The terms used in construction can be confusing, especially when it comes to items like subfloors. There are several different types of subfloor and a number of different materials can be used in building subfloors. But whatever your choice of materials, a subfloor will have the effect of making your room more comfortable by eliminating springy, uneven flooring. Building sub-flooring can even help turn an unfinished basement, porch, or sunroom area into extra living space.

1. What is a Subfloor?

Sub floors are used throughout most buildings to provide a proper level surface to support the finished flooring. In basements, building sub-floors can be an opportunity to add extra insulation to make the finished room both warmer, drier, and more soundproof.
Everything that lies underneath your top flooring, whether that is to be wood, laminate, carpeting, or ceramic tile, should be regarded as sub floor. In basements, that means everything between the concrete slab and your top flooring while in upper stories it means everything between the joists and the finished flooring.

The subfloor is part of a building's substructure. Basically, a subfloor is the material that the top flooring material, such as carpet, tile, or hardwood, is laid on, and when you understand this you can see why it is necessary for the subfloor installation to be done carefully and for the correct materials to be used. The kind of subfloor and subfloor material installed varies both in the geographic area your home is in, and in the kind of top floor you intend to install.

If you own an older home and plan to replace your flooring with a newer material, you must check the subfloor installation. The requirements for subfloor materials have changed over the years and some subfloor installations would not be considered adequate in a modern home for the new materials that are available. This is particularly true of the newer types of hardwood or tile floors that are now so fashionable. If you are installing these you may find that your subflooring is too uneven to get a good finish, or possibly not strong enough to bear the weight of the new flooring.
If this is the case, you may have to look at replacing your subfloor.

2. What are the Benefits of Installing Subfloors?

Sub flooring can be used to make your house more comfortable. Properly installed and insulated subflooring can be used to make previously cold and irregular floors, including basement floors, more even and comfortable to walk on. It can add value to a house, keep moisture at bay, and allow previously unused space, such as that in basements to be turned into comfortable living space.
Building a sub floor helps to insulate the space as well, but the most important aspect is that it provides a good foundation for your expensive floor finishes, whether these are ceramic tile, vinyl tile, slate, or carpeting.
The subfloor is the material that the flooring, which may be carpeting, hardwood, tile, or some other material, rests on. If the subfloor is not straight and even, then the new materials will not lay properly. Carpeting with underlay may compensate for uneven flooring to some degree.

But if you are laying a fashionable new hardwood floor, or using ceramic or vinyl tiles, the subfloor installation becomes even more important. Imagine carrying out all the work of laying hardwood planks or panels, or cutting and laying tiles, only to have the edges sticking up dangerously and the whole thing looking 'lumpy' and uneven! Ceramic tiles may even work loose as the grouting cracks if laid on an uneven subfloor.

3. What Kinds of Subfloor Are Available?

A variety of materials and proprietary products can be used in sub flooring. These include various types of mastic, polyethylene film, and other compounds, which help to keep moisture at bay. These include adhesives, cement based smoothing compounds, cement board, plywood, laminated wood and conventional, dimensional, soft, and hardwood lumber.
Subfloors come in a variety of materials but usually are made of 1 by 4 or 1 by 6 inch lumber or, more often, plywood panels are used when building subfloors.

The subfloor must be strong enough to carry the type of top floor you want to install, so it is vital to ensure that you are using plywood, etc., which is thick enough to carry the top flooring material. It is not uncommon for homes to be built on a concrete slab and in this case, the concrete itself will act as a subfloor for most types of flooring. It is a good idea to check that there is a vapor barrier, or else to have one installed before you put in your new flooring material. This will prevent moisture seeping through the concrete and damaging your new floor.

In fact, it's very important to ensure that all your flooring materials are protected from moisture or condensation damage. The use of a subfloor membrane or plastic vapor barrier will offer this protection over a cement subfloor and will help your flooring material last longer.

4. What Materials are best in a Subfloor?

When building a subfloor, the materials to be used are determined by the end use, location, and the condition of any existing sub floors. If the existing subfloor is very uneven or damaged, the installation of a new subfloor may be necessary. Ceramic tiles for example, need a firm, flat surface as well as a good load bearing sub floor. To install ceramic tiles, slate, or other rigid tile materials, you need to have a subfloor strong enough to carry the weight and one that is inflexible. Tiles and grouting may crack if the surface upon which they rest is too flexible.
Materials used in building subfloors in bathrooms, utility rooms, and basements should be moisture resistant. Eliminating moisture problems is essential for all sub floors to be long-lasting and effective.

