A low-slope section of a St. Louis roof often sits under a membrane that nobody on the homeowner side ever sees, and the layer under that membrane is usually a rigid board. Polyisocyanurate, usually called polyiso, is the board most roofers reach for under single-ply and modified bitumen systems. The spec should name it, along with its thickness, its facer, its compressive strength class, and whether the boards are tapered to move water toward the drains. Those four items decide how the roof handles a February thaw, a July sun on black membrane, and the water that sits on the flat areas after a heavy rain.
Material and spec
Polyiso comes in standard flat sheets and in tapered sets, where the thickness changes across the board to create a slope. A flat-only layer gives a low roof its R-value but leaves the water to find its own way across the deck. A tapered layer builds the fall in from the start. The tapered package is cut to a plan, numbered by location, and laid in a set order so the joints stagger and the slope runs toward the drain or scupper. Ask for the tapered plan in writing, with the board sizes and the slope in inches per foot, before the material is ordered.
The rating on a polyiso board is a long-term thermal resistance figure, and it is lower in cold weather than the number printed on the label. Polyiso also loses some of its insulating value when it is wet, which is one reason the membrane above it and the flashing around every penetration matter. Check the product data sheet for the long-term R-value per inch and the minimum temperature it is rated for. A spec that says only "polyiso, R-30" gives the crew no way to check what was actually delivered.
Compressive strength is the number that matters on the roof. Foot traffic from the crew, rooftop units, and the weight of gravel or pavers all press on the board. The data sheet lists the compressive strength class under ASTM C1289, and the spec should name that class, not just the brand. A board that is too soft will dent under a condenser pad or a walking crew, and the dent telegraphs through the membrane as a low spot that holds water.
The facer is the paper or fiberglass mat on each face. Many polyiso boards come with a coated glass-fiber facer, and some are supplied with a plain facer that the membrane contractor may prefer for adhesion. Name the facer on the spec. If the membrane is adhered to the board, the adhesive has to match the facer, and a mismatch shows up as blisters after the first hot week.
Thickness is a spec number too. A two-layer system with staggered joints is common on a St. Louis low slope because it covers the joints of the first layer and reduces the chance that a single seam lines up with the deck seam below. Ask the roofer how many layers the plan uses, what the total thickness is at the low point and the high point, and where the tapered sets change. If the roof has a parapet, the height of the tapered sections has to clear the top of the membrane turned up the wall.
What it costs
Polyiso is priced by the square foot for flat boards and by the set for tapered packages, and the tapered plan adds a design step. A cover board or a second layer adds material and labor. A contractor who quotes "insulation included" without naming the board, the thickness, and the taper has left the number open. Ask whether the quote includes the tapered design, the cutting, the adhesive or fasteners, and the disposal of the old board if it is removed.
Removing old insulation is a separate cost. If the existing board is wet, crushed, or has a soft spot, the crew has to pull it out to find the deck, and the deck repair is priced by the sheet or by the foot. Ask the roofer how a wet board is handled and what the unit price is for deck replacement. A quote that says "deck repair as needed" with no unit price is a number that moves after the first pull.
Polyiso can be the right board for a low slope and still be the wrong choice for the budget. Some homeowners compare a higher-density board to a standard board, and the difference in cost is small compared with the cost of replacing a membrane that failed on a soft layer. Other homeowners find that a second layer adds cost without a matching gain in the roof's performance. The useful comparison is the same roof, with the same taper plan, quoted two ways.
What to ask
- Which polyiso board is named, by manufacturer and product name, and where is the data sheet?
- What is the total thickness at the low point and the high point, and how many layers are laid?
- Is the tapered plan drawn to the roof, with the slope in inches per foot and the drain or scupper locations marked?
- What is the compressive strength class under ASTM C1289, and does it match the load on the roof?
- Which facer is supplied, and does the membrane adhesive match it?
- Is the deck condition checked before the board goes down, and what is the unit price for deck replacement?
- What happens to wet or damaged board found during tear-off, and who approves the change?
Those questions produce answers that fit in one line each. If the roofer cannot name the board or the taper plan, the spec is still open, and the crew will fill the blank on the day. Call the Keys office at (314) 220-2333 and ask for the board name, the taper plan, and the deck unit price in writing before a date is set.