Building Envelope -- Waterproof Design: Roof Design

Ill. 117 Ways to insulate the roof system.

Vented Pitched Roof System Traditional, Vented Ceiling Cavity Air Comes in at the Eaves, Out at the Ridge or Gable Passive Ventilation Reduces Delta T to Occupied Space and Limit Moisture Build up on Framing, Sheathing, etc.

Ventilated Pitched Roof System Mechanical System Ventilation Conditioning System Introduces Supply Air Adequate for Pressurization and Dehumidification. Surplus may be Returned, or Exhausted

* Actual CFM will Vary Depending Upon Volume, etc.

Vent Hot Gases Out at Ridge Ceramic tile for Example, Air Comes in Below Tiles at Eaves Non-Vented Ceiling Cavity Apply Planks to Top of Roof Beams, then AIB and Rigid Insul. Between Vertical Sleepers, then Apply Sheathing and Roofing Membrane, Final Roofing Materials

Even Low Slope Roofs Need Venting Hot Air Escapes from Base Sheet at Wall, Preferably Under Flashing

There are many different ways to insulate a roof, just as there are ways to design a roof system. You can group roof insulation into three main approaches. You can insulate above the roof decking, right below the roof surface, or on the ceiling plane (see ill. 117). The proper choice depends on the form and materials used, the climate, and the method of ventilation. You can classify roof pitch or slope into four main categories: flat, low slope, intermediate slope, and steep.

Flat roofing can be as low as 1/2” per foot (1 percent, or 0.5 degrees) and up to about 1/2 inch per foot (2 degrees). Low-slope roof systems start there and increase up to about 3 inches per foot (12.5 degrees). Intermediate-slope roofing goes up to about 6 inches per foot (22.5 degrees). Steep roof pitches are in excess of 6 inches per foot. Different materials are used in the different sloped systems. It is just as important to prevent condensation being formed in the roof system as it's in the walls, (see Fgr. 118). We can calculate insulation and temperature gradients in the roof like we did in the earlier wall examples. Using perm ratings of materials, we can also calculate dew point and by careful application of our basic principles minimize moisture formed and therefore reduce mold and mildew.

Fgr. 118 Dew point avoided in roof. Shingles Roof Membrane Sheathing Insulation Ceiling G. W. B Paint Wall Insulation

Flat roof design

Flat roof systems are the kind you find on large commercial or industrial projects. Depending on live and dead loads, the flat roof structure may vary. Most commonly, you see economical bar joists on about 5-foot intervals with metal decking or purlins. Single- or multiple-ply roofing systems such as EPDM or modified bitumen are installed on top of insulation. If ratings and loads dictate, the metal deck may have lightweight concrete topping. When possible, perforated G-90 galvanized metal deck is used in combination with lightweight concrete. The top chord of the bar joists and decking may be installed level or sloped to drains. If the deck is level, the concrete may vary in thickness to attain the desired slope or be installed at a uniform thickness. If the concrete is level, then tapered foam insulation is installed over the lightweight concrete. The average R value for the composite whole needs to equal or exceed the value used in the energy calculations. Insulation R values vary with the thickness of the tapered insulation. The minimum thickness typically occurs at roof drains, with maximum thickness at the ridge or crickets. The best condensation protection is provided by using the minimal insulation value instead of the average value that was used in dew point analysis.

Fgr. 119 Roof section at overflow scupper. Line of Parapet Wall (Typical) Line of Cant (Beyond) Slope at 1/4" Per Foot, Min. Roof Membrane Lightweight Concrete Insul. Bds. (Tapered) Roof System

Another common roof system is constructed of concrete. If it's being used on other floors, a post-tensioned (PT) concrete roof system could be used.

