Should Your Garage Floor Level Be Lower Than Your House? By How Much?
Firas Naji
Senior Building Surveyor - Fire Safety (Grade A1 Unrestricted All Classes of Building) @ Campbelltown City Council | Town Planner & Building Code (BCA) Consultant @ Silver End Pty Ltd
I recently encountered an interesting question from an architect: Should the floor level of an attached garage be lower than that of the main house? This query piqued my interest and prompted me to investigate whether such a requirement is specified in the Building Code of Australia (BCA).
Disclaimer: This article specifically addresses attached garages with slab-on-ground construction, not detached garages or dwellings with suspended floors.
Clause 3.3.3 of the ABCB Housing Provisions standards states that “Surface water must be diverted away from a Class 1 building as follows.” According to the BCA Volume 2, ‘Surface water’ is defined as “All naturally occurring water, other than sub-surface water, which results from rainfall on or around the site or water flowing onto the site.”
This clause focuses on managing surface water, which pertains to external ground surfaces, rather than internal garage areas. Terms used such as “finished ground level adjacent to a building” and “external finished surface surrounding the slab” suggest that the clause is concerned with external aspects rather than internal garage levels.
We can also see that Figure 3.3.3a, refers to “external surface”.
Further analysis of Australian Standard AS 2870-2011 and the New South Wales Guide to Standards and Tolerances 2017 did not reveal specific requirements for garage floor levels relative to the dwelling house.
However, a closer examination of the BCA’s performance requirements in Section H, particularly “Part H2 Damp and Weatherproofing,” clarifies the intent. Objective H2O1 aims to protect occupants and buildings from moisture damage, including surface water. Functional Statement H2F1 specifies that buildings must be designed to protect people and property from the harmful effects of redirected surface water.
Objective H2O1 is to protect occupants from harm and safeguard the building from damage due to surface and external water.
Functional Statement H2F1 also indicates that a building must be designed to protect people and property from the harmful effects of redirected surface water.
Upon closer examination, it becomes clear that the clause applies to any external area around the dwelling house, including concrete footpaths and attached garages. These areas are all considered "external" to the house. The first sentence of the clause states: “Surface water must be diverted away from a Class 1 building.” This means the clause is focused on redirecting surface water away from the house to prevent any potential damage caused by water entering the dwelling. Regardless, if these “external” areas are attached garages or not.
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Part H2 primarily addresses weatherproofing for Class 1 buildings. It does not apply to Class 10 buildings (such as garages) unless they contribute to the weatherproofing of the Class 1 building. For example, if the garage driveway slopes towards the house, proper drainage mechanisms, such as grates or a hobs, are essential to prevent water from reaching the dwelling.
Conclusion:
Given the limitations outlined on pages 106 & 107 of the BCA, particularly regarding the applicability of H2F2 and H2P1(2) to Class 10 buildings, it appears that any requirement for the garage floor level to be lower than the house is context-specific, so long as it contributes to the weather proofing of a Class 1 building.
As such, to contribute to the weatherproofing of a Class 1 building, it is important to consider features such as the slope of the driveway for an attached garage. If the driveway slopes towards the building, it's crucial to have proper drainage mechanisms in place to prevent surface water from reaching the dwelling’s floor level. This might include installing drainage grate or a hob (a raised barrier) to effectively redirect water away from the building.
It is ultimately up to the Principal Certifier (PCA) to determine the appropriate measures based on site-specific conditions which must be designed and sufficiently detailed at CC stage, prior to commencement of works.
In summary, while there is no explicit BCA requirement mandating that garage floors be set lower than the main house, ensuring effective drainage and weatherproofing is crucial for protecting the dwelling from potential water damage. As outlined in the Practice Standard For Registered Certifiers Volume Two – Class 1a Buildings
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A certifier ensures public health and safety by assessing and issuing Construction Certificates (CC). They need broad knowledge of the planning system and construction, consult specialists when needed, and must ensure that all aspects of a development comply with relevant regulations.
A certifier must ensure a building complies with the current Building Code of Australia (BCA) before issuing a Construction Certificate (CC). The certifier must objectively evaluate and assess the plans and specifications, request more information if needed, and ensure compliance to avoid invalidating the certificate.
Please share your thoughts and professional opinions with us on this platform so we can learn from each other and make informed decisions.
Disclaimer:
This article is not to be taken out of context. It is general in nature and does not relate to any particular site. You are strongly advised to seek your independent town planning or a legal advice for your site and not to rely on the opinion of this article or any comments within.
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Regards
Firas Naji
31 August 2024
Contracts Manager, B Eng MIEI
6 个月Great Analysis! Having come across this situation on many occasion wher ‘American’ type Garages adjoined to the house are becoming increasingly popular the rule of thumb used was always to remove the floor insulation depth from the FFL. I.e. If 125mm floor insulation was used then the garage floor would be 125mm lower as the use of insulation would not be required in an uninhabitable area. this ensures the build up of the substructure would be uniform and thus unaffected by differential settlement
National Purchasing and Estimating Manager at Integrity New Homes Franchise Group
7 个月Great post Firas Naji. I did similar research about 5-years ago and came to the same conclusion but stupidly didn't save all my evidence. I really appreciate you putting the time into this publication and have shared with our builders should their certifiers ask why we don't have a set down on all our homes.
Builder at Ionic Construction
7 个月Great analysis.
Director | Tier One Certification
7 个月Interesting one Firas. I’ve always wondered how they resolve the problem of wall frames along the external wall of attached garages being on the low side of the slab, without a hob this would essentially raise a PS.I would like to see other thoughts on this.
Licensed Builder/Director, at Craft Built Homes, Building Your Home As If It Were Our Own
7 个月Great insights. It also highlights why our Building Codes, Standards and Regulations cause so much misunderstanding and varying interpretations. If garages have to be lower than the house slab because they're considered "external surface" why isn't that clearly stated in the Code?