From the Deck Up: Accessories
What size expansion joint closure system should I use?
Bellows-type expansion joint closure systems should be a minimum of one-and-one-half times the joint opening or one-and-one-half times the spans of the flanges for roof-to-roof. For roof-to-wall conditions with the same performance requirements, the bellows can be two times the span or larger.
Metal closure systems also require sizing to perform up to expectations. This involves the overall cover size to extend to one or both sides of the joint to remain engaged with the rest of the system. Under all circumstances, for both types, the anticipated building movement also must be taken into consideration, such as allowing for extremes in temperature, wind or seismic movement.
How much shear can an expansion joint handle?
The two common considerations for shear are “roof-to-roof” and “roof-to-wall.” When a
roof-to-roof joint with a bellows is closed, it can handle the bellows size of shear. The
roof-to-wall is more complex. When the joint is closed, the distance between the flanges is still that of the roof-mounted flange and the wall-mounted flange, which greatly reduces its ability to handle shear. This translates into the need for a larger product.
For metal closures, there should be clearances or slip zones to handle the relative extension and compression that shear may develop between vertical “end” conditions. Remember that whenever a joint changes direction, one joint will be in either expansion or contraction and the joint at right angles to it is in shear or a combination of shear expansion and contraction at the same time.
How is an expansion joint terminated?
True structural expansion joint closure systems do not terminate, except in rare conditions, anywhere other than at the foundation of a building. They will transition to another condition. Transitional issues surrounding expansion joint closure systems challenge almost every aspect of design, specification and construction. These intersections must be carefully planned to be both functional and aesthetic. Bellows closures must never be flattened to provide a low profile, as their ability to adjust to movement lies in the flexibility inherent in the curve of the bellows. The expansion joint must be allowed to continue over the edge of the roof (and down the side-wall expansion joint) or transitioned properly into a wall expansion joint. Metal joints also need to adhere to these principles. Never terminate them into a solid wall using caulking as it will pull apart from the anticipated movement and fail prematurely. If these important details are not anticipated and addressed in the earliest phases of the project, they can turn into a coordination nightmare on site, or worse yet, fail when performance is required.
Does outdoor temperature play a role in the size of expansion joint cover I should order?
Temperature is a very important consideration. In very cold weather, expansion joint openings will be larger than normal, whereas in the summer, they are likely to be much smaller. These differences should be taken into account when measuring and ordering materials. The size of the bellows required is based on nominal conditions together with movement.
What is the fire rating of expansion joint closure systems?
All types of closure systems achieve a fire rating with the use of an additional secondary product assembly that is inserted to the opening under the primary weather resistant “exterior” system. These secondary systems consist of many different high-temperature materials to provide the fire barrier and ratings typically from one to four hours. It is very important to coordinate these systems with the rest of the building constructions.
What is the most important consideration for expansion joint performance?
One of the keys to all roof-related expansion joint closure system transitions is the continuity of seal through the full range of expected building movements. Closure systems must be capable of being turned in various directions or even looped around building features. The closure system must be capable of transition to the edge of the waterproofing system/membrane with the same movement characteristics as the rest of the roof joint. Under no circumstances should the closure system be allowed to flatten out at the roof edge or wall, as this will create a shear failure.
What should be considered when designing the application and selection of an expansion joint closure system?
- Movement in/out, up/down, back/forth and rotational.
- Desired percentage of survivability in severe events.
- Joint opening size.
- Curb or parapet mounting width/size.
- Wall clearances/obstructions (for roof-to-wall styles).
- Fire rating(s) if needed.
What are some of the most frequently encountered problems?
One frequent problem is not using a structural joint covered by an expansion joint when the roof dimensions exceed 200 feet. The expansion joint provides relief from membrane failure due to excessive roof movement over long runs. Another frequent problem is utilizing a metal expansion joint to accommodate transitions with three or more legs, such as Ts and Xs. Joints at right angles are moving in/out in one direction and shear in another. Any metal closure that is not symmetrical makes three-way closure difficult, as there is no good way to seal the odd leg to the two that are connected. Most bellow closure systems can mix sizes and configurations to achieve a proper seal. Finally, radiuses, circular or arched conditions deserve special attention because very few situations permit the simple act of bending any closure system around an arc. Consult the supplier for a custom-made closure.
How can splice failures be patched?
Never patch a bellows tab failure with industrial roof cement or any asphaltic-based mastic system. Instead, it always should be patched and sealed with appropriate manufacturer- approved materials. For metal closures, manufacturers’ recommendations also should be sought.
Which materials make the best bellows?
Neoprene has been used the longest, has a great track record and exhibits excellent aging characteristics. It handles problems with severe exposure to hydrocarbons, oils, fats and certain industrial environments better than EPDM. However, EPDM weathers well and has good ultra-violet resistance. PVCs also are available in a variety of styles.
If the curb(s) are wide and the joint is small, what can be done?
There are several alternatives. One is to use the traditional curb-form style product with a larger bellow to span the distance. Another is to use a product with a proportionate bellows size held in place with a nail strip that is covered with a flexible membrane flashing flap to extend over and down the curb.
Why do you need to use rated roof -edge treatments (fascia, coping, gravelstops and gutters)?
The survival capability of these items should be as good or greater than the roof system in order to eliminate the weak link. Remember that corners and edges of a roof system require additional fastening for the higher forces there. Be sure to use an appropriate rated termination metal assembly at these locations.
How important is the size, quantity and style of roof drains?
Roof drain sizing, quantity and location are critical to prevent too much water from accumulating on a roof without proper drainage. Careful consideration should be given to maximum rainfall, area drained, overall plumbing capacity and secondary or overflow options. The style of drain also may need to be addressed for membrane type (weldable, clamping or strip in).
Are there any special considerations for installing drains on a reroof?
When using drain inserts on a reroof, always be sure to inspect the condition of the existing pipes and their ability to accept and seal to the new inserted drain body and sealing system. In addition, a drainage calculation must be run as the inserts always restrict the flow capability of a drain due to reduced diameter. The result may dictate additional drains or other means of discharge such as scuppers.
Accessories are an integral part of ensuring a successful roofing system. It is critical that full consideration is given to how they are used and to how they are integrated into the overall project.