Depending on the spans and loads, the PT concrete might be in the range of about 7 to 9 inches thick. Most PT slabs are formed level, but mono-pitched PT can be done by many crews. It is usually easier to set the bottom elevation form level and , if needed, taper the thickness to achieve the slope. This may save concrete and weight. We typically see lightweight concrete applied to the top to slope to roof drains. In the cold climates, we would like to place insulation above the roof membrane. One way to achieve this would be to apply a bi-level drain system. Install one set of drains at the PT slab elevation, and apply a semi permeable vapor retarder at that level (say, about 0.8 perm). Then apply the lightweight concrete and tapered foam insulation to get the R values required and 1/4-inch slope to drain (see Fgr. 119). The rooftop membrane would be applied above the tapered foam, with a perm rating of about 0.03. This would be in keeping with our paradigm of increasing by a factor of 4 from the lowest permeability membrane to the next. Membranes can be screwed down, mopped down, torched, ballasted, or some combination. Four- or three-ply hot-mopped modified bitumen roofing is the standard for areas with high annual rainfall.

The advantages of multiple-ply roof membranes are in their strength and redundancy. The first ply is often the dry-in layer on which other subcontractors work. It often gets damaged. Subsequent layers get lapped on each other to reinforce the corners as well as to resist abuse. Cap sheets are what really stand up to UV light. Modified bitumen roofing product lines are expanding to meet special demands. These products are now available in several finishes and colors, including white and aluminum finishes. Single ply thermoplastic roof membrane systems are increasing in use because of their excellent heat rejection properties and low initial cost. Other single-ply roof membranes are avail able, such as EPDM. Each has pros and cons, and should be selected first and most importantly upon the flame spread rating required. Some products are available to withstand more than 125 mph winds, and exceed FM600 rating.

The difference between class A and B roofing is the difference in flame spread ratings. One component used to achieve the better, Class A rating is in the rein forcing fabric, but there are other differences as well, such as underlayments, decking, and fastening methodologies. Class A system membranes using woven polyester may be tougher but may be difficult to get to lay flat than a more pliable Class B or C system if there are a lot of bends and corners, especially at drains. Class B is easier to bend and can be applied over sumps at drains with less likelihood of fish mouths. Fish-mouths are opening caused by puckering material when everything is not flat and planar. They can permit water intrusion and should be avoided. Single- and two-ply roofing is specified where initial cost is more of an issue than long-term performance. Both put a premium on protection of installed roofing membranes. If the design is such that other trades are kept to a minimum after the base sheet is applied, or if the construction manager (CM) can protect the membrane successfully, these lower-cost systems can be acceptable. Most roofing systems can be war ranted for more than 10 years.

Many building codes still permit coal tar roofing to be installed at 1/8 inch per foot. We believe that the reason most codes don't still allow 1/8 inch per foot for any other membrane is that ponding water is more likely to occur if the design value is 1/2 inch. When plies get lapped, such as at roof drains, the additional plies make the roof membrane thicker. This extra thickness can result in a relative high point in the flow path of water, effectively trapping water. The best way to prevent water intrusion from a roof is to get the water off the roof as fast as you can-all of it. The higher the pitch, the faster is the flow, and the fewer dams will occur. When designing a roof, we try to show 1/2 inch per foot in the area surrounding roof drains for 4 feet in every direction (see Fgr. 120). In this way, lapped plies usually result in the desired 1/4 inch per foot taper.

Low-slope roofing

Low-slope roofing often will be done in many of the same materials as flat roofing. Roll goods often are mopped or torched down on sheathing as a membrane to stop moisture and with another material for protection against the sun.

Starting at about 1½” per foot, metal roofing is a common product line.

Metal roof systems can be installed over sheathing but typically get clipped onto purlins. These roofing products include standing-seam metal products with varying profiles depending on spans and loads. These products are available in a variety of colors, materials, and coatings. The lowest-cost applications are painted steel. Upgrades include galvanized steel, aluminum, stainless steel, and copper. Insulation most commonly is rolled out on top of the purlins in long rolls before the metal goes on. Standing-seam metal roofing is popular because the fasteners are concealed, and it goes on quickly. Each successive panel gets crimped over the preceding one with a special tool that captures the clips between two pieces of bent metal. The clips need to be spaced close enough to prevent rippling, as well as to resist uplift. The clips get fastened to the purlins.

