Most projects start with an innocuous need – someone reports a leak coming from the roof during a rainstorm, or the ice buildup starts to melt in the spring and there is water seeping down someone’s office wall. No problem, we’ll just replace the roof and everything will be back to normal – nice and dry. This article aims to outline the relevant Code provisions to consider when reroofing is undertaken – not to scare anyone away, but to eliminate surprises when the permit application is submitted.
The most prevalent question will be your jurisdiction, which will inform which Codes apply and what actions need to be undertaken. Applicable Codes vary widely by state (sometimes even more locally), so to create an example, let’s assume this project is happening in Massachusetts.
In Massachusetts, the applicable Codes are the 2021 International Energy Conservation Code (IECC), the 2021 International Existing Building Code and the 2021 International Building Code (IBC), each with Massachusetts-specific amendments. Upon re-roofing an existing building, there is a requirement C503.2.1 Roof Replacement that the replaced roof shall meet current code R-values and U-factors, as though it was a roof for new construction. This target insulation value in Massachusetts Climate Zone 5 is R-30. This effectively increases the amount of insulation beneath the new roof membrane to limit heat loss through the roof. The reduction in heat loss means that more snow will be retained atop roofs between snow events, and that the statistical likelihood of having more snow on a roof increases, leading to larger design snow loads for buildings. The current Code amendments in Massachusetts recognize this effect, though in a somewhat narrow way, in Section 706.2:
Further, this is the referenced Table 7.3-2 from ASCE 7-16 (current in Massachusetts) which describes changes in the roof Thermal Factor (Ct) only in broad terms of gross categorization:
In summary – the Code indicates a need for further insulation, and the Code also indicates that more insulation may increase Ct – which is linearly related to the total snow load – to the point where snow loads are increased on roofs. In practice, because Ct is assigned by building type, it is usually takes a change of use to make the jump between values shown in ASCE 7-16; the upshot is for most buildings that there isn’t a change to Ct, and thus no change to the snow load. So, in Massachusetts right now, there are limited instances where reroofing – and adding insulation – will require consideration of larger snow loads.
The challenge then comes with future Codes, or Codes that have been adopted elsewhere in the country. In some jurisdictions – and likely in the future for Massachusetts – is the International Building Code 2024, which brings in ASCE 7-22, where the table for Ct (now Table 7.3-3) reads in quite a bit more detail:
Notably, the table for determining Ct values is no longer assigned by type but solely by the roof’s thermal resistance. For a building being reroofed now, it is likely that the existing roof assembly predates the relevant Energy Code, and thus has minimal (if any) insulation, and increasing the insulation to R-30 correlates to a substantial increase in the snow load. For example, take a building with a ground snow load (pg) of 40 pounds per square foot (psf). If the present-day roof has little insulation (i.e. <R-20) the corresponding Ct per Table T.3-3 is 1.01; when it is replaced with Code-required insulation to achieve R-30, the new value of Ct is 1.16. Reroofing in this scenario leads to a 15% increase in snow load on the roof, which is certainly high enough to warrant an evaluation of the roof framing and connections; depending on the building type, it may warrant localized or wide-spread strengthening of the roof for the increased snow loads. This is just a representative example – there are scenarios where the Code-required addition of insulation could lead to up to a 20% increase in snow loads.
Some states have already adopted IBC 2024 which brings in the relevant provisions for Ct described in ACSE 7-22. Some of those states are in southern portions of the country where snow load is less prevalent, though some are northern states where snow load is often a driver in determining how roofs are structured.
For now, in Massachusetts, in 2026, there are limited applications where a reroof project would lead to increased snow loads, evaluation of roof framing, and/or structural strengthening. When Massachusetts adopts IBC 2024 – as some other states have done previously, this will become a more prevalent topic.
Owners may consider the use of roof coatings, liquid applied assemblies that are installed over the existing roof membranes. These may become more popular as they provide additional lifespan to the roof’s watertightness without triggering the Code requirements instigated by a reroofing effort.
Be sure to inform your client – when someone calls you with a leaky roof, they might need to consider snow loads and how their roof structure will perform as well.
Takeaways
- If you need to reroof, consult a professional familiar with the Code.
- Determine which provisions of the Code apply in each unique case.
- Make a plan to ensure your reroof project can be successfully permitted.
- Move forward confidently that your new roof replacement will be dry and safe.