Foundation Venting and Waterproofing When Installing a Raised Terrace

Foundation Venting and Waterproofing When Installing a Raised Terrace

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Constructing a raised terrace on an existing structure can have some complications. The crawlspace must be vented and waterproofed, and crawlspace doors and other items may need to be moved. Below is an excerpt of an analysis we performed for one of our clients.

Existing Conditions 
The foundation is brick and block with a series of traditional 8x16 foundation vents. There is a crawlspace door and what appears to be a stove vent. The existing crawlspace grade sits 12+ inches below the outside grade. The earth under the house is wet. There is no indication of mold on the floor joists, which may be in part due to a plastic vapor barrier, but the moisture should be addressed.

Proposed Alterations 
The existing wood deck will be removed and a new stone/concrete terrace installed. Finish elevation of the terrace will be approximately 4” below finish floor of the house. At issue is the venting of the foundation, relocating the crawlspace door and other vents, and waterproofing the foundation.

Alternative A – Structural Slab 
A platform is constructed and concrete slab poured on top. The net result is much like the wooden deck in that there is no reason to relocate the foundation vents, crawl door and other elements. An additional benefit is that the foundation does not need to be waterproofed. The downside is the cost. A structural slab will be the most expensive to build of the three alternative discussed here. Some of the cost may be off-set by not having to move the crawl door or install waterproofing membrane. Another concern is that the moisture levels under the house remain as they are, which is too high.

Alternative B – Slab on Fill with Vented Crawlspace 
The terrace is filled with compacted soil and a slab is poured on top. Foundation vents are extended through the fill to daylight on the face of the terrace and a waterproof membrane is installed on those parts of the existing foundation which are covered. The crawl door and other vents will be relocated. The benefits of this approach is cost (this is the least expensive of the three) and possibly eliminating the moisture under the house. A concern that will need to be addressed is the length of the vent extensions. The vent pipes end up being about 20-feet long. I do not know if code will require the vents to be powered.

Alternative C – Slab on Fill with Conditioned Crawlspace 
This alternative is preferred by the architect who designed the house. It is substantially the same as Alternative B except that the foundation on the entire house is sealed and conditioned. Many of the better home built today are designed with conditioned crawlspace and retro-fitting an existing structure is not that difficult. All foundation vents area sealed as are any other points where air flows in or out of the crawlspace. The foundation walls are typically insulated, although this step is sometimes left out in a retrofit. Finally, HVAC vents are added in the crawlspace to create positive pressure. The idea is to isolate the crawlspace from outside air in order to reduce moisture levels and thereby extend the life of the floor system. Compared to Alternative B, the cost will be the same with the additional cost of sealing and conditioning the crawlspace. 


Hertzler & George

 is a design, build and maintenance firm in Williamsburg, Virginia dedicated to the development of extraordinary gardens and properties.

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