Roll insulation comes with plastic inside facing referred to as scrim. The scrim can be painted (see Fgr. 121) or left unpainted. It can be ordered with a rein forcing mesh that acts as safety netting for installers working far above the ground. The mesh-reinforced scrim also reduces deflection of the insulation between the purlins over time. We have used metal roofing products over exposed metal decking and plywood sheathing to give a nice finished aesthetic to the underside in a fast, cost-effective manner. Metal roofing companies pro vide a series of closure pieces for ridge and exterior wall conditions, as well as fascia and soffit systems, louvers, integral windows, skylights, etc. One additional advantage to this is sole-source responsibility for all these installations. Another is compatibility of colors, finishes, etc.

Fgr. 121 Painted scrim-faced insulation.

Different shapes of metal roof products are available. Some replicate barrel tile, others, slate. One of our personal favorites is the rustic look and long-lasting performance of galvanized V-crimp metal. This is applied in a variety of ways depending on the space below. We like to use it above battens on 90-pound granulated roll roofing that has been hot-mopped to 15/32-inch four-ply exterior plywood. An alternative is to dry it in with 30-pound fiberglass membrane and add SAM strips under the battens. One by four battens are installed over the SAM at 10 to 14 inches on center with wood screws through the plywood right into joists or trusses. These battens will have drainage channels or notches cut into them before installation. The metal is finally screwed into the sheathing and through the battens into the joists at 12-inch on-center (OC) spacing horizontally. Batten and fastener sizes and spacing must be designed for wind loads, which is especially critical on corners and at end zones. These fasteners face the greatest loads in resistance to hurricane-force winds. Accessory pieces close off the sides to prevent wind from getting behind or under the panels at the drip edge and along the ridge. We typically specify stainless steel wood screws with neoprene-gasketed washers to better seal against the weather. In hot climates, the insulation is below the sheathing. In cold climates, you might use rigid foam above the membrane with the battens on top of the insulation.

Longer fasteners (approximately 4 inches) would be required to fasten the metal through the battens and , say, 2 inches of foam and sheathing into the joists. In some jurisdictions, the V-crimp can be fastened directly over the granulated roof membrane and does not require stripping.

Intermediate-slope roofing

Starting at about 3-in-12 pitch, you start to see shingle roofing and lapped membranes. This is the minimal slope recommended for most shingle applications. For best performance, SAMs are used for shingle underlayment with their self-sealing aspect again as one of its best features. Each shingle is to receive at least six nails per tab, which could mean several square inches of opening per 100 square feet of roof (referred to as squares). A simple home may have 20 squares of roof shingles.

Since the membrane is functioning as the air and vapor barrier in most cases, you need it to be as sealed as possible. In using the SAM, you lose the ability for moisture to dry by passing through the membrane, as it could through traditional roofing felts. This moisture needs to be dealt with in another way.

There are three common ways to construct the intermediate-slope roof system.

Each needs to meet (or exceed) the building codes for insulation and ventilation. The traditional way is to have eave and ridge vents with a ventilated attic.

Variations on this theme include turbine ventilators, gable vents, etc. In each, ambient outside air is brought into the attic in an attempt to keep the sheathing and framing from moisture buildup and rot. Insulation typically is placed above the ceiling (see Fgr. 122). In cold climates in the winter, vapor drive could cause condensation on the warm side of the ceiling if insulating values and perm ratings of materials weren't selected correctly and /or the materials weren't installed correctly. This situation is affected by operating temperatures in the space and ambient conditions, as well as the volume of outside air admitted per minute. In hot, humid climates, condensation risk is in the attic.

An alternative means of design is the sealed-off, actively ventilated attic. In this solution, the soffit, fascia, and exterior walls are not open to the attic.

