Subfloor Construction
For Your Small House

So now your small house foundation has been constructed, the concrete has cured, and the forms have been removed. It is time to think about the subfloor structure that will cover your basement and provide the basis for building your small house.

The first thing you want to consider is what type of flooring you have planned for the rooms that will be built over the basement level. The type of flooring you select for the rooms will be the deciding factor in the design of the subfloor that will cover your basement foundation. Different types of flooring materials require different types of subfloor structures

Another thing to consider is whether or not you would like raised flooring (if you decided to go with a slab) which provides space for plumbing, electrical wiring. Raised flooring is a good idea because it allows space for plumbing and electric. Additionally, it will allow you the flexibility of making revisions in the plumbing or electrical infrastructure in later years if you want to pursue renovations or additions to your small house. Renovation projects often require you to re-route plumbing and electric so this will provide you with the flexibility to do so.

Once you have taken these factors into consideration you may begin the subfloor structure. The first step is to attach sill plates to the top of the concrete foundation walls. The sill plates serve as a base to the subfloor structure and are usually made of treated lumber such as pressure treated wood to discourage rotting, termites, and other nuisance factors. Sill plates are attached to the protruding metal rods that were installed in the concrete during the foundation building process. This prevents the sill from slipping off the foundation.

The subfloor structure itself consists of a framework that includes wooden joists running parallel to each other within the existing frame of the subfloor. The measured distance apart of the joists will depend upon the local building code for your jurisdiction. In general the joist size is usually 2 by 10; however this size could vary depending upon the size of the subfloor and the load that it is going to support. The joists are connected by rim joist hangers, a nifty little piece of hardware that makes securing the joists so much faster and more secure.

You will design the joist structure with the type of flooring you have in mind for each room that will rest on the subflooring. Floor covering such as laminate, vinyl and carpet can tolerate a flexible floor structure that will give a little when weight is applied. If you will be installing this type of floor material, a joist structure that is spaced farther apart will be sufficient (if code allows). These floor materials work well with a wood subfloor structure.

If you are choosing to install tile flooring, the subfloor will need to be built to accommodate a heavier load (joists have to be closer). This is due to the extra thick plywood and concrete-type board that will be necessary to use so the floor does not expand and contract and inevitably crack the tile floor. You will definitely need cement backer board to eliminate the flex in the floor... otherwise your mortar will eventually crack, and maybe even the tiles themselves.

You want to be sure that you discuss the type of flooring materials you have in mind for your small house with your subcontractor so the subfloor structure can be designed accordingly. Keep in mind that you want to include the details of the different flooring material that will be used for each room because this also affects the overall design of the subfloor joist framework and structure.

Other Resources on Subfloor Structure

Here's a page from a course on Contruction Technology offered in North Canton, OH. Pretty basic, but a good starting point.

Hometips.com has a nice page on floor and ceiling framing.

Here's a 55 page pdf document from the American Wood Council; probably more than you wanna know.

Some good basic tips on subfloor construction from Home Time.

Better Homes and Gardens has some tips on selecting a subfloor.

Pictures From Our Basement Construction

Laying down the sill foam before the treated sill plate goes on.
The central laminate beam is also in place.



Here you can see the framing for the basement windows starting to go in.
Gravel has also been poured into the basement "floor".



Another perspective.



The south end of the laminate beam is supported by 4 contiguous, vertical 2x4's. I think I would've been more comfortable if there would've been concrete poured in that spot to hold the beam (and the above south-face wall), but the building inspector let it pass.

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