Home Features Technical Buildings on the move – deflection heads in drywall

Deflection heads in drywall partitions are quite often the topic of discussion, and for Steve Halcrow, technical consultant to FPDC, the subject has arisen numerous times, so what are the major points.

The principle of the deflection head is simple: during normal use, all buildings move around, and part of this movement is the deflection of floor elements as they literally bend under the load. Any partition built underneath that floor will have load imposed upon it by the structure above, causing the wall itself to bend, potentially cracking finishes and causing other problems. We therefore build a telescopic head arrangement which allows the deflection to take place without putting any load on the wall below.

The problem is the deflection head is a relatively complicated, and hence expensive detail to construct. The question often arises as to whether it is really required at all; I know of numerous cases of clients asking the drywall contractor whether they need to be used, and if so how big a deflection needs to be accommodated.
The answer to whether or not they are needed lies with the structural engineer. In their design they will have calculated a THEORETICAL maximum to which they expect any given floor construction to deflect, and it is this figure, plus factors of safety and perhaps other add-ons that eventually arrives in our drywall drawings.
Typically we are asked to accommodate 15, 20 or 25mm deflection (sometimes more), and realistically when might we expect a concrete slab of perhaps 250mm thick to move that far?

The answer is never, unless there has been a catastrophic structural failure! But if that’s what the engineer calculates as a maximum POTENTIAL deflection then that’s what we have to cater for, and unless you are prepared to employ an engineer to disprove the original calculations (highly unlikely, to say the least) then you must adhere to the requirement.

Of course there are times when the structure to which you are attaching partitions may deflect quite a lot. Portal frame roofs, for instance, are capable of a great deal of movement under wind and snow loadings, etc., and in those ‘non-standard’ cases the detail becomes even more complex.

As a drywall contractor you cannot say whether a deflection head is required. You can only advise how to construct the detail to cater for the amount of deflection stated by the engineer in any given case. In all cases it is imperative that you build the detail correctly, as the performance of the wall depends on it. Typical faults include:

– Studs and boards not cut sufficiently short to permit deflection
– Screws penetrating the head track, rendering the detail useless
– Flat plates being omitted or installed too low; they MUST be as detailed
– Insulation and mastic not included correctly as per the design detail
– Wrong boards used as fillets beneath the head track

The plasterboard manufacturers have a range of different details available depending upon the system in use and the performance required. It is vital that you adhere to their details and do not assume a principle applied by one manufacturer will necessarily apply when building partitions from another; there are some differences.

In my experience engineers and architects are unwilling to dispense with, or significantly reduce, the need for a deflection head, even if the indications are that they are not necessary. They remain therefore a fact of drywall life, and whilst this is the case they need to be built carefully and correctly. The risk has been effectively delegated from the engineer to the subcontractor so do not treat it lightly!

Steve Halcrow  is technical consultant to FPDC