Conditioned supply air is introduced into the attic. Depending on envelope tightness and mechanical design, this supply air may positively pressurize the attic space relative to the outside (see Fgr. 123). You would like it to be slightly negative to the occupied spaces. In most designs, return air is ducted back to the air handler. If this space is fed by a dedicated air-conditioning unit, you may be able to use temperature and humidity sensors to control when the system introduces conditioned air. In humid climates, it may be sufficient to have a dehumidifier run 24/7 and just use a temperature sensor to control an operable damper to sup ply the attic only if temperatures become problematic relative to the dew point.

Insulation is placed more effectively at the roof rafters than at the ceiling, but calculations for both summer and winter may reveal that insulating both ceiling and roof planes would avoid condensation in all seasons, even at record-high and low temperatures.

Fgr. 122 Insulation above the ceiling.

Fgr. 123 Ventilated pitched roof system.

The third scenario is where foam insulation is sprayed to the underside of the roof sheathing. This foam skins over and acts like a vapor retarder and air barrier.

We have heard that some manufacturers are not warranting their asphalt-based roofing products if used in these applications. Always confirm that manufacturers will warrant the application around which you design. Temperatures can build up at the roof in excess of the operating limits for some roofing products. We would imagine that there should be no problem with a high-temperature-tolerant membrane and ceramic, concrete, or metal roofing products. However, we could imaging temperatures of metal flashings, copings, and valley metal reaching in excess of 175ºF. Whenever specifying materials, be sure to read the manufacturer's warranty exclusions and recommendations for operating conditions. This may apply to high-salt or high-chemical content in rainfall resulting in a voided warranty as well.

Steep-pitched roofing

The differences between intermediate- and steep-pitched roofs are many. It is much easier to work on a roof that's less than 6 inches per foot (6-in-12, or 22.5 degrees). Safety precautions increase and productivity decreases as you get steeper. The good news is that snow and rain don't stay on a steep roof very easily. The steeper you get, the more surface area is exposed to the elements.

Therefore, air-barrier leaks get multiplied times a greater surface area to calculate infiltration or exfiltration. The same three kinds of insulation schemes explained earlier are permitted in most jurisdictions. If none of these fit your architectural aesthetic, there are always other ways to solve the problem. A cathedral ceiling with large exposed beams and plank ceilings can be designed to meet any climatic constraints. In the section provided in Fgr. 124, you will find a solution that was used in Florida's hot, humid climate. In this example, 6- by 16-inch glulam beams were used at 80 inches OC. Then 2- by 6-inch tongue-and-groove no. 3 southern yellow pine (SYP) planks were nailed to the top using 20d spikes.

Thirty-pound fiberglass roofing membrane was installed next. Rigid extruded polystyrene (EPS) foam insulation 3 inches thick and 22 1/2 inches wide was installed at 24 inches OC on top of the dried-in planking with 2- by 4-inch nailers (3 1/2 inches in vertical axis) at 24 inches OC. That 1/2-inch airspace between the top of the foam and the bottom of the plywood was used as the ventilating airspace. Air was introduced at the bottom under the fascia and expelled at the top under the cap flashing. Next, sheets of 5/8-inch CDX plywood was nailed to the 2 by 4s on end. Ninety-pound hot-mopped roll roofing was applied next. This served as the air and vapor barrier. Strips of rolled roofing were applied next, where the purlins were going to be at about 10 inches OC. This dimension is critical and should be matched to the width of the pans and cap tiles. Next, 1¼- by 3-inch purlins were toe-nailed through the roofing into the plywood. The strips were used to help seal the nail holes. Finally, the pans were set in place by using a compatible adhesive where the tile sat on the roof. Caps were nailed and mortared into place, trim tiles installed, etc. This resulted in a well-insulated, long-lasting roof system that looked good on top and bottom.

Roof penetrations and roof-to-wall conditions

If it were as simple as installing membranes on a rectangular, fully completed substrate, roofing would be easy. If it were as simple as unrolling a SAM from bottom to top and walking away, anybody could do it. In the real world, roofing is never this easy. Even the simplest shed roof has edges that need drip metal.

