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All About Joist and Concrete Floor Structures

Not every house is built from wood frame construction, and some houses are built with a combination of materials and techniques.

Excerpted from Do It Yourself Home Improvement

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Typically the building practices for residential construction vary according to region, availability of materials, and climate. Block houses, made of stone, concrete, or brick, are found more often in regions with older construction. When gutting an older block house to begin a major renovation, you may find that in addition to building new floors and walls, the shell of the house needs insulation and waterproofing to improve the home’s performance. Here are a few alternatives to a standard wood frame building.

Sawn Timber Joists Method

In this traditional floor construction, the ends of joists or beams are built into the walls of a block building, and are therefore directly supported by the wall structure. Sometimes joist ends rest on wooden wall plates secured to the wall surface. Old lumber joists secured within a wall may eventually become damp, and might need replacing over time. Joist size can vary according to the floor span, and this is an important consideration when building a new house or creating an addition.

Traditional Construction
Here, the joists are braced using herringbone struts and blocking, and are covered using straight-edged or tongue-and-groove boards.

Courtesy of © 2007 Dorling Kindersley Limited

Joist Hanging Method

In modern buildings, metal joist hangers are often used to support the joists. Joist depths, widths, and designs vary, so there is a wide range of hangers available to match. It is also an option to use joist hangers when renovating an old floor as these do not require large holes in walls, and are straightforward to fix.

Metal lateral joist straps are used to brace joists in position. One end of the strap is attached to the exterior wall, and the other end is attached to joists (either across or in line with them), to secure their position. Lateral joist straps are mainly used in new building projects.

Modern Construction
Here the joists are attached to the wall using joist hangers, and the joists are braced with lateral restraint straps and metal herringbone struts.

Further Information
Another alternative to wood frame construction is incorporating steel into the design. For example, steel beams are an alternative to wood girders. The wide flange (commonly called I-beam) used most often in residential applications. The appropriate size of the beam is dependent on the calculated load of your house and the length of span.

Courtesy of © 2007 Dorling Kindersley Limited

Concrete Floors

Ground floors made from concrete have been common for many years in residential construction, but as concrete grows in popularity as a building material, upper floors also are being constructed from concrete. If you build an upper floor from concrete, you will most likely use concrete beams and blocks, as opposed to the slab we commonly find in basements and ground floors of homes without basements.

When preparing a bottom floor for concrete, it is essential to insulate and prepare the floor for moisture control. Your floor will need compacted fill, possibly a drain tile, vapor barrier, and perimeter insulation (see below) to effectively combat potential moisture problems. Vapor barriers not only help keep the house sealed from moisture, but they also help speed the curing process of the concrete. A 6-mil polyethylene vapor barrier is laid over any mechanical systems, but the connections need to be accessible and above the finished floor level.

Pouring in the concrete and leveling it off by "eye" is possible, but it is far more accurate to create some shuttering within which a level floor can be laid. To achieve a smooth surface, concrete floors are usually covered with a topcoat of screed. Screed is a 3:1 mix of flooring sand and cement, and ready-mixed screed is available.

Floor Alignment
When altering floors — for example, in a renovation project where two rooms are to be made into one — you may need to align the floor level. To level between two concrete floors, adjust the screed using mortar or self-leveling compound, depending on the degree of change. Rather than leveling across a large area, you may need to build a small step. If there is only a small height difference, it may also be possible to simply create a small slope across the floor surface.

Solid Concrete Floor
The structure shown here is suitable for renovating or replacing an old floor. However, this type of structure is also used in new properties. In this case the damp-proof membrane would be lapped into the main wall structure.

Courtesy of © 2007 Dorling Kindersley Limited

Further Information

- The strength of your floor frame depends on the accuracy of your cuts and how well the pieces fit together.
- When building a floor frame, first set the trimmer joists in place. Joists are usually spaced at 16" on center. Headers are then attached before tail joists are installed.
- The parts of a floor frame can be assembled using a pneumatic nailer. Joist hangers and framing anchors can also be used.
- Caulking compound can help seal the joint between the header and the sill in colder climates.
- Wood is not always perfectly straight. Always lay the crown (hump) side of a board up when nailing it in place.
- While you are working on the floor frame, lay a piece of plywood across nearby joists to provide a more stable work area.

Excerpted from Do It Yourself Home Improvement

© Dorling Kindersley Limited 2009

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