Exposed beam grid, terracotta tile ceiling, and thick pine floorboards?

[Lilly_Pines]

I'm trying to figure out an efficient (both environmentally and on the wallet, although I have a fair bit of tolerance for spending on the structure) structure for achieving a somewhat idiosyncratic combination of structural and aesthetic goals. I would like to achieve the following:
 

• The load-bearing structure of the floor is exposed to the room below as much as possible.
• The floor is reasonably stiff, without noticeable bounce or sag, and has load-bearing capacity significantly in excess of usual residential standards.
• The structure wastes as little space as practicable between the ceiling surface between the beams, and the floor surface above.
• Noise attenuation is decent but doesn't need to be amazing.
• UFH pipes can be accommodated, for heating/cooling both the floor and ceiling, with a low temperature delta.
• Room for other services is not needed, they will be accommodated elsewhere.
• All walls can be assumed to be load-bearing. The largest distance between them (in the short direction) can be assumed to be no more than around 4-5m, less in most cases.
• Sufficiently compliant with building regulations to avoid trouble.

 

To meet these goals, I'm considering using heavy primary beams (e.g. 200x400mm glulam, oak, douglas fir, etc.) across the room in the short direction, with secondary thinner beams/joists (e.g. 100x250mm) connecting them orthogonally. For example, in a 5x3m room 2 or 3 primary beams could effectively divide the 5m span into three spans of 1.5m or four spans of 1.1m. The secondary beams would be dovetailed into the primary beams and ring beams at the edges of the room, so that the tops of both beams are level with each other; this can be achieved with a set of templates/jigs and a router. The beams would be thick and deep enough to provide adequate structural stability in the case of fire, and this excess thickness in unburned beams would contribute to stiffness. The beams should count as "narrow members" for building regulations, so fire-resistant coating shouldn't be needed.

 

The secondary beams would be at a spacing of roughly 350mm of empty space between them (450mm centres with 100mm wide beams), and 400x300x30mm terracotta tiles laid across them. The tiles provide a finished ceiling surface with no further work required (convenient when ceiling height is in excess of 3m), and satisfy fire resistance requirements for the ceiling surface. Lime mortar or similar could be used between the tiles and to hold them in place.

 

In the spaces between the tiles, 2x4" battens would be laid along the length of the secondary beams. The space between the battens would be filled with heavyweight pugging for both aiborne sound insulation and thermal mass, and UFH pipes can be laid within the pugging layer. A decoupling felt either above or below the battens & pugging layer provides impact sound insulation; if the felt is above the battens but extends down below the pugging between them, it also helps avoid pugging leaking from any cracks that might form in the ceiling.

 

On top of the battens and across them, heavy floorboards (e.g 45x120-200mm, or even thicker if available for reasonable cost) act as both the structural subfloor and the finished surface. This should be significantly stiffer than the common solution of subfloor and thinner boards, due to the greater thickness of the layer and the optimal orientation of the wood fibers. T&G to constrain their movement relative to each other and to prevent gaps from opening between them, softwood for cost and being easier on the feet than hardwoods. Wear and patina on the surface is acceptable or even desirable. If carpets are used on parts of the floor, a thinner subfloor (e.g. plywood or non-visual 25-30mm boards) would be used on those areas to allow for carpet and underlay where applicable.

 

All together, this structure would consume roughly 15cm between the ceiling surface (sans beams protruding below) and the floor, which compares very favourably with conventional constructions. An even thinner construction could be achieved by laying the floorboards straight on top of the beams, or even dovetailing them in between the beams, but that doesn't provide space for UFH and requires fire retardant treatment to satisfy building regs (or some other way to convince them to allow it), and could have issues with noise transmission.

 

One thing I'm not entirely sure about is whether terracotta tiles would be strong enough to hold the pugging without risk of breaking. An alternative would be to reduce the spacing of the secondary beams, or to make the floor into a timber-concrete composite where the concrete is in compression and the timber beams below in tension. The latter would allow greater stiffness and/or shallower beams, but adds concrete, the shear connectors, and the complexity of engineering the structure. It is also not clear what would be the best solution for the floor surface in this case.

 

Does this buildup look viable? Are there ways to simplify/improve it that I've missed, or alternative ways of achieving similar performace and aesthetic goals?