However, most roofing jobs are not simple. Roof-to-wall conditions, parapets, roof drains, plumbing vent stacks, and a number of other things above the roof make it challenging to complete a roof installation. Rooftop mechanical equipment, exhaust ducts, chimneys, skylights, lightning protection, expansion joints, and other objects penetrate roof sheathings, decks, and membranes. The manner in which roof penetrations are protected has a lot to do with the pitch and products being used, as well as the wind speed anticipated. It is safe to say that most roof leaks happen at penetrations and vertical projections meeting the roof.

Fgr. 124 Steep roof section.

Fgr. 125 Plumbing vent stack.

Fgr. 126 Minimum 8" height.

Fgr. 127 Pedestrian topping.

Steep- and intermediate-pitch roof systems typically don't have membranes that are sealed at their seams. Proper lapping of the membranes works with gravity to keep most of wind-driven rain from going uphill far enough to get behind the membrane. This is all that prevents rain from wetting the sheathing. Vertical surfaces such as fireplace chimneys on a sloped roof are installed with crickets that divert the water to the sides instead of letting the force of water flowing down the roof, striking the base flashing of the chimney at the level of the roof plane.

Crickets are used wherever the vertical face of an intersection surface extends more than 8 inches past the ridge of the cricket. This leaves room for the vertical leg of flashing to extend up the wall behind the wall membrane.

Plumbing vent stacks are sealed by applying bent flashing (referred to as a boot) over the top of the pipe (see Fgr. 125) and under the shingles at the high side. At the low side of the boot, the shingles are installed under the boot.

Low-slope roof penetrations are dealt with in a similar fashion. As long as membranes are lapped from the bottom up and flashings are applied to keep water from getting behind joints, penetrations can be sealed with few problems. Parapet walls should be avoided on all but flat roof systems.

Flat roof penetrations are potentially easier to seal than on sloped roof systems.

Flat roof systems typically are sealed at the seams and laps to act as a single sheet of membrane material. Penetrations need to be sealed to the roof membrane. This is accomplished in one of two common ways. The preferred method has been with membrane; the other is with liquid-applied sealing products such as asphalt or tar. Most roofing manufacturers offer a wide range of products, some of which are designed for sidewall applications. These can be applied in the same manner as the base and cap sheets. We recommend mop-down and torch-down membranes. For skylights and roof access hatches, the minimum sidewall height above the roof is 8 to 12 inches (see Fgr. 126). In every case, the top of the sidewall membrane should be higher than the overflow scuppers by at least 4 inches.

Flat roofing also can serve as activity areas, pool decks, or patios. These kinds of applications require different roof coatings and membranes for their protection. For patios, you often see liquid-seal coatings applied directly to the concrete structural slabs. Hot-liquid-applied asphalts and modified bitumens make good water-protective coatings but aren't attractive or particularly nice to walk on with bare feet. There are special topping products for pedestrian use that can be warranted for up to 20 years. They also serve to seal the floor-to wall condition (see Fgr. 127). Other areas may need a concrete paver system for poolside lounging. Pavers typically are installed on pedestals or on sand beds.

In both instances, the structural deck may require waterproof coatings to prevent incidental water from causing problems with slab-reinforcing steel or damage to spaces below. Liquid or sheet membranes typically will be installed on the structural slab, followed by tapered lightweight concrete sloped to drains. Then a coarse sand or soil cement bed and pavers with sand or screenings as fillers between the brick or cement pavers (see Fgr. 128) is applied. Similar details are provided for amenity deck planter beds to prevent stains or water damage from soil and rainfall adjacent to million-dollar condominium units (see Fgr. 129).

Fgr. 128 Pavers on sand fill. Structural Slab Hot Liquid Applied Membrane on Slab and Up Wall Lightweight Concrete Coarse Fill Mat'L Pavers Vibrated in Place

Fgr. 129 Planter detail at exterior wall. Hot Liquid Applied Membrane Stucco on Lath Air and Vapor Barrier Drain Board Rigid Insulation Exterior Wall