Gapping Recommendations for SECUROCK High Performance Roof Boards

Throughout our experience with roofing, many of us have seen movement on roof decks as materials interact with the environment during and after installation. The two main environmental factors are changes in temperature and moisture content. It is common practice to install control joints in the building industry. We see this in sidewalks, walls, floors and so on, allowing for the expansion and contraction naturally occurring in materials. Left unrestrained these materials do not crack or buckle. When panels are butted tightly over large areas and are allowed less than necessary accommodations for expansion; buckling, ridging or crushing is the end result. It is typical to recommend allowances for movement as all building materials are subject to dimensional changes in temperature and moisture.

When looking at physical properties of products there is usually data with the following verbiage like “Linear Variation with change in moisture, “Coefficient of Thermal Expansion,” or “Linear Variation with change in Temperature.” This is basically saying that all of these building materials (including SECUROCK High Performance Roof Boards) are affected by changes in temperature and moisture content. If a product doesn’t explicitly say what its data is, it is a good idea to find out so you can understand how moisture and temperature will effect installation.

It is clear that the environment affects building materials. In order to plan for movement, when installing SECUROCK High Performance Roof Boards, both thermal and hygrometric consequence should be taken into consideration. We would like to help out the community with a quick discussion of how to use these values. As always, design of proper expansion relief is the responsibility of the design professional; please also refer to organizations like NRCA for their guidelines with respect to treating gapping.

The following is an illustration of how to use expansion coefficients for hygral and thermal effects.

Securock High Performance Roof Boards have the following expansion coefficients for hygral and thermal effects:

	Securock Gypsum-Fiber Roof Board	Securock Glass Mat Roof Board
Thermal	8E-06 in/in ºF	8.5 E-06 in/in ºF
Hygral	8E-06 in/in %RH	6.3E-06 in/in %RH

 

 

 

The following tables show gapping allowance for runs with 4’x4’ and 4’x8’ panels for given additive temperature and humidity changes expected; values have been extracted and placed in the table for example. The numbers have been calculated to stay within NRCA guidelines where ¼” is the maximum allowable gap before insulation should be filled (see NRCA for further reference). In this table, ΔT can be defined as initial installation temperature and maximum expected rooftop temperature during install or directly following. The overall rise would be used. Similarly, the overall expected rise in % RH would also be used.

Gap needed per board using 4’x8’ Securock Gypsum-Fiber Roof Boards:

	ΔRH = 0%	ΔRH = 10%	ΔRH = 20%
ΔT = 25°F	.02”	.03”	.04”
ΔT = 50°F	.05”	.06”	.08”
ΔT = 75°F	.08”	.13”	.13”

 

Gap needed per board using 4’x4’ Securock Gypsum-Fiber Roof Boards:

	ΔRH = 0%	ΔRH = 10%	ΔRH = 20%
ΔT = 25°F	.01”	.02”	.02”
ΔT = 50°F	.03”	.03”	.04”
ΔT = 75°F	.04”	.07”	.07”

 

For example:

Given the table above, panels are kept in the shade at ground level and are noted to be 75ºF, then see temperatures at lunchtime on a hot day during installation with a black roof of 150 ºF, your ΔT = 75ºF. If no humidity increase is expected, your ΔRH = 0%. Looking at the table you would gap every 4’x8’ panel by .08” or every 4’x4’ panel by .04”.

So our recommendation is to butt board edges and ends loosely in typical installations and for long, uninterrupted runs, there should be some slight gapping due to changes in temperature and moisture. More tips to help ridging on roofs include installing dry materials into a dry roof, storing boards on the rooftop to avoid thermal swing, and always consulting a design professional and use industry guidelines before installation.