Dogtrot Foundation Design
We are planning a Dogtrot home design. Traditionally these homes were built entirely separate (including the foundations) and joined by a common roof. We live in a Northern region where 4’ frost walls are required. Our concern is heaving between units that would cause enough shifting to damage the joining roof. My first question is, what is the best way to utilize this design in a Northern climate? Can you secure two separate buildings with one roof, or would the units need to share a perimeter concrete frost wall joining the two units through foundation?
Secondly, if they do need to be joined through a common concrete foundation, what is the best way to mitigate water issues or moisture wicking from the center breezeway?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Emal,
You are fine using two separate foundations. No properly designed and built foundation should move at all - and certainly not enough to cause problems with the roof.
Dogtrots are great. Having covered outdoor space is a benefit in every climate. I've only had the chance to build one. I wish I had designed more into the houses I have done.
The only caveat I would add: don't mix different footing types, which is bad. Don't use a friction pier footing with a spread footing... etc as you can get differential settling. But obviously that is not what is proposed, just wanted to note.
On a few occasions I have extended reinforced footings between structures, which won't limit vertical movement but if you're on a slope of unstable soil, that would help keep things tied together. But otherwise I agree with Malcolm.
Designing a Dogtrot-style home in a Northern climate with required frost walls presents unique challenges, especially concerning foundation stability and moisture management. The traditional separation of living spaces connected by a breezeway under a common roof must be adapted to prevent differential heaving and ensure durability. Here are the considerations and recommendations for your project:
1. Foundation Design for Northern Climate
Shared Perimeter Frost Wall: In regions requiring deep frost walls, constructing the two units with a shared perimeter concrete frost wall is advisable. This approach ensures both structures are subject to uniform ground movements, minimizing the risk of differential heaving that could damage the connecting roof. The shared foundation should extend below the frost line (at least 4 feet in your case) to prevent frost heave.
Structural Integrity with One Roof: Securing two separate buildings under one roof is feasible if the foundation is designed to accommodate the thermal and moisture conditions of your region. The key is to ensure that the foundation and the roof structure are engineered to handle any potential movement as a single cohesive unit, reducing the risk of damage from heaving.
2. Mitigating Water Issues in the Breezeway
Drainage and Waterproofing: Proper drainage around the perimeter and beneath the breezeway is crucial. Consider installing drainage tile or perforated pipe at the base of the foundation walls, leading to a sump pit or daylight discharge to remove water from the foundation area. Waterproofing the exterior of the foundation walls with a membrane and protecting it with a drainage board can also prevent moisture intrusion.
Slope and Surface Treatments: Ensure the grade around the home slopes away from the foundation to promote surface water runoff. For the breezeway floor, consider a slight slope to direct water away from the buildings, possibly incorporating drains or a graded channel in the center to handle any water that does enter.
Capillary Breaks: To prevent moisture wicking, use capillary breaks between the foundation and the structure, such as a layer of gravel under the slab and damp-proofing or waterproofing membranes along the foundation walls. For the breezeway, a vapor barrier beneath the finished surface can prevent ground moisture from migrating upward.
3. Design Considerations
Enclosed vs. Open Breezeway: Depending on your climate and how you plan to use the breezeway, consider whether it will be open or enclosed. An enclosed breezeway can offer additional living space and a controlled environment but may require additional considerations for insulation, heating, and ventilation.
Insulation: If the breezeway is enclosed or if there are concerns about thermal bridging through the foundation, insulating the foundation walls and the breezeway slab can be beneficial. Rigid foam insulation can be applied externally on the foundation walls and beneath the slab to improve thermal performance.
Consultation with Professionals: Given the unique aspects of a Dogtrot home and the specific challenges of your climate zone, consulting with an architect and a structural engineer familiar with local conditions and requirements is essential. They can provide tailored advice on foundation design, roof structure integration, and moisture management strategies to ensure the longevity and comfort of your home.
In summary, for a Dogtrot home in a Northern climate, a unified approach to the foundation with careful attention to drainage, waterproofing, and structural design is key to managing frost heave, moisture, and ensuring the long-term stability and comfort of the home.