Our dome roof designs utilize more insulation, provide for roof ventilation, incorporate long-lasting roof coverings and tend to have longer construction cycles. The many variables that contribute to roof top temperatures; insulation, facing, pitch, color, etc., make it difficult to predict what kind of heat profile the roof top will experience. Choosing an underlayment that will perform under all of these demanding conditions is essential to a successful roof design.
We DO NOT recommend the use of 15# or 30# regular felt roofing paper anymore. GCP Tri-Flex or GAF Deck-Armor™ are far superior products to standard felt paper and they weigh less than 30# for a 1000 sf roll (the equivalent of 5 rolls of felt paper). Our 70 page roofing chapter in our Construction Manual includes a listing of 33 synthetic roofing underlayments and indicates their permeability rating. This is critical to understanding what type of underlayment is best for your climate zone and roofing material.
Building codes require the use of a “water and ice protection” roofing underlayment in various parts of the roof. We specify the use of GCP Ice and Water Shield Membrane because it has a 250% to 300% elongation quality and fantastic adhesion. It’s rubberized asphalt seals around nails and provides a level of protection that traditional felt underlayments just can’t match. We use their HT (High temperature) version for valley flashing, the very top of the dome and as an “extended” flashing around our skylights and cupolas. With our ventilated dome shell system, if you are in a wet, windy, or heavy snow climate zone, you could consider covering the entire roof in Ice & Water Shield.
GCP TRI-FLEX® XT Roofing Underlayment
GCP TRI-FLEX® XT is an innovative spunbonded polypropylene coated with a layer of U.V. stabilized polypropylene on both sides. This technology, available only from GCP, creates a very durable and tough weather resistant fabric. It has excellent tear resistance, designed to withstand even the strongest wind conditions. It is inorganic, contains no asphalt, and is up to 15 times more resistant to tearing than 30# felt. Lightweight and easy to install. Remains pliable at temperatures as low as -60°F (-51°C) and weighs only 28 lbs for a 1000 sf roll. Thermal stability up to 240°F (116°C). Tri-Flex XT has a permeability rating of .04, so it is considered a vapor barrier. Depending on climate zone and insulation type, you may need to ventilate below the exterior plywood roof triangles or use an underlayment with a higher perm rating, like GAF Deck-Armor™.
GCP Ice & Water Shield®
GCP Ice & Water Shield® is a premier self-adhered membrane composed of two waterproofing materials – an aggressive rubberized asphalt adhesive backed by a layer of high density cross laminated polyethylene. The rubberized asphalt surface is backed with a foldless release paper that protects its adhesive quality. During application, the release paper is easily removed, allowing the rubberized asphalt to bond tightly to the roof deck. In addition, embedded in the membrane is a split release on demand feature called Ripcord®. Thermal stability up to 180°F (82°C).
GCP Ice & Water Shield® HT
We use Ice & Water Shield HT as a flashing and waterproofing membrane around our skylights, venting products, cupola roofs, valley flashing and on other portions of the dome roof subject to wind driven rain or low slopes. It is a premier self-adhered membrane designed to deliver premium in-place performance for high temperature applications. For desert and high mountain areas with hot summers, see below. GCP Ice & Water Shield®HT is a premier self-adhered membrane designed to deliver premium in-place performance for high temperature applications. It is composed of two waterproofing materials – an innovative and proprietary rubberized asphalt adhesive combined with a high performance polymeric film with UV barrier properties. Thermal stability up to 260°F (127°C).
GCP Ice & Water Shield® Ultra
GCP Ultra is designed to protect sloped roofs from the effects of wind driven rain and ice dams in applications where the self adhered membrane must withstand high, in-service temperatures for extended periods of time. This membrane offers excellent performance at elevated temperatures, in hot desert southwest climates or any application where superior heat resistance is a requirement. We use Ultra as the flashing around our skylights, venting products, cupola roofs, valley flashing and on other portions of the dome roof in the climate zones mentioned. Thermal stability up to 300°F (149°C).
GCP Bituthene waterproofing membrane (for foundations)
We use Bituthene waterproofing on all our Permanent Wood Foundation Systems. We also use it on some low slope roof areas not subject to heat buildup by the sun. Bituthene is a self-adhesive, cold-applied sheet membrane consisting of a top surface of high strength, 4 mil. cross laminated, high density polyethylene film adhered to 56 mils of rubberized asphalt. Rubberized asphalt promotes full adhesion to the substrate while simultaneously providing crack-bridging and crack-cycling properties. This 60 mil thick membrane is supplied in 36″ wide rolls 66.7′ long inter-wound with a disposable coated paper release sheet. Thermal stability is up to 135°F (57°C).
