Technical Bulletin #1 : Evaluating roofs for solarsolarpermits.ca/bulletin1-web.pdf · 2012-09-12 · Will the solar panels stay attached to the roof based on the local design provisions

The Solar Permits Initiative has been developed by the Toronto Atmospheric Fund (TAF) to share knowledge gained through the largest single-city solar hot water project in Canada — Solar Neighbourhoods.

Solar Permits is made possible by an investment by the Government of Ontario and the Government of Canada.

Technical Bulletin #1 : Evaluating roofs for solar

IntroductionFor the majority of residential solar energy systems — whether it is photovoltaics (PV) for producing electricity or solar hot water systems used for pool heating, hot water heating or space heating — the solar panels1 that collect the sun’s energy are usually installed on the home’s roof.

As such, where the solar panels are supported by and connected to a building they are normally covered under the jurisdiction’s building code, which means there is a need to carry out a review to ensure conformance to the structural requirements as outlined in the building code.

Through the work done in the City of Toronto’s pilot solar hot water deployment program — the Toronto Solar Neighbourhoods Initiative — Toronto has developed a simple methodology to evaluate rafter roof conditions and to confirm that the solar installation is in compliance with the building code (the Ontario Building Code in the case of Toronto). This bulletin outlines the solutions that have been developed by the City of Toronto.

Roof Considerations for the Installation of Solar

There are two concerns that must be addressed when considering the structural capacity of roofs for the installation of solar panels:

1. Can the roof structurally support the solar panels?

2. Will the solar panels stay attached to the roof based on the local design provisions outlined in the building code?

The second issue can be satisfactorily dealt with through the use of a P.Eng-stamped structural drawing of the system’s mounting system showing the details of the connection to the roof. Good engineering design will include:

a. Wind loading — will the solar panels blow off the roof due to winds?

b. Freeze-thaw cycling — will the panel’s mount be loosened by repeated cycles of thermal expansion and contraction?

c. Weather proofing — will the roof continue to provide protection from water penetration?

The issue of identifying the roof’s capacity to hold solar panels, however, is more complex and relates to both the roof conditions and the mounting method proposed.

1 PV panels are commonly referred to as “modules” while solar thermal panels are referred to as “collectors.” As this bulletin applies to both solar technologies it uses the term “solar panels.”

May 2010


2 TORONTO ATMOSPHERIC FUND - SolarPermits.ca

May 2010

In Canada there are two primary roof construction methods:

• Rafters (sloped roofs) or joists (flat roofs)

• Trusses (sloped and flat roofs)

Figure 1: Roof Construction Details

Rafter roof Truss roof

While rafter (and joist) requirements are defined in the building code (i.e. their size, spacing and maximum allowable span), trusses are a product that is engineered (i.e. they are P.Eng stamped for use in a specific application). Thus a solution for rafters can be standardized whereas a solution for trusses is unique to the specific truss design.

In evaluating solar for roofs there are three issues:

1. The weight of the solar panels (the dead weight or distributed load).

2. The height of the solar panels above the roof (the active load or the uplifting load due to wind).

3. The connection points to the roof (the point-load conditions).

1. Solar Panel WeightsAs shown in Table 1, the weight of solar panels varies between technologies and, for hot water collectors, between products. In most cases solar panels are below the weight of two layers of asphalt shingles (roofs under the Ontario Building Code are designed to accommodate up to three layers of roofing shingles).

Table 1: Solar Panel Weights on Roofs

System or Panel Size (m2)

Full Load (kg) Distributed Load (psf)1

Seasonal SDHW System (typical) * 3.5 270 15.7Solar Hot Water Collectors (as measured in the Toronto Solar Neighbourhoods Initiative)*

4.6 – 5.9 94 - 140 3.3 – 5.3

Photovoltaic Solar Modules (typical) 1.2 each 14 2.4Solar Pool Heating Collectors (typical) * 3.8 each 17 0.9Asphalt Shingles - 2 layers of shingles 3.8Standard Asphalt Shingles - 190 lbs/100 ft 1.9Weight of average Canadian male (codified structural live load for a worker)

83 20.8

* Note – weight of solar collectors when filled with fluid

May 2010 Technical Bulletin #1 : Evaluating roofs for solar

TORONTO ATMOSPHERIC FUND - SolarPermits.ca 3

May 2010

2. Height above the Roof

To maximize performance solar panels need to be directed at the sun. This is done for both the direction (south) and angle or tilt (from the horizon). The optimal angle for performance is typically equal to the latitude or slightly less (due to the higher percentage of solar energy received in the summer months).