5. How is a Subfloor Installed?

If you are planning to lay a new floor in your home, particularly if you are using tile, wood, or vinyl, then you should find out what condition the existing subflooring is in first of all. A subfloor is usually constructed from lumber or with joists that have plywood panels nailed across them.

If you are laying a plywood paneled subfloor, you should place the panels in a staggered way rather than side by side, and the ends and edges should be closely aligned. The panels are then nailed to the joists, of if you prefer, construction glue can be used instead of nails.

When installing a subfloor over a cement floor, such as in a basement, some people prefer to build a wooden frame and add plywood panels with insulation and a vapor barrier between the concrete and the new subfloor. This has the effect of creating a warmer, more comfortable basis for most flooring materials.
As mentioned earlier, the weight of the new top floor dictates how thick the plywood or lumber subflooring must be, as you need to build subflooring that is strong enough to hold your new floor.

One way to get an idea of the condition of the subfloor before you start tearing out the existing flooring is to examine it from underneath. Go into your basement or crawl space and look at the underside of the floor. This will tell you whether it is planks laid over lumber, or whether it is plywood paneling. If it is plywood paneling, you should try to find the manufacturer's grade stamp. This will indicate whether the thickness of the plywood is suitable for your new floor or whether you will need to install a new subfloor. It's a good idea while you are examining the underside of your existing sub floor, and later when you have taken off the upper floor finish, to check for staining, damp marks, sagging, or signs of rot, which may indicate that there are structural problems that will need to be taken care of before your home improvement project goes any further.

6. Can I Install Subflooring Myself?

While it is possible for the do-it-yourself handyman to carry out a subfloor installation, it's important to remember that this is a big job and one that will require a lot of time and work. You may have to remove the existing top floor and the subfloor and make sure that all beams and floor joists are in good condition.

If installing an entirely new subfloor, it is essential to ensure that all angles are square and joists are firm and undamaged. You must check for possible signs of structural damage. If you install a subfloor that is not suitable for the flooring you plan to lay, you could find that your expensive new flooring is ruined and all that work, time, and money is wasted. It may well be wise to bring in an experienced subflooring contractor to advise and do at least part of the work

7. Can I Use a Subfloor in My Basement?

Finishing the basement and turning it into extra living space not only adds a lot of comfort to your home, but it also adds a lot of value, too, making your home more attractive to prospective purchasers.

Some people install carpeting directly over the concrete floor in the basement, but this can be very cold underfoot as well as having the potential for mildew and other problems. To eliminate this problem, it may be worthwhile to put in a vapor barrier over the cement, and then install a subfloor of plywood on top of this, provided your basement has sufficient headroom to allow for the floor to be raised a few extra inches. This will make the floor warmer to walk on, provide a better surface for the new flooring, and the entire space will feel more comfortable.

8. Using Your Existing Flooring as SubFlooring

Instead of installing new subflooring, you can, in some instances, use your existing flooring as a sub floor. This may be suitable in a new home or if the existing flooring is very level, even, and firm. It may well be wise to bring in an experienced subflooring contractor to advise and do at least part of the work etc., if the existing flooring is firmly hammered in, otherwise it will interfere with the level placement of your new flooring material.

One problem you may encounter if you want to use existing flooring as subflooring is the lack of headroom. The existing flooring is at least an inch thick on top of a subfloor, and your new flooring on top of this may make the room unpleasantly low-ceilinged. This can be especially true in a basement, where the headroom tends to be quite low anyway.

9. Can I Install Hardwood Flooring Over A Concrete Subfloor?

Because basements tend to have low ceilings, making it difficult to install a subfloor over the concrete floor, and then to install a ceiling as well without leaving too little headroom, many people simply carpeted their basement rooms because hardwood flooring could not be installed directly over concrete. The carpet was placed directly over the concrete subfloor. This was undesirable as moisture tended to cause mildew which stained the carpeting and led to rotting and mildew.
But modern developments in flooring techniques now mean that even basements or low-ceilinged rooms with cement floors can have fashionable hardwood floors installed. Instead of framing in the subfloor with two by fours fastened to the concrete floor, it is now possible to fasten the plywood panels directly to the cement using concrete fasteners such as concrete nails, concrete screws, and other specialty fasteners.

Yet another way to provide a solid, level subfloor for hardwood flooring over a cement floor is to build a floating subfloor. This involves laying down a moisture barrier, then adding two layers of plywood laid in opposite directions, overlapped and screwed together. In this way, the subfloor 'floats' as it is not screwed to the cement floor and so does not penetrate the vapor barrier and allow moisture to pass through into the plywood subfloor and, ultimately, to ruin the new hardwood flooring. Either of these methods can provide a solid, level subfloor to which hardwood flooring can be safely attached.
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