IWA Publishing

In Oslo, the snow-covered periods (SCPs) of the year will typically not coincide with the previously defined seasons. Hence, to address the functionality during SCPs solely, it is necessary to evaluate the characteristics of the winters and the roof performances with respect to water retention during periods when snow was observed on the roof. The experimental site had durations of SCPs approximately 5 months per year, usually ending in April. Snow could melt several times through the winter season resulting in a runoff. The first five SCPs were colder than the six that followed (average −3.3 and −0.5 °C, respectively; Table 1). Through the eleven SCPs, the average temperature increased by 0.46 °C annually (r² = 0.38 and p = 0.02). Temperature sensors below the roofs followed the average daily air temperatures.

Table 1

Days between first snowfall to the last sign observations of snow, including maximum snow depth and air temperature. Precipitation and runoff in real and relative numbers on each of the three roofs

SCP	09/10	10/11	11/12	12/13	13/14	14/15	15/16	16/17	17/18^a	18/19	19/20
Observations
Days with snow	119	113	121	172	104	168	136	190	166	192	154
Precipitation (mm)	142	181	206	338	366	350	286	325	340	417	349
Runoff roof 2	143	210	218	415	456	312	251	271	209	327	368
Air average (^oC)	–5,8	−5,8	−0,7	−3,5	−0,6	0,7	−0,8	0,1	−2,2	−1	0
Air min. (^oC)	−20,3	−17,5	−14,7	−15,4	−12,3	−13,3	−14,4	−12,3	−13,1	−12,3	−6,9
Air max. (^oC)	5,4	5,7	11,3	6,7	5,3	14,4	7,9	15,1	8,8	8,1	4,3
Max snow depth
REF (cm)	48	30	54	41	37	–	–	–	–	–	–
GR1	52	30	64	50	38	38	29	28	80	46	19
GR2	–	–	–	–	–	30	20	25	75	39	14
GR3	42	30	49	40	27	28	23	24	74	34	18
Runoff in rel. nu.
Precipitation	99	86	95	82	80	112	114	120	163	128	95
REF	100	100	100	100	100	–	–	–	–	–	–
GR1	98	72	83	75	92	110	107	104	106	119	106
GR2	–	–	–	–	–	100	100	100	100	100	100
GR3	95	66	81	79	83	107	111	114	111	118	108

SCP	09/10	10/11	11/12	12/13	13/14	14/15	15/16	16/17	17/18^a	18/19	19/20
Observations
Days with snow	119	113	121	172	104	168	136	190	166	192	154
Precipitation (mm)	142	181	206	338	366	350	286	325	340	417	349
Runoff roof 2	143	210	218	415	456	312	251	271	209	327	368
Air average (^oC)	–5,8	−5,8	−0,7	−3,5	−0,6	0,7	−0,8	0,1	−2,2	−1	0
Air min. (^oC)	−20,3	−17,5	−14,7	−15,4	−12,3	−13,3	−14,4	−12,3	−13,1	−12,3	−6,9
Air max. (^oC)	5,4	5,7	11,3	6,7	5,3	14,4	7,9	15,1	8,8	8,1	4,3
Max snow depth
REF (cm)	48	30	54	41	37	–	–	–	–	–	–
GR1	52	30	64	50	38	38	29	28	80	46	19
GR2	–	–	–	–	–	30	20	25	75	39	14
GR3	42	30	49	40	27	28	23	24	74	34	18
Runoff in rel. nu.
Precipitation	99	86	95	82	80	112	114	120	163	128	95
REF	100	100	100	100	100	–	–	–	–	–	–
GR1	98	72	83	75	92	110	107	104	106	119	106
GR2	–	–	–	–	–	100	100	100	100	100	100
GR3	95	66	81	79	83	107	111	114	111	118	108

Note: Precipitation and runoff from roof 2. Roof 2 had no vegetation (REF) during the first 5 years. Relative runoff compared to runoff from roof 2 (e.g., runoff in SCP 14/15 was 312 mm from GR2 and is set to 100). Runoff was 10 and 7% higher from GR1 and 3, respectively. Precipitation was 350 mm, which is 12% more than runoff for GR2.

^aHalf of the snow height had to be removed because the snow load could harm the roof.

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