For solar water collectors there is the additional requirement of a minimum slope to allow proper circulation of the heat transfer fluid — this is usually 10-15o minimum. Where the collector’s heat transfer fluid needs to be drained when not in operation (for freeze or overheating protection), this is usually increased to 20o minimum.

When considering solar panel heights above the roof there are three possible conditions:

a. Flush to roof — typically this is on a sloped roof but can, for PV modules, also include flat roofs;

b. Angled mount on sloped roof — slightly angled above the roof line to increase tilt of the panels on a low tilt roof;

c. Angled above the roof on flat roofs.

Figure 2: Roof Mount Types

A B C

Generally solar panels are not installed directly onto a roof for the following reasons:

• PV modules require air circulation to keep the modules cool, which will increase their performance;

• Installing solar panels directly onto the roof can increase the heating of the roof covering, causing premature degradation of the covering;

• A direct mount (which is not weather sealed) would allow moisture between the solar panels and roof resulting in a potential weakening of the attachment method due to the thermal freeze-thaw expansion cycle.

However there are some exceptions where solar panels may be directly mounted onto the roof:

• Solar pool collectors are generally installed directly (often supported by straps underneath the collectors) on the roof as the mounting design allows the collectors to slightly shift about on the roof due to temperature expansion and contractions of the plastic solar collectors. The low temperatures that these collectors operate at prevents roof covering degradation.

• PV “shingles” which replace conventional roof coverings;

• Solar panels installed like skylights — integrated into the roof structure (shingles and sometimes sheathing removed) and flashed in on all sides.

One of the primary considerations of roof mounting of solar panels is the wind loading — often referred to as the active or uplifting load. As the height of the solar panels above the roof increases the wind loading also increases. Above 18 inches the potential impact of wind loading increases significantly. Thus a flush roof installation is generally defined as one that is less than 18 inches above the roof.



May 2010

3. Solar Point Load

Unless the solar panels are directly mounted onto the roof, their weight will be concentrated at the point of connection to the roof — referred to as their point load. Thus a 100 kg solar panel with four roof connection points will have a point load of 25 kg at each connection. As such, the design of the mounting structure is a critical element in determining the types of roofs that can adequately support a solar panel.

Table 2 below illustrates the importance of good rack design. Collectors A and B have about the same distributed load, but Collect A’s rack design distributes this weight more evenly on the roof, allowing it to be installed on a wider variety of roofs. Collector A could be installed on a roof (composed of 2x8” rafters with spacing of 16”) to a maximum rafter span of 6.6m while collector B can only be installed up to a maximum span of 4.42 m on the same roof. Collector D — which is considerably heavier than the other collectors — can be installed on roofs with spans up to 4.92m due to the effective distribution of its load across the entire roof.

Table 2: Maximum Roof Spans for Solar Water Collectors

Defining a Solar Span TableBuilding codes define conditions (design provisions) that will influence the structural loading of a roof. Conditions such as wind load, snow load, and strength of rafters are defined and can be used to calculate the specific roof conditions2 that will accept the point load weight of a specific solar mounting design. Once defined for a standard condition, then it is simply one further step to calculate the acceptable roof rafter span with different rafter sizes and spacing. This then results in a “solar span table” showing the roof rafter construction details that will accommodate the specific solar panel and mounting design.

