Basics of Building Construction

Building Components And Types

Lesson 21 : Upper Floors

Timber Floors

Timber floors, though quite light in weight, have poor fire resistance and sound insulation properties. They are quite costly, except at those locations where local timber is cheaply available. It is also highly vulnerable to termite attach.

Timber floors are basically of three types:

1. Single joist timber floors.
2. Double joists timber floors.
3. Framed or triple joists timber floors.

1. Single joist timber floors

This is the simplest type of timber floor used for residential buildings, where spans are short or moderate (say up to 4 m) and loads are comparatively lighter. The floor consists of wooden joists (also called bridging joists) spaced 30 to 40 cm apart and supported on end walls, over which timber planking or boarding is fixed. The width of joists are kept 5 to 8 cm wide. The depth of the joists is determined from the thumb rule:

Depth (cm) = (4 x spanin metres) + 5 cm.

The joists are supported on wall plates 10 x 7 cm to 12 x 7 cm in size, at the end walls. A space of about 5 cm is kept at the ends for air circulation. When the span exceeds 2.5 m, it becomes essential to strengthen the timber joists by providing herring bone strutting at the mid – span, by means of inclined pieces of timber of size 5 x 3 cm to 5 x 5 cm. End wedges are provided between the wall and joist.

The end of the joists are nailed, cogged or notched to the wall plates. If the joists of adjacent room run in the same direction, they may be overlapped and nailed to each other. Planking consists of wooden boards of 4 cm thick and 10 to 15 cm width, which are fixed to the bridging joists.

2. Double Joists Timber Flooring

This type of flooring is stronger, and is used for spans between 3.5 to 7.5 metres. The bridging joists are supported on intermediate wooden supports, called binders. Thus, the loads of bridging joists are first transferred to the binders and through them to the end walls in the form of highly concentrated loads. This is a disadvantage of this type of flooring. Also, the overall depth of the flooring is increased.

Because of intermediate supports, the bridging joists are of smaller sections, and are spaced at 30 cm centers. The spacing of binders is kept 2 to 3.5 m, and they rest on stone or wooden bearing templates which are not less than 0.75 to 2.5 m in length. In order to reduce the overall depth of the floor, bridging joists are cogged to the binders, with depth of sinking equal to 113 rd depth of bridging girders and bearing not less than 2.5 cm. Alternatively , the ends of the bridging girders are cut, and they are jointed with the help of fillers provided along the two sides of the binder.

3. Framed Or Triple Joists Timber Floor

This type of floor is suitable for spans greater than 7.50 m, in which intermediate supports, known as girders are provided for the binders. There are four elements of flooring. (i) floor boards, (ii) bridging joists, (iii) binders, and (iv) girders. The bridging joists support the floor boards. The binders are staggered and connected to girders by tusk and tenon joints, to increase the rigidity of the floor and to decrease the overall depth of floor.