Direct Answer
Bid alternates are separately priced options that allow a building owner to add, remove, or substitute defined portions of a commercial roofing project without requiring contractors to prepare entirely new bids. They help owners manage budgets, compare roofing assemblies, and evaluate optional improvements before awarding the contract.
Roofing bid alternates generally fall into three categories: additive alternates, which increase the project scope and price; deductive alternates, which remove defined work and reduce the price; and material or system alternates, which price an approved alternative to a specified product, assembly, or installation method.
Each alternate should have a unique number, a clearly defined scope, coordinated drawings and specifications, a separate price, and a stated effect on the base bid. The base bid should remain a complete, functional project, and the bid documents should explain how alternates will be evaluated, prioritized, accepted, and incorporated into the contract.
1. What Is a Bid Alternate in Commercial Roofing?
A bid alternate is a separately priced option that the building owner may choose to accept or reject under the bidding and contract requirements. It allows the owner to evaluate optional roofing work while keeping the core project scope consistent across bidders.
For example, a commercial roof replacement project might include a complete specified membrane and insulation system in the base bid, with separate alternates for additional insulation, a different approved membrane system, or replacement of an additional roof section.
Alternates are useful when the owner needs flexibility because of budget limits, uncertainty about optional improvements, or a desire to compare different technical solutions.
However, an alternate should not be used to omit work essential to a complete, code-compliant, weather-resistant roofing system. Required flashing, attachment components, drainage provisions, and other necessary work must remain included in the appropriate base or alternate scope.
2. What Are the Main Types of Roofing Bid Alternates?
| Alternate type | Purpose | Commercial roofing example |
|---|---|---|
| Additive alternate | Adds defined work to the base project. | Add a specified cover board or replace an additional roof section. |
| Deductive alternate | Removes defined work from the base project. | Omit a separately identified optional roof area or decorative feature, where technically appropriate. |
| Material or system alternate | Prices a defined alternative to the specified product or assembly. | Price an approved alternative membrane system or insulation configuration. |
| Phasing or area alternate | Prices a separately defined roof section or project phase. | Replace an additional wing or separately identified building roof area. |
Material or system alternates need particular care. The bid documents should state whether the alternate price represents the full cost of the alternative assembly or only the difference between that assembly and the base specification. For a material substitution, a clearly defined differential price often makes comparison easier.
Relevant guidance: Federal Highway Administration — Additive and Deductive Alternate Bidding.
3. Which Roofing Items Make Good Bid Alternates?
The most useful alternates are specific, measurable, and capable of being accepted or rejected without leaving the roof incomplete or creating unnecessary conflicts with adjacent work.
| Potential alternate | What should be defined | Why it may be useful |
|---|---|---|
| Additional insulation | Insulation type, thickness, area, attachment, and associated assembly requirements. | Allows the owner to compare additional thermal performance and upfront cost. |
| Cover board | Board material, thickness, attachment, and roof areas covered. | Allows comparison of specified assembly configurations. |
| Alternative membrane system | Membrane type, thickness, attachment method, flashing, and approved assembly. | Supports a documented comparison of suitable roofing systems. |
| Additional roof section | Exact roof boundaries, removal, replacement, drainage, and tie-in details. | Allows the owner to extend the project if funding permits. |
| Roof coating or restoration | Eligible roof areas, preparation, repairs, coating system, and specified application requirements. | Allows comparison of an appropriate restoration option against other defined scopes. |
| Enhanced roof protection | Specified protection components, locations, attachment, and compatibility with the assembly. | Allows evaluation of optional protection where the design team considers it appropriate. |
| Additional drainage improvements | Drain locations, slopes, tapered insulation, plumbing interfaces, and related work. | Allows evaluation of separately defined improvements to roof drainage. |
Not every item should automatically become an alternate. If a component is required to satisfy the approved assembly, applicable building requirements, or the project’s performance criteria, it should be included in the mandatory scope rather than presented as an optional upgrade.
4. How Should Bid Alternates Be Structured?
Every alternate should be written so that contractors can price the same work using the same assumptions. A one-line description without clear boundaries, specifications, and pricing instructions can produce bids that are difficult to compare.
Step 1: Give each alternate a unique identifier
Use a consistent numbering system, such as Alternate 1, Alternate 2, and Alternate 3. Give each item a short descriptive title that identifies the proposed change.
Step 2: Define the exact scope
Identify the affected roof areas, materials, quantities, drawings, specification sections, and interfaces with adjacent construction. State what remains in the base bid and what changes if the alternate is accepted.
Step 3: Specify the complete assembly
For roofing work, identify the required materials, thicknesses, attachment methods, flashing details, edge conditions, and other components necessary for a complete installation. Require supporting documentation for proposed substitutions.
Step 4: State how the price must be submitted
Require each bidder to identify whether the price is additive, deductive, or a differential for an alternative assembly. State whether labor, materials, equipment, disposal, overhead, profit, and related work are included.
Step 5: Establish the priority and evaluation method
If the owner intends to select alternates in a particular order, state that order in the bid documents. Explain how the selected alternates will be applied when determining the evaluated bid amount and awarding the contract.
Step 6: Explain acceptance and contract incorporation
State when alternates may be accepted, whether they must be selected at award, and how accepted alternates become part of the contract. Identify any applicable deadline for selection or pricing validity.