If the solar panel and the mounting design remain constant then this solar span table can be used to evaluate all rafter roofs in a jurisdiction to determine if they have the adequate structural elements (rafter size, spacing, and span) for solar. This is the solution offered in New Zealand3 and adopted by the City of Toronto’s building department to allow for a simple assessment of roofs for solar. Table 3 provides an example of a solar span table using the Solar Neighbourhoods Standard Sloped Mounting System for flat plate collectors found in Appendix Appendix 2.

2 Roof conditions are generally rafter size (i.e. 2x6s), rafter spacing (generally 16” or 24”) and rafter span (the distance between structural supports of the rafter).3 Manual for Structural Assessment for Installation of Solar Water Heating in Domestic Dwellings; www.solarindustries.org.nz/documents/030149_Manual_for_Structural_Assessment.pdf

Collector A Collector B Collector C Collector D

Type flat plate flat plate evacuated tube tank on roof

Distributed Weight (psf) 3.7 3.3 5.4 15.8Maximum Rafter Span (2x8 rafters w 16” spacing) 6.60 4.42 3.18 4.92

Information for this table was derived from the structural drawings and solar span table of solar collectors in the Toronto Solar Neighbourhoods Initiative.



May 2010

Table 3: Example of a Solar Span Table

Maximum span (ft) for roof rafter with solar installedRafter Size Rafter Spacing

Mm Nominal 16” 24”

38 x 89 2 x 4 9.0 7.5

38 x 140 2 x 6 12.8 10.6

38 x 184 2 x 8 15.6 12.9

38 x 235 2 x 10 19.1 15.8

38 x 286 2 x 12 22.3 18.4

Developing the solar span table (and the accompanying structural drawings of the solar panel mounting design and attachment to the roof) is generally the responsibility of the solar system supplier and must follow the design provisions determined by the local regulator. In the case of the Toronto Solar Neighbourhoods Initiative, it was determined that the majority of solar hot water flat plate collectors in the program were of similar dimensions and weight and could be installed on a sloped roof in a similar fashion. Thus the program developed its own set of structural drawings and span table, which can be used as a “generic” design for installing solar hot water collectors in Toronto. The standard conditions are defined in Table 4 below and the full drawings, engineer’s letter of compliance and sample engineers’ calculations can be found in Appendix A4.

Table 4: Conditions for use of the Solar Neighbourhoods’ Standard Slope Roof Drawings

Solar Collectors

Type Flat Plate

Max Dimensions 8.5 ft x 8.5 ft (72.25 ft2) – 6.71 m2

Max Weight (Dead) 131 kg – 289 lbs

Connection Points to roof Either 6 or 8 roof connections points

Connection Method 3/8” x 4” SS lag screws or bolts into middle of rafters

Distributed Load 4.0 psf

Point Load 36.1 lbs (8 points) or 48.2 lbs (6 points)

Roof Conditions

Roof Slope 30o or steeper



May 2010

Verifying that Roof Conditions are Adequate for SolarThe final step is to verify the roof conditions at the proposed site of installation and compare them to the solar span table of the product that is being considered. Toronto Building has developed a SDHW Roof Structure Report (found in Appendix 1) which can be used to do the evaluation of the roof components (rafter size, spacing and span).

Once completed the roof structure report is compared to the solar span table to ensure that the roof conditions are adequate as outlined in the following example.

Example: Evaluating a Roof for SolarThe roof is evaluated and found to consist of 2x6 rafters with 400 mm (16” spacing) and the span of the rafters at the proposed solar panel installation is 2.3m.

The solar span table for the proposed solar installation shows that this type of roof (2x6s with 16” spacing) can hold this system up to a rafter span of 2.52m. Thus the roof is adequate for this proposed solar installation

Do all Solar Panels Need Roof Structural Confirmation?From the work that the Toronto Solar Neighbourhoods Initiative has done in partnership with Toronto Building it appears likely that the vast majority of solar panel installations do not represent a structural concern for the integrity of a residential roof. Various other jurisdictions have also come to a similar conclusion. For example, the City of Palo Alto, California only requires a structural analysis to be undertaken if the additional roof load exceeds 5 psf. 4 The Solar America Board of Codes and Standards (SolarABCS) also recommends that a structural permit not be required where the PV array is less than 5 psf (and point loads are under 40 lbs per connection).5

However there are some conditions that should be considered in making an assessment of whether a structural review is required. The following table can provide guidance on conditions that would minimize the potential impact of a solar panel installation on the structural integrity of a residential roof based on the experiences in Toronto and elsewhere.

