking beneath it is damaged or covered with residue, sections of wood may need replacement before a new membrane will bond properly.

Roof complexity changes the amount quickly. A plain roof with two eaves and no valleys uses much less than a roof with intersecting additions, dormers and several wall lines. Chimneys and skylights require cutting and fitting. A wide chimney may also need a cricket on its uphill side, which is a separate construction detail rather than an extra patch of membrane.

Ask for the membrane cost to be included in the base price for stated locations. If additional coverage might be recommended after tear-off, the proposal should give a unit price or explain how approval will work before the material is installed. That keeps a vague allowance from becoming an open-ended charge on the day of the job.

Also compare the decking terms. Self-adhering membrane needs a suitable surface. If one roofer has allowed for likely deck repairs and another has not, the lower total may simply postpone part of the bill. The written proposal should state the rate for replacing damaged sheets or boards and how the contractor will document the quantity.

Material specifications matter. Ask for the membrane manufacturer and product name, not merely a generic description. If the roof is being quoted with Atlas or Malarkey shingles, the contractor should confirm that the complete underlayment and flashing plan follows the applicable installation instructions. Keep the final product list with the contract so substitutions require a conversation rather than an assumption.

On an older three-tab roof, this detail should be considered with the condition of the entire assembly. Once the shingles are brittle, losing tabs or near the end of their useful service, opening a valley or eave for a limited repair may disturb adjacent material that cannot be resealed reliably. At that stage, repeated repairs can stop making practical sense. The roofer should explain what can be repaired without promising that an old field of shingles will behave like new material.

What to ask

1. Exactly where will the membrane go? Ask the bidder to mark the eaves, valleys, walls, chimney, skylights and penetrations on a roof diagram or annotated photograph.
2. How wide will the eave coverage be? A phrase such as "one row" is incomplete because products come in different widths and roof edges are built differently. Request the intended width or square footage.
3. Does the price include every valley? Confirm whether short valleys over additions and dormers are counted, not just the obvious main valley.
4. What flashing goes above the membrane? Membrane beneath bad flashing does not make a durable wall, chimney or skylight detail. Ask whether step flashing, counterflashing and valley metal will be replaced or reused.
5. What happens if the deck is wet, rotten or uneven? The proposal should give the replacement rate and describe how photographs and measurements will be provided before concealed work is covered.
6. Which permit desk applies to this address? St. Louis County contains many municipalities with their own procedures. The contractor should identify the jurisdiction instead of treating the entire county as one permit office.
7. Will the final invoice identify any added coverage? If conditions discovered during tear-off change the membrane plan, require the location, quantity and price in writing.

Put the answers beside each other before comparing totals. A useful comparison might read "eaves only," "eaves and three valleys," or "eaves, all valleys, chimney and two roof-to-wall lines." That wording tells you what the money buys. A check mark beside "ice and water" does not.

Keys Roofing can inspect the roof, photograph the relevant details and prepare a written scope using Atlas or Malarkey shingles. To compare the membrane plan on a proposal you already have, call (314) 220-2333. Have the roof diagram, product list and stated coverage locations available so the conversation begins with the actual assembly rather than the total at the bottom of the bid.