Construction guidance from the Federal Highway Administration and the Engineers Joint Contract Documents Committee emphasizes clear alternate definitions and transparent evaluation procedures. See EJCDC — How to Specify Alternates.
5. Example of a Well-Structured Roofing Bid Alternate
The following example shows how an owner could request an alternate for additional cover board. The quantities and specifications should be replaced with the project’s actual requirements.
| Bid item | Example requirement |
|---|---|
| Alternate number | Alternate 1 — Additional Cover Board |
| Type | Additive |
| Scope | Install the specified cover board over the designated roof areas shown on the alternate drawings. |
| Technical requirements | Follow the specified board material, thickness, attachment method, membrane compatibility, and approved assembly. |
| Included work | Materials, labor, fastening or adhesive, coordination with adjacent work, and required installation details. |
| Price format | One separate additive amount, excluding the alternate from the base bid. |
| Evaluation | Apply the alternate according to the priority and evaluation rules stated in the bid documents. |
| Acceptance | Include the alternate in the contract only if the owner formally accepts it. |
This structure makes the alternate easier to compare because each bidder prices the same defined addition rather than interpreting a general request for an upgrade.
6. How Should Additive and Deductive Alternates Be Prioritized?
Alternates can help owners adjust project scope to fit the available budget, but the order of selection should be determined before bids are received.
Additive alternates can allow an owner to include more work when funding permits. For example, the owner may prioritize adding a specified roof section, followed by an optional insulation improvement.
Deductive alternates can identify defined scope reductions if the base bids exceed the available budget. Any deduction must preserve a complete and technically acceptable project. Required system components should not be removed simply to achieve a lower price.
When multiple alternates are used, construction guidance commonly recommends keeping their pricing and evaluation rules consistent and listing them in the intended order of priority. The specific procedure should comply with the applicable procurement requirements.
Reference: EJCDC Construction Documents — Base Bid Plus Alternates.
7. How Do Bid Alternates Affect Bid Comparison?
The lowest base bid is not necessarily the lowest evaluated price after the owner’s selected alternates are applied. Owners should compare bids using the same defined base scope and the same alternate-selection rules.
For example, consider the following hypothetical prices:
| Bid component | Contractor A | Contractor B |
|---|---|---|
| Base bid | $480,000 | $495,000 |
| Alternate 1 — Add cover board | +$24,000 | +$15,000 |
| Evaluated total with Alternate 1 | $504,000 | $510,000 |
In this hypothetical example, Contractor A remains lower when Alternate 1 is selected, even though Contractor B offers a lower price for that particular alternate. The owner should also confirm that both bidders have priced equivalent specifications and complete scopes before making a decision.
The example is illustrative only. Actual bid evaluation must follow the published procurement rules, contract requirements, and any applicable public-sector bidding regulations.
8. Common Mistakes to Avoid
- Using vague descriptions: Terms such as “better insulation” or “upgrade roof system” do not define a priceable scope.
- Leaving drawings incomplete: Bidders may assume different roof boundaries, quantities, or interfaces.
- Mixing full prices and differential prices: Clearly state whether the alternate is the total cost of the alternative scope or only the price difference from the base specification.
- Allowing the base bid to be incomplete: Essential waterproofing, flashing, attachment, and drainage work must remain covered.
- Using too many alternates: Excessive options increase estimating complexity and can make proposals harder to compare.
- Failing to state selection priorities: Contractors may not know how their alternates will be evaluated.
- Ignoring warranty and compatibility requirements: A cheaper alternative may not meet the required roofing system or warranty conditions.
- Failing to document acceptance: Only accepted alternates should become part of the contract under the applicable agreement.
Bottom Line
Well-structured bid alternates give commercial property owners flexibility to manage roofing budgets, compare approved systems, and prioritize optional work without compromising the required base project.
Define each alternate precisely, price it separately, coordinate it with the drawings and specifications, establish selection priorities, and document acceptance in the contract. This makes contractor bids more comparable and helps reduce confusion, change-order disputes, and unexpected costs.
Related Questions
- What Allowances Are Typical for Deck Replacement in a Bid?
- Which Unit Prices Should Be Requested in Every Roofing Bid?
- What Is a Roof Lifecycle Assessment, and How Does It Feed Capital Planning?
- When Is It Better to Replace a Commercial Roof Rather Than Repair It?
Sources
- Federal Highway Administration — Additive and Deductive Alternate Bidding
- Engineers Joint Contract Documents Committee — How to Specify Alternates
- EJCDC Construction Documents — Base Bid Plus Alternates
- Proctor Recreation Center Roof Replacement — Example Roofing Bid Alternate Specifications
Last reviewed: October 2026
Related Resource
Planning roofing work around the building’s condition, expected service life, and future capital requirements can help owners make better-informed decisions about optional improvements. Learn more about Commercial Roof Maintenance & Preventative Maintenance Programs.
Protect Your Commercial Roofing Investment
Clear roofing scopes and documented project requirements help building owners compare proposals and plan appropriate repairs. If your commercial property has suspected roof leaks or needs an assessment before a roofing project, explore ShieldLine Roofing’s Commercial Roof Leak Detection & Repair services.