4 Solar Water Heating Program Technical Handbook; City of Palo Alto Utilities; May 17, 2008; www.cityofpaloalto.org/civica/filebank/blobdload.asp?BlobID=12062 5 Expedited Permit Process for PV Systems; Solar America Board for Codes and Standards; May 2009; www.solarabcs.org

Maximum Spans (m) For Roof Rafters — Solar Thermal Structural LoadRafter Size Rafter Spacing (mm)

mm Nominal 400 600

38 x 89 2 x 4 1.69 1.74

38 x 140 2 x 6 2.52 2.56

39 x 194 2 x 8 3.18 3.17

38 x 235 2 x 10 4.08 3.98

38 x 286 2 x 12 4.89 4.69



May 2010

Table 5: Minimizing Roof Structural Concerns for Solar

1. The solar panel’s distributed weight is less than or similar to other products with long-term use on roofs (i.e. 2 layers of shingles). Typically this structural acceptability is between 3-5 psf in many jurisdictions.

2. The solar panel’s connections to the roof results in the panel’s weight being fairly equally distributed amongst the rafters under the solar panel. Toronto Solar Neighbourhoods experience for solar hot water collectors that fall under (1) is that this is under 50 lbs per connection.

3. The mounting structure must provide direct connection to the roof to prevent wind uplift and anchorage penetration must be directly to rafters.

4. The solar panels are installed within 18” of the roof (i.e. flush to roof or slightly above the roof).

5. The mounting structure is an engineered product specifically designed to mount the solar panels to roofs similar to that at the proposed site.

6. There are installation instructions provided by the supplier of the mounting structure and they follow good engineering practices.

7. The roofing material is lightweight (i.e. asphalt shingles, cedar shakes, metal, etc.).



May 2010

Building SDHW Roof Structure ReportThis site review report may be sent directly to an inspector or faxed directly to the District Inspection Office.

North York 416-696-4179 Toronto and East York 416-696-4151 Date Permit No. Scarborough 16-696-4166 Etobicoke York 416-696-4169 Day Month Year

Project Location Street No. Street Name Unit No.

Roof Assessment Assessment is done underneath the proposed solar collector installation location.

Roof Slope (angle from horizon, flat =0°): If roof is sloped - direction of roof slope (i.e. S, SW, SE):

Roof Structural EvaluationRafter Spacing (on centre): Rafter type: 38 x 89 (2” x 4”)

38 x 140 (2” x 6”) 38 x 184 ( 2” x 8”)

38 x 235 (2” x 10”) 38 x 286 (2” x 12”) Rafter Span (mm or feet) at proposed location of solar collectors:

Rafter span: is the distance, measured along the length of rafter, between the two structural supporting members of the rafter.

Roof Plan

Please sketch out the roof (in bird's eye view) showing

dimensions of roof elements, the direction of the rafters and the proposed location of the solar

collectors. Show compass points. Assessment of the roof is done underneath the proposed

location of the solar collectors as shown on this sketch.

Applicant I have reviewed the roof joist size, spacing and span at the proposed location of the solar collectors and the information contained in this report is true to the best of my knowledge.

.....................................................................................

......................................................................................

................................................................................Print name Signature Date

If homeowner does not sign below then a Building Code Identification Number (BCIN) is required

Individual BCIN: Firm Name:

(or) Firm BCIN: Phone No.:

Note: BCIN is not be required if the Designer is an Ontario licensed P.Eng or Architect (proof required).

Owner

.....................................................................................

......................................................................................

