ASHRAE Journal - February 2011 - 51

Figure 2 (left):	Pressure	Differences.	Notice	the	large	pressures	 at	 roof	 perimeters.	 From	 Leutheusser2	 and	 the	 Department	 of	 Mechanical	 Engineering,	 University	 of	 Toronto,	 1964.	 Some	of	us	recall	that	1964	was	 a	good	year.	The	Toronto	Maple	 Leafs	won	the	Stanley	Cup.	Tim	 Horton	played	defense	on	that	 team	long	before	he	got	into	the	 donut	business.	Figure 3 (right):	 Effect	 of	 Parapets	 on	 Pressure	 Differences.	Parapets	dramatically	reduce	these	pressure	differences	 at	 roof	 edges.	 This	 is	 from	a	beautiful	bit	of	work	from	 Leutheusser	 at	 my	 alma	 mater	 the	 University	 of	 Toronto.	 Of	 course,	this	happened	in	1964,	 more	than	a	decade	before	I	got	 there.	I	am	old,	but	not	that	old.

Figure 4:	Parapet	Water	Management.	Keep	rainwater	from	getting	 into	the	top	of	them.	Slope	the	top	of	them	inward	so	they	don’t	stain	 the	 building	 façade.	 Make	 sure	 there	 is	 a	 waterproof	 membrane	 under	the	coping.	And	always	have	drip	edges—front	and	back—so	 they	don’t	stain	the	building	façade. stress of the membrane through sometimes multiple layers of insulation before it got to the structural deck (Figure 9). If you didn’t get it right, you concentrated the stresses at roof edges, and you could suck in a parapet (Figure 10) or tear or rip a membrane at the parapet (Photos 3 and 4).

Photo 1 (left):	 Excellent	 Water	 Management.	 Notice	 the	 membrane	 under	 the	 coping.	 Notice	 the	 slope	 to	 the	interior.	And,	notice	the	drip	edges.	Photo 2 (right):	 Stains	at	Parapet.	No	drip	edges.	A	beautiful	building	 is	becoming	ugly.

Transferring loads in multilayer compact roofs is quite controversial. There are lots of opinions, and I want to point out right from the start that only I am right. Let’s start out in the field of the roof. This is how a compact roof would be constructed if I was in charge (Figure 11). There should be a con-

*	The	Italians	have	claim	to	the	word	“parapetto,”	which	comes	from	“parare,”	which	means	“to	defend,”	and	“petto,”	which	means	“breast.”	The	military	calls	 “parapet	fortifications”—defensive	stonewalls—“breastwork.”	The	dictionary	meaning	of	“breasted”	means	“to	confront	boldly.”	So,	low	stonewalls	historically	 are	called	parapets	and	are	military	in	origin.	“Stonewall	Jackson”	was	also	called	“Parapet	Jackson.”	OK,	so	that’s	not	true,	but	with	the	way	textbooks	seem	 to	be	written	today	I	bet	I	could	get	away	with	it	if	I	decided	to	write	one.	So	how	did	they	come	to	be	located	on	the	edge	of	roofs?	Ah,	we	can	thank	the	 English	for	that.	In	the	old	days,	London	tended	to	keep	burning	down	and	that	tended	to	irritate	the	folks	who	lived	in	London.	So,	projecting	wooden	eaves	 were	banned	in	the	Building	Act	of	1707	as	a	fire	risk.	Instead	an	18	in.	brick	or	stone	parapet	was	required,	with	the	roof	set	behind,	as	fire	protection	(http:// en.wikipedia.org/wiki/Parapet).
†	Check	out	“The	Perfect	Wall,”	ASHRAE	Journal,	May	2007.	Or	go	to:	http://tinyurl.com/2upg3fv.	It	was	my	first	column	for	ASHRAE	and	it	was	inspired	by	Max	 Baker’s	marvelous	book	Roofs.1	The	book,	now	out	of	print,	was	sponsored	by	the	National	Research	Council	of	Canada	and	it	brought	together	information	into	 one	document	from	Canadian	Building	Science	Digests,	Research	Papers	and	Building	Science	Seminars	and	Workshops	from	the	Division	of	Building	Research.	 Much	of	this	information	is	online	(http://tinyurl.com/2eezyth).	Awesome.

February 2011

ASHRAE Journal

51
ASHRAE Journal - February 2011

Table of Contents for the Digital Edition of ASHRAE Journal - February 2011
