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ASK THE INSPECTOR: Structural wood floors require proper installation

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QUESTION: I read your column every week in the Winnipeg Free Press. Although I don't have a specific question regarding my current home, I do have a question about current new-home-construction methods, specifically with regard to structural wood floors (SWFs).

More and more builders are promoting the use of SWFs in basements. The benefits of HVAC ducting under the floor and less impact from ground movement are obvious, but I would like to know about the long-term integrity and any disadvantages of SWFs.

For example, are there any perceived long-term issues with respect to the structural members and HVAC ducting being exposed to the higher humidity that comes from the soil? Is pressure-treated lumber used throughout with galvanized or stainless-steel fasteners?

Also, are present construction techniques doing anything to restrict or manage radon-gas ingress? I suspect when it comes to radon gas, an SWF would provide almost no barrier to the gas ingress as opposed to the barrier supplied by a sealed concrete floor.

Looking forward to seeing your comments on this topic.

Darren Besler

ANSWER: I'm more than happy to oblige your excellently articulated request to find out more information about structural wood floors in homes.

Structural wood floors replaces the traditional concrete-slab basement floor, with a heated crawl space underneath. The benefit of this type of construction is to provide a more comfortable floor and prevent heaving and sloping caused by soil pressure, typical of concrete basement floors.

Most homes built with this system have deep concrete piers under the beams and perimeter foundation, rather than traditional footings. This, combined with engineered floor systems, can provide a perfectly straight and non-moving floor system that should not require telepost adjustments or other periodic repairs.

Because many of these systems incorporate engineered floor joists, often with manufactured or steel beams, they can span greater distances between vertical supports. This allows for easier installation of plumbing, mechanical and electrical components in the crawl space. It also makes it easier for contractors or other individuals to access some components that are often installed below the main-floor ceiling in a traditional home. This gives additional headroom to the lower level and makes some systems more accessible than in a basement with drywall-covered ceilings.

The downside of this type of construction is that crawl spaces tend to be cooler than the living space above, and are more prone to condensation and moisture issues, especially if the soil inside is not well-sealed.

To combat this, the crawl spaces should be deep enough to provide a gradual slope to the centre, while maintaining enough room for a person to crawl underneath the joists at the entire perimeter. If ombined with a well installed and sealed polyethylene air/vapour barrier over the entire floor area, there should be few moisture problems.

A sump pit is normally located at the lowest area in the crawl space, to further help drain any moisture that seeps into this area from surrounding soil. The sealed poly sheathing is also necessary to prevent soil gases from infiltrating the crawl space from beneath, which should address your concern about radon.

Maintaining a reasonable space between the covered dirt floor and the underside of the joists should prevent moisture damage to the joists from wet soil, but it must be combined with other items to be fully effective.

The floor joists in this style of construction are often supported by joist hangers or a ledger board attached directly to the concrete grade-beam foundation around the perimeter. This allows direct contact between the ends of the joists and the concrete, which may be below grade. This can cause wicking of moisture from the cool concrete to the joists or the ledger boards.

To combat this, the perimeter grade beam must be well-insulated and properly air-sealed to prevent excessive condensation and damage to the joists. I recently saw a newer home experiencing this problem, mainly because of a lack of insulation in this area.

To further compound the potential for moisture damage to wooden components in crawl spaces, many builders are using manufactured TGIs, or I-joists, in structural wood floor systems. These I-joists are high-quality materials, but they can be more prone to structural damage from high moisture levels. Because the centre web of the I-joists is normally only 10 to 11 millimetres thick, it doesn't take much moisture damage to weaken the joist.

This contrasts with traditional framing lumber that may retain it strength even with some visible surface rot. For this reason, some builders are using traditional spruce or fir joists and beams in the structural floor, while incorporating I-joists into the upper floors of the home. Exposed lumber attached to the grade beams, whether for ledgers or nailing purposes, should also be pressure-treated to prevent easy rotting.

Many homes have been built with these items fully addressed and have few problems with their structural wood floors. But there are also others that have experienced premature rot and moisture damage in their structural floors because adequate attention hasn't been paid to these critical components.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at .

Republished from the Winnipeg Free Press print edition December 8, 2012 F12

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