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21 November 2022

Should We Be Designing Intermediate Floors to Inter-Tenancy Standards?

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Medium density housing (MDH) has introduced us to a whole lot of new terms, like inter-tenancy walls (ITW) and inter-tenancy floors (ITF), but not all floors have to be ITF. Some floors are just, well, ordinary floors, the way we used to build them, where the sound could creak through from upstairs to downstairs, ruin many a night’s sleep, and introduce adults to the latest sounds of teenage angst. So, maybe ordinary old-fashioned floors are not such a good idea any more! To really make a MDH dwelling work well, consider putting in some extra effort into the intermediate floors as well, so everyone can sleep well at night.

If you’re building a two-storey townhouse, as many people are doing, you will have one intermediate floor, probably with two bedrooms and a bathroom upstairs, and a whole living/dining/kitchen area downstairs. Sometimes, you might have the reverse, with bedrooms below and living up above. Either way, you may not have the fire requirements of a full ITF, but you will still want to limit the amount of sound coming through. Our standard means of flooring, with timber floor-joists and a single layer of plasterboard as a ceiling fixed directly to the ceiling joists, is simply not going to cut the mustard.

The lessons we have learnt about acoustic transmission still operate, even if there is not a formal requirement for acoustic control. What we need is three things: some mass/density, some absorbent fluffy stuff, and some clear physical separation between layers. That means it is important to keep up with the basics, like suspending ceilings on resilient rails, double layers of plasterboard with all the joints staggered between layers, avoiding penetrations like downlighters, and most of all, getting a good solid flooring system to link it all together. Standard thickness particle board should be swapped out for thicker, heavier products like Strandfloor, which is a full 20mm thick, as well as heavier and denser. Standard timber joists will only reach a certain distance under NZS3604, but designed flooring products like hySPAN LVL and hyJOIST can stretch longer and give you a stiffer floor as well.

Three storey townhouses are even more important, as often the living floor will be in the middle floor, with sleeping both above and below, perhaps in the form of a granny flat downstairs. Inter-generational housing is a really important growth area within MDH, and the easy provision of a suitable space for a member of the extended family can work well, as long as there is no noise transmission. If you think that this granny flat may be sold off separately later, you should design a full ITF system now, because it is hard to go back and retrofit that later, but in most cases, it is still worth just beefing up the acoustic standards of this floor now.

Get used to working with systems like Strandfloor, particularly when they interact with other parts of the flooring system, like Batten & Cradle’s rubber mounted floor system, which really helps bring noise transmission right down. But remember, of course, that if you still have a stairwell open to each floor, that will still act as the most direct route for noise to travel up and down, so make sure that your design can still work in a surrounding wall and doors to the stair, if you should need them later.

Bathroom areas are a little easier to achieve on intermediate floors than they are in full ITF systems. Plumbing can be run through the joist space below the flooring, but plan the falls carefully so the drainage pipes can run direct to the plumbing riser without having to cut across from one joist to another. StrandfloorH3.1 comes as a treated panel that will resist dampness but regardless, any flooring surface needs a full waterproof membrane layer and careful laying to falls to achieve a dry floor for the life of the building. As always, plan your WC fittings clearly so that no one has to listen to the noise of the plumbing. A happy long lifetime of enjoyment is available in MDH with great attention to all the details. 

Medium Quick-Fire Quiz — Be in to Win!

Enter the draw to win a bottle of champagne, Laminex HI-Macs Cheese Platter and a $50 Westfield voucher thanks to Laminex New Zealand by answering two simple questions! Entries close 5pm, Wednesday 30 November 2022.

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