GAF Deck-Armor™ Premium Breathable Underlayment
It provides a critical extra layer of protection between your roofing shingles and your roof deck — to help prevent wind-driven rain (or water from other sources) from infiltrating under your shingles and causing damage to your roof structure or to the inside of your home. In addition, Deck-Armor™ will:
- Breathe…To allow moisture from inside your roof plywood or unvented, insulated areas below your roof plywood, to escape (to help prevent structural decay, mold, and mildew). If you use spray foam directly under your dome’s exterior plywood you must have a breathable underlayment.
- Provide Longer-Lasting Leak Protection…Than conventional “felts” (which can rot or become brittle before your shingles reach their useful life)
- Lay Flatter…So your shingles will lay flat and uniform. Special slip-resistant surface provides enhanced walkability. Light blue surface is cooler in hot weather.
Select the correct Underlayment for your climate zone:
Where winters are generally mild but will still experience wind driven rain, GCP
Tri-Flex XT® can offer sufficient protection. However; this is a vapor barrier and should not be used if you are applying sprayed-in-place closed cell foam to the underside of the exterior plywood. Instead, use GAF Deck-Armor™.
In mild winter climates, if it is expected that the roof will be subjected to very high temperatures for extended periods of time, then GCP
Ice & Water ShieldUltra should be the product of choice. Southwestern USA is a good example, especially higher altitudes.
In areas of the country that experience snowy winters, especially with wind driven rain and snow, GCP Ice & Water Shield® would provide maximum protection.
In snowy winter climates, if it is expected that the roof will be subjected to high temperatures for extended periods of time, then GCP Ice & Water Shield® HT (high temperature) should be the product of choice.
In an on-going test at our Bear Creek Dome 1 hour north of Minneapolis / St. Paul, Minnesota, temperatures in the 2″ ventilated air space between the exterior roof 3/4″ plywood sheathing (with asphalt shingles) and the 16″ of high density fiberglass insulation, with the outside Summer temperature at 95°F / 35°C, the air space was 165°F / 74°C. This is why we suggest using GCP Ice and Water Shield high temperature underlayment.
If using metal roofing (such as Reinke Aluminum Shakes) you must use GCP Ice & Water Shield Ultra. In high altitudes, or in hot desert areas, use GCP Ultra under all types of roofing. These roofs can reach extremely high temperatures due to low reflectivity, high absorption, and high conductivity of the materials. It is a vapor barrier and should not be used if you are using spray foam insulation.
For more Frequently Asked Questions regarding these GCP products, click here. NOTE: Grace Construction products are now brought to you by GCP Applied Technologies.
Prices Updated 4-1-2023
| Title/Description | Price | Shipping |
| GCP Tri-Flex 15 Underlayment 48″ x 250′ (1000 sf) – 7 mil thick – 28# / roll (240°F / 116°C) |
$110 | Call for shipping |
| GCP Tri-Flex XT Underlayment 48″ x 250′ (1000 sf) – 7 mil thick – 28# / roll |
$150 | Call for shipping |
| GCP Ice and Water Shield Membrane Underlayment 36″ x 67′ (200 sf) – 40 mil thick – 55# / roll (180°F / 82°C) |
$185 | Call for shipping |
| GCP Ice and Water Shield HT Membrane Underlayment 36″ x 67′ (260°F / 127°C) (200 sf) – 40 mil thick – 55# / roll |
$235 | Call for shipping |
| GCP Ice & Water Shield Ultra Membrane Underlayment 34″ x 70′ (300°F / 149°C)(198 sf) -30 mil thick – 42# / roll updated 9/27/23 |
$395 | Call for shipping |
| GCP Bituthene 3000 Membrane (foundation waterproofing only) 36″ x 67′ (200 sf) – 60 mil thick (1.5mm)- 83# / roll (135°F / 57°C) |
$360 | Call for shipping |
| GAF Deck-Armor™ Premium Breathable Roof Underlayment 54″ wide x 223′ long (10 squares) 1371mm x 68M (93 sq. M) – 37#/16.8 KG/roll |
$287 | Call for shipping |