................................................................................Print name Signature Date

Appendix 1: Toronto Building’s SDHW Roof Structure Report



May 2010

Appendix 2: Solar Neighbourhoods’ Standard Sloped Roof Mounting System for Flat Plat Collectors

mm Nominal 400 60038 x 89 2 x 4 2.74 2.3038 x 140 2 x 6 3.89 3.2338 x 184 2 x 8 4.74 3.9238 x 235 2 x 10 5.84 4.8138 x 286 2 x 12 6.80 5.59

Maximum Spans (m) For Roof Rafters - Solar Thermal Structural LoadRafter Size Rafter Spacing (mm)

Inch Nominal 16 241-1/2 x 3-1/2 2 x 4 9.00 7.531-1/2 x 5-1/2 2 x 6 12.77 10.591-1/2 x 7-1/4 2 x 8 15.56 12.861-1/2 x 9-1/4 2 x 10 19.14 15.781-1/2 x 11-1/4 2 x 12 22.30 18.35

Maximum Spans (ft) For Roof Rafters - Solar Thermal Structural LoadRafter Size Rafter Spacing (Inch)

mm Nominal 400 60038 x 89 2 x 4 2.68 2.2838 x 140 2 x 6 3.78 3.1838 x 184 2 x 8 4.61 3.8638 x 235 2 x 10 5.68 4.7438 x 286 2 x 12 6.63 5.52

Rafter Spacing (mm)Maximum Spans (m) For Roof Rafters - Solar Thermal Structural Load

Rafter SizeInch Nominal 16 24

1-1/2 x 3-1/2 2 x 4 8.78 7.461-1/2 x 5-1/2 2 x 6 12.40 10.441-1/2 x 7-1/4 2 x 8 15.11 12.671-1/2 x 9-1/4 2 x 10 18.64 15.551-1/2 x 11-1/4 2 x 12 21.75 18.10

Rafter Size Rafter Spacing (Inch)Maximum Spans (ft) For Roof Rafters - Solar Thermal Structural Load

Continued on page 10



May 2010

Appendix 2 continued: Solar Neighbourhoods’ Standard Sloped Roof Mounting System for Flat Plat Collectors



May 2010

Appendix 3: Sample Engineers’ Letter of Structural Compliance

Sustainable EDGE Ltd.

250 MERTON STREET SUITE 502 TORONTO ONTARIO M4S 1B1

T 416.481.0400 E [email protected] WWW.S-EDGE.COM

c o l o g i c a l e s i g n

r e e n n g i n e e r i n g

October 8, 2009 Wade Tam Toronto Building, City of Toronto 100 Queen Street West Toronto, ON M5H 2N1 RE: SLOPED ROOF SOLAR COLLECTORS

Dear Mr. Wade Tam, The attached sealed drawings, S-1 to S-3 are based on Sustainable EDGE’s structural design calculations, which considered solar system dead loads, live liquid loads, roof structure self weight, specified snow load, and wind 1/50 year live load including a 2.5 gust factor, as per Limit State design provisions of the Ontario Building Code. These drawings are only for flat plat type solar hot water collectors. This structure is designed to hold two typical solar hot water collectors of a maximum array size of 8.5’ x 8.5’ (72.25 square feet) and a maximum dead weight filled with heat transfer fluid of 131 kg. Sloped roof installation is considered to be for roofs at a slope of 30° or steeper. If installed as shown in the enclosed documents for sloped roof installations and using the materials specified within those documents, and provided that the existing roof rafter size meets the span requirements of Tables 1 and 2 for 6 connectors or Tables 3 and 4 for 8 connectors on drawing S-1, the installed system would meet the structural requirements of Part 4 of the Ontario Building Code. I trust the above is sufficient information for meeting the structural permit requirements of the City of Toronto Buildings Department. Sincerely, Mario Kani, P.Eng. President

SAMPLE



May 2010

Appendix 4: Sample Roof Calculations

Documents

Technical Bulletin #1 : Evaluating roofs for solarsolarpermits.ca/bulletin1-web.pdf · 2012-09-12 · Will the solar panels stay attached to the roof based on the local design provisions