Method of supporting joists and DPC Q3 - Estate Agent's Examination Part 1&2

Q.
(a) With the use of diagrams, explain the methods of supporting the ends of joists and the types of joist strutting in the construction of the timber upper floor. (15 marks)

(b) Explain the suitable materials used for damp proof membrane in ground floor construction. (10 marks)

(25 marks, 2016 Q3)

A.
a) Timber upper floor is a common question:

Using diagram, explain timber upper floor

However, this question asked about methods of supporting the ends of joists and the types of joist strutting, not about drawing of timber upper floor.

First, the joist - which are long timber beams, forming skeleton frame for the entire support of the flooring on top. The end of joists must be anchored into the wall which hold these joists up.

Timber joists used for upper floors are supported at ends by either the external wall or by internal loadbearing partition walls. As with ground floors, the joist section size will be determined by the span length and the floor loading.

The joists used may be of solid timber, be constructed of composite plywood I-beams or composite metal web beams. How it sits into the wall and other loadbearing timber beams is what is asked about in this question.

1. The most common method is to penetrate the wall as to sit inside the wall like below.

Then, the completed diagram would look like below.

2. If it does not sit inside the wall it can be secured onto the wall with hanger devices like below:

Or sits onto a "ledger" which is a plank of timber beam fastened to the wall with wall bolts.

A long beam of solid timber is fastened into the wall by using screws (wall bolts) like above, and the joist timber beams sits on this fastened timber beam. It can be secured by i-joist hanger or notched trimmer like below.

i-joist hanger

In the most simple form, it looks like below after completion.

It can be reinforced by an i-joist hanger or long leg joist hanger.

3. If the joist are secured during the wall construction it can be supported by i-joists as the bricks are being laid. I-joist with restraint strap like the diagram above would carry the load of the perpendicular joist beam which the timber upper floor will lay on.

Finally, the completed timber upper floor when viewed from underneath.

These lines of joists would line-up parallel to each other. To prevent them from moving side ways, herringbone joists are used to keep them together in proper span.