Which green-roof assessments are worth paying for before requesting quotes?

I’d be careful with the phrase “waterproofing evaluation” in an installer’s quote. Ask for a written scope that separates visual inspection from invasive testing. A walkover can document exposed membrane, seams, flashing, outlets, and obvious ponding, but it usually can’t confirm the condition of concealed layers or how much service life the membrane has left.

If they’re making a firm recommendation about the existing roof, ask whether that requires core samples, moisture testing, or opening up edge and penetration details, and whether those are included or priced separately. Otherwise “membrane inspected” may just mean someone looked at it and declared it acceptable, which is a fairly expensive way to learn that nobody knows.

I’d probably get the structural review independently, then have the roofing or green-roof contractor price the membrane, drainage, and any proposed repairs against that report. The useful deliverable is an itemized list distinguishing observed conditions, assumed conditions, and items that still need invasive confirmation.

The proposed build-up should be part of the assessment fee, not something left until the installation quote. Could the reviewer provide a preliminary saturated-load calculation based on the actual layers being considered, including membrane protection, drainage board, growing medium, vegetation, retained water, and temporary construction loads?

A generic “green roof allowance” may not reflect the difference between an extensive system and a deeper build-up, especially where drainage is restricted or water can pond. I’d also want the drainage concept tied to that same build-up: outlet capacity, overflow routes, and whether the proposed layers could obstruct or slow drainage.

That gives you a useful basis for comparing installers. The scope could list the assumed materials and depths, the saturated weight per square metre, any construction-load assumption, and which drainage details still need confirmation. If the design changes later, it should be clear what needs recalculating rather than treating the original assessment as universally valid.

The independent structural review should verify the load path, not just calculate the saturated green-roof weight. A roof deck may appear adequate while the joists, beams, bearing walls, columns, connections, or foundations below limit what can actually be added.

I’d ask the engineer to state whether the review includes:

- existing framing and its spans, supports, and condition
- how the new load transfers through beams, walls, posts, and foundations
- concentrated loads from planters, pavers, access routes, or stored materials
- temporary construction loads, not just the finished saturated build-up
- any assumptions that still need site verification

That’s especially important if the proposed system changes from the “generic green roof allowance” Susan mentioned. A revised saturated weight might be acceptable at the deck but still require checking a beam or supporting wall. The installer can provide the build-up and drainage proposal, but I’d have the engineer confirm the complete load path before treating the roof as suitable.

The structural scope should cover access and future maintenance, not just the finished roof. Could the reviewer identify:

- permanent walkways, pavers, inspection pads, and edge zones
- safe routes to outlets, overflows, and any roof equipment
- edge protection or guardrail requirements
- access points and areas where maintenance materials might be staged
- concentrated loads from maintenance workers or equipment

Those items can add more than a uniform load and may change where the growing medium or planting zones can go. They also affect the installer’s price, so they should be shown separately from the basic green-roof build-up. A quote that assumes maintenance access should show how that access is provided and supported.