[Lilly_Pines]

Construction time considerations:

 

Once the structural beams have been laid, a temporary floor can be placed across them. Apart from plastering the walls of the room below, there seems to be no further need to work at heights as the ceiling tiles and any fixtures, penetrations, etc. can be installed from above. Fixtures in the ceiling can either be attached to the beams, or they can be attached through a hole in a tile to a cross-batten placed above it to take the load. Because the 2x4" battens are mainly to act as spacers for the floorboards above, cutting them for pipes or wiring shouldn't be a huge problem. For mounting heavier objects through the tiles, a cross-batten can be placed across multiple battens with notches cut appropriately.

 

If the floor surface is removable (harder to do with the heavy boards, but very easy to do with carpeted sections by having the carpet lift off and the subfloor screw off), the pugging space can be accessed for servicing and inspection. Loose pugging enables easy installation of additional services or wiring; conduits could be buried in the floor to accommodate this. New fixtures in the ceiling would naturally require installation from below unless tiles are loosened to create an opening for access.

[Lilly_Pines]

Performance considerations:

 

Unglazed terracotta tiles aren't amazing acoustically, but the grid structure of beams should help diffuse echoes. Alternative materials with good acoustic and aesthetic properties could be nice, but I'm not sure I know of any that work here. Woodwool boards most likely require backing structure, and don't necessarily look great. Hempcrete seems like a pain to cast within a ceiling like this if permanent shuttering isn't used, and permanent shuttering means woodwool. Cork again requires a backing structure, and may look weird in colours other than brown.

 

The UFH would heat and cool the tiles significantly more than the floorboards. That is fine because softwood boards are quite insulating already so a large amount of heating is not needed for good foot-feel. Cooling is limited by how much the wooden floorboards can tolerate. Thick wood has okay thermal capacity but is not amazing. A timber-concrete composite with an oiled earth finish would probably be significantly better at dealing with excessive solar gain, with greater thermal conductivity to quickly pull excess heat away.

 

Airtightness layer would be on the exterior of the ring beams, so the floor structure doesn't matter for it.

Edited September 28, 2022 by Lilly_Pines

[ProDave]

Why do you think the sand / cement pug mix needs to be 100mm thick?  Mine is just 25mm thick.  That is a lot of dead weight your floor would have to be designed to support.

[Lilly_Pines]

1 minute ago, ProDave said:

Why do you think the sand / cement pug mix needs to be 100mm thick?  Mine is just 25mm thick.  That is a lot of dead weight your floor would have to be designed to support.

It is not, with 30mm thick tiles and 89mm battens the space remaining between the tiles and the floorboards would be 59mm because the tiles sit on the edges of the secondary beams, between the battens. Reducing the battens to 63mm would leave 33mm.

 

However, with such a thin layer I'd start to get concerned about the acoustic implications; I modeled the pugging after acoustic products like Hush-Fill (except without the price of slapping a proprietary label on a bag of tiny stones, as regular coarse sand/fine gravel/whatever should do pretty much the same job for less cost), so the dead weight is an intentional feature and cutting the layer thickness would obviously cut the weight as well. If the pugging being loose is a problem for the UFH, the pipes could perhaps be fitted to the tiles with more solid material and loose fill added on top.

 

Insofar as the dead weight itself goes, my rough calculations are giving 7.4mm deflection for a 5m long 20x40cm scots pine primary beam loaded with 5,000kg uniformly over its length, equaling roughly 600kg/m² if the beams are at 1.7m centers, or 5.2mm deflection for a 4m beam loaded with 1,000kg/m². The secondary beams in turn are deflected far less due to their shorter length. 80kg/m² is peanuts compared to the capacity of this floor design, which is intended to be excessive for any normal residential specification.

[Lilly_Pines]

Now that I think of it a bit more, 80 kg/m² might be excessive and a specification in the range of 40kg/m² might be perfectly sufficient for airborne insulation in the same dwelling, so the 33mm afforded by 2x3" battens would be a bit thinner and cheaper.

Edited September 28, 2022 by Lilly_Pines

[Lilly_Pines]

Fire resistance of the buildup as drawn might actually be marginal? With a conservative charring rate of 1mm/min, and the tiles sitting on 25mm of beam on each end, fire damage would reach past the part they sit on in less than 30 minutes. Assuming the charred wood cannot carry loads, there could be the possibility of the tiles and pugging falling off and exposing the thinner structures after 30 min. Covering the beams entirely with the tiles and using 2x2" battens on top might be an alternative with better fire resistance, but even 30 minutes should satisfy Approved Document B so this may not be an issue in practice.