Technical library
Roof Decks A to Z – Part XV: Haydite and Nailcode
Haydite lightweight aggregate and Nailcode nailable concrete roof decks: how to identify them, test fastener withdrawal and plan attachment before a reroof.
Article by L.D. Hogan, F-IIBEC, RRC, PE and Don Kilpatrick
Read the full article (PDF)Background on this topic from Inspec
Two trade names, one family of roof decks
Haydite and Nailcode are trade names rather than generic deck descriptions, and both turn up on drawings, specifications and old roofing bid documents for buildings of a certain vintage. Haydite is a lightweight aggregate made by expanding shale or clay in a rotary kiln, so concrete batched with it weighs less than concrete made with ordinary gravel or crushed stone. Nailcode is one of the proprietary names carried on older documents for a nailable concrete roof deck, cast so that roofing nails driven into it will hold.
The two categories overlap. A nailable deck is often a lightweight aggregate concrete, so a drawing may name the aggregate, the proprietary system, both, or neither. What governs a reroof is not the name on the sheet but what the deck will actually do: what it weighs, what it carries, how much moisture it holds, and whether anything driven or drilled into it will stay put.
Where these decks sit in the roof assembly
These roof decks rarely stand alone as a simple slab. Many were placed over bulb tees with cementitious or fiberboard form boards left permanently in place below, which produces a layered assembly: concrete above, form board below, and a joint plane between them that can behave very differently from either component. Others appear as precast plank, or as a topping poured over another structural system. Each configuration changes where a fastener actually gains its holding power and where water tends to collect.
Two companion articles cover the neighbouring cases. Our article on structural fiberboard roof deck planks addresses the form-board side of that layered assembly, and cast-in-place structural concrete covers the conventional normal-weight pour.
Reading the documents, then checking them
Original drawings, specifications and shop drawings, where they survive, may name the mix or the proprietary system and give a design thickness. That is a starting point and not a conclusion. Proprietary nailable deck products were discontinued long ago, batch proportions varied by supplier and region, and published physical properties for a given trade name are often no longer obtainable. Decks are also patched, overlaid or partially replaced during earlier reroofs, so a single original note can describe only part of the roof area.
Earlier roofing contracts, maintenance records and photographs from previous tear-offs are worth collecting for the same reason. They tend to show where the deck has already been repaired, which is usually where the next set of surprises is found. Terms encountered in those old documents are defined in our building envelope glossary.
Identifying the deck in the field
From the roof surface
A full-depth test cut through the roof assembly is the most direct method. In a lightweight aggregate concrete, the cut face shows rounded, porous particles distributed through cement paste rather than the angular faces of crushed stone. A nailable mix tends to be more open at the cut edge and may crumble under a chisel where a structural mix would fracture. Record the thickness, whether the concrete is continuous or spans between tees, and what lies directly beneath it.
From inside the building
The underside shows the framing, the form board and the tee spacing, along with staining, sagging and spalling that the roof surface conceals. Observations are worth planning as a route across the building rather than as a single opening, because deterioration in these decks is usually local: over bays with a history of leakage, under old drains, and along the perimeter.
What to record
A deck investigation is only as useful as its mapping. Cut and core locations, thicknesses, aggregate character, form-board type and condition, moisture findings and framing direction all belong on a roof plan, keyed so the same plan can later carry the attachment pattern and the repair quantities. That plan becomes the basis of the construction documents.
Building an attachment test program
Holding power in these decks varies with mix, thickness, moisture content and local deterioration, so it is established on the building rather than taken from a table. A useful program tests the fastener and plate actually proposed for the project, spreads locations across the roof area so that variation between bays is captured, and deliberately includes perimeters and corners, where uplift demand is highest and where years of edge leakage have usually done the most damage. Suspect areas identified during the field survey should be tested as well, so the low values are known before bidding rather than discovered during installation.
Moisture evaluation runs alongside it. Lightweight and nailable concretes absorb water and can stay wet long after the leak that wetted them is gone, which affects fastener holding, adhesive bond and any new insulation placed in contact with the deck. Moisture detection services and core sampling together give a defensible picture of how much of the deck is dry enough to build on.
When the results do not support the intended design
Low or inconsistent values are not an obstacle so much as a design input. The options generally include a tighter fastening pattern where values allow it, an adhered assembly subject to bond and moisture evaluation on the prepared surface, a structural overlay, or removal and replacement of the deteriorated deck areas. Each option changes dead load, drying behaviour and sequencing, so the choice belongs in the design phase, with structural input where the assembly is getting heavier.
Carrying the findings into the documents and the field
What the investigation establishes only protects the owner if the specification says it. Removal means and methods, protection of the deck surface during tear-off, treatment of nails and base sheet left behind, unit prices for deck repair, and the contractor's obligation to stop and report damaged deck rather than cover it are all drafting decisions. The deck itself is visible for only a short interval between removal of the old roof and installation of the new one, which is why construction observation and administration matters more on these projects than on most.
How Inspec approaches these decks
We investigate and document the existing deck, arrange and observe testing, and prepare construction documents that reflect what the deck can actually support. We do not perform the work; our role is to define it and then observe the contractor's execution against the documents. Related services include roof consulting, commercial roof inspection and forensic roof investigation where the deck condition is tied to an active leak or a dispute. Further articles in this series are collected in our educational resources.