Mechanically fastened, adhered or ballasted single-ply roofs
Updated 23 October 2026 · SEO Dons Editorial
This guide is for owners and specifiers comparing single-ply proposals for a commercial roof. It sets out what each method asks of the deck and the building, and what to request in a tender. It gives no wind or fixing figures, because those come from a calculation for the individual roof. Membrane materials are compared separately in the EPDM, TPO and PVC guide.
What is the difference between the three methods?
Holcim Elevate, which supplies EPDM and TPO systems, lists the three attachment methods as ballasted, adhered and mechanically attached. Soprema’s synthetic membrane page describes the build-ups, and notes that hot-air welding is always used to secure overlaps and that laps are probed with a seam probe to check integrity.
| Method | How it is held down | Limits the manufacturers state |
|---|---|---|
| Mechanically fastened | Insulation fixed to the deck with plates and fasteners, membrane fixed mostly in the seam area | Fastening systems can act as thermal bridges. Installation is little affected by temperature or humidity |
| Fully adhered | Membrane bonded to the insulation or substrate with adhesive | Adhesive is weather-sensitive. Holcim says self-adhered membranes remove the need to apply adhesive on site |
| Ballasted | Insulation and membrane laid loose, held by gravel or pavers over a protection layer | The deck must carry the ballast. Most suitable where there are few openings and penetrations |
Holcim also describes non-penetrating variants of in-seam fixing, such as induction welding and a linear strip attachment system. Soprema’s fully adhered build-up runs primer and vapour layer, insulation adhesive, insulation, then adhesive and the waterproofing layer. The full single-ply system is described on the single-ply membrane roofing page.
How does wind uplift decide between them?
By calculation, not preference. BSI describes BS EN 1991-1-4 as the European standard for wind actions on structures, covering the whole structure and components such as cladding units. SPRA, the Single Ply Roofing Association, publishes a wind loading protocol for calculations (S11b) and a checklist for wind loading (S11a), and says developing competence is necessary to get consistent, accurate wind load results. Its training course expects software complying with BS EN 1991-1-4 and the UK National Annex.
Manufacturers then turn the result into a system. Sika’s data sheet says fastener spacing must follow project-specific Sika calculations. For ballast, it says to cover with the appropriate roof material according to the roof design and the local wind loading conditions. BRE publishes separate Digests on how loose-laid insulation boards can be restrained using ballast or mechanical fixing (DG295) and on estimating the wind speeds at which gravel ballast begins to scour (DG311).
One timing point. BSI says UK adoptions of the first generation of Eurocodes will be withdrawn on 30 March 2028, with first generation documents treated as applicable until then unless a project says otherwise. Ask which edition and National Annex the calculation used. The guide to UK wind exposure and flat roof uplift covers the regional picture.
Which method suits which situation?
| Question | What the sources say | Points towards |
|---|---|---|
| Can the deck carry added permanent load? | Holcim says the deck must support the ballast, which may mean strengthening, and that old buildings may not suit ballast without modification | If not, fastened or adhered rather than ballasted |
| Will fixings hold in the deck? | SPRA publishes a site pull-out test protocol (S15) and guidance on attaching single ply to concrete decks (S16) | Fastened only once the site tests support it |
| Is the substrate firm, and is the weather workable? | Holcim says adhesion gives strong uplift resistance on a firm fixed substrate but is weather-sensitive | Adhered, with a weather plan |
| Many penetrations or plant bases? | Holcim says ballast suits roofs with few openings | Away from ballast |
| Exposed site? | The calculation decides, per SPRA and BSI | No method on exposure alone |
| Roof accessed regularly, or a green roof or solar planned? | Holcim lists access, a green roof and a solar installation as factors in the choice | Tell the designer before the specification |
Sika lists concrete, OSB, plywood, timber boards and metal decking as substrates for its AT-15 membrane, and says it can go over existing bituminous roofing that is clean and level. That is a compatibility statement for one product, not a method choice.
What changes if plant or solar goes on later?
The roof side is the membrane, the build-up and the load. Ballast is permanent weight the deck already carries, so a later load from plant bases, walkways or panels adds to it. Say what is planned before the wind calculation and structural check are done. This guide does not cover mounting. The guides on whether a flat roof can carry solar panels and a worked example of a re-roof engineered for ballasted PV cover the roof-side checks.
How does the method affect manufacturer cover, inspection and re-roofing?
Manufacturer cover is tied to the system the manufacturer has specified. Sika says its membrane must be applied by an applicator trained or approved by Sika, and that fastener spacing follows its own calculation. SPRA publishes a guide for clients and designers on manufacturer cover (S04). Treat a change of method after tender as a change to the specification and ask for written confirmation of cover. The site’s page on manufacturer cover explains how it works.
Inspection differs. Holcim says gravel ballast makes a roof harder to inspect and maintain, with damage harder to detect and leaves collecting unseen. A maintenance contract should say how ballast is lifted for checks.
For a future overlay or re-roof, Holcim says a long-lived membrane such as EPDM can be reused from a ballasted roof because it can be easily removed and used on another roof. The pages checked do not say what a later re-roof involves for the other two methods, so ask the designer to describe it. The overlay and strip-and-recover guide covers the decision. If bitumen layers stay under a new membrane, the asbestos in old roofing felt guide applies.
What should a tender show?
- The wind load calculation for the roof, stating the standard and edition used.
- The fixing or ballast layout, with pull-out test results where fixings go into the deck.
- For ballast, the structural engineer’s confirmation that the deck can carry it.
- Written confirmation from the membrane manufacturer that the method is within the system it covers.
- The allowance for inspection and maintenance. To compare proposals, you can send roof details through the form on the industrial flat roofing page.
Sources
- Holcim Elevate, Ballasted, fully adhered or mechanically attached roofing system?, for facts about Holcim Elevate EPDM and TPO systems (checked 22 October 2026)
- Sika, Sarnafil AT-15 and AT-18 product data, for facts about Sika products (checked 22 October 2026)
- Soprema UK, waterproofing, for facts about Soprema synthetic membranes (checked 22 October 2026)
- Single Ply Roofing Association, technical library and wind loading protocol sessions (checked 22 October 2026)
- BSI Knowledge, BS EN 1991-1-4 and BS EN 1991-1-1 (checked 22 October 2026)
- BRE, Digest 311 and Digest 295 catalogue descriptions (checked 22 October 2026)
Common questions
Is a ballasted roof the cheapest or simplest option?
Holcim describes a ballasted system as relatively easy to install, but says the deck must support the ballast and that strengthening can increase the overall project cost. No price comparison is given here, because the pages checked offer no sourced figures. Ask for the structural engineer's view before comparing.
Does fully adhered mean no fixings at all?
Not necessarily. Holcim says upstands can be mechanically attached or fully adhered, while Soprema's fully adhered build-up uses adhesive for both the insulation and the membrane. Ask the manufacturer for the exact build-up and what is fixed at edges and details.
Who calculates the fixing pattern?
Sika's data sheet says fastener spacing follows project-specific Sika calculations. SPRA publishes a wind loading protocol and runs training because accurate results need competence. Ask who carried out the calculation, which standard edition they used, and whether the membrane manufacturer has approved it.
Can I change the method after the tender?
You can, but treat it as a change to the specification. The wind calculation, any deck tests or structural check and the manufacturer's cover were all prepared for the original method, so ask for each to be reissued in writing.