MortarThePoint

Opinions seem a bit divided over 12.5mm plasterboard on 600mm c/c: https://www.plasterersforum.com/threads/600mm-centres.61316/ Seems a bit perverse to go for 15mm plasterboard @600mm c/c to save timber cost, but may end up the better choice. 600mm saves 33% over 400mm 480mm saves 17% over 400mm but is not a standard spacing so a bit weird

600mm centres would be great. I thought 12.5mm needed 400mm c/c, perhaps that's 9.5mm plasterboard then.

Just finished phoning around for prices and the timber for the void would cost: 50mm void (47x50) £4.13/m2 (£1.65/m) 38mm void (38x50) £3.23/m2 (£1.29/m) 25mm void (25x38) £2.01/m2 (£0.81/m) In more normal times all of those prices would probably be halved. Painful to pay more than 4x2 prices for 2x2.

I had considered not having a ceiling void under our hollowcore (see below) but think I do need it really, but plan to keep it to a relative minimum. That raises the question of what the minimum is. Is there a reg that needs to be followed? There are lots of downlighters that support shallow voids (e.g. https://www.downlightsdirect.co.uk/shallow-downlights.html?depth_range=1-50), so I don't think lights will set the minimum void size. I have seen people mention downlighters that are only around 20mm. Any wiring needs a minimal gap. An insulated 15mm water pipe ends up 13+15+13=41mm (22mm --> 48mm) so that suggests a 50mm ceiling void. Not needed, but waste pipes are "40mm" typically, but at solvent well joints mean it would need 50mm plus any fall. I don't need any of this though. Seems like the water pipes are the driving factor. I could take a long route to avoid having the water pipes in the ceiling void. Would it be bold to have a 25mm ceiling void? I was thinking 50mm was best, but with timber prices having gone nuts, I expect 2x2 to be double the 90p/m I paid in December and it needs a fair length as I expect to install at 400mm c/c. I could have a larger void (50mm) in the rooms that the water pipes would need to pass through and a smaller void (25mm) in the other rooms.

Wow, that was close. It came down to having less than two rails to spare and I feel I put it in everywhere it was needed. I also worked out that you can ream the Nu-Heat 14mm clip rails out to 16mm using a 16mm forstener bit. Thanks for the suggestions, but looks like I'm good and the screeders are now doing their thing. Pressure test was started last night at >9pm.

Amazon have sticky back for Wednesday and Screwfix is 24-48 hour unfortunately. The JG-Speedfit is 15mm. Ambiente may be an option. But time isn't on my side to head to the other side of Hatfield. Fingers crossed.

I urgently need a small quantity of clip track. I thought I had over ordered but think I have come up half a dozen short. If anyone has some left over from their install near South Cambridge it would be awesome if I could buy it off them. I've been using the Wunda clip track, but as long as it is for 16mm pipe and self adhesive it should be OK. It can't have barbs on he back for sticking into insulation. Thanks

So the edge foam was outside the constructional hearth area? I have a inglenook so think anything inside that counts as the constructional hearth area

I'm trying to make it one. The other WBS can have 12mm but one unsure so want constructional hearth

75mm < required 125mm minimum noncombustible depth

It's not fully below slab, but comes to the surface. It will then have a noncombustible 75mm over the top. Looks like Knauf do mineral wool perimeter insulation so that could work but it expect the screed would soak in.

Back wall is brick so not getting plasterboard. I'm supposed to have something between the screed and wall to cope with 'expansion'. Sure it doesn't need to be thick, but should be noncombustible which the foam isn't. Is there something thin and compressible that is? I thought of trying to creat this with wood sheet. 'oilboard' could be an option as has a smooth face, but I have to be certain I can get it out afterwards so I'd rather something noncombustible I can leave in.

I think I need something there for expansion

The depth should be fine, it's round the edge between wall and floor I'm concerned about. I've seen some links that say 125mm depth for constructional hearth and I'll have that. One WBS will definitely be 12mm compatible but not certain of the other.

We've two fireplaces to receive stoves (WBS). Screed going in very soon and finalising UFH in the rooms. Am I right in assuming Cemfloor will be fine here? There will be an additional hearth material laid on top (e.g. 75mm stone).. I have standard expansion perimeter strip elsewhere, but what should I use in the fireplace? It needs to be noncombustible for around the fireplace.

Just a large pile of wood and relocated chassis. It's now a post build project.

In the spirit of over engineering. I could use both. The slate would serve well to spread the load as well as adding stiffness. I'd need something thin and compressively strong between the slate and the HardieBacker. I wonder though if this concentrates the stress on the screed at the end of the slate? Would it want to crack between the two Es of SCREED? Not is there's no movement going on I suppose. Should convince the wife that a slate floor would look nice. ?

Reconsidering slate as I have some on site: Rearranging the equation for Flexural Strength, the fracture force is F = (2 * width * thickness^2 * flexural_strength) / (3 * span). Popping some numbers in: thickness = 0.005m (roofing slate is 5 - 7 mm thick) width = 100mm = 0.1m (complicated this as there is obviously more to the side of the loaded the area, but if there is load spreading under a point load...) span = 100mm = 0.1m flexural strength = 50MPa (various sources state a range of 50 - 80MPa, link, link) This gives a fracture force of F = 833N = 83kgf. Almost identical. Thermally, the thermal conductivity of slate looks to be around 1.6W/mK so 10x more conductive then HardieBacker and being half as thick it will be 5x overall. That's a shame. Despite the worse thermal properties I am tempted by slate as I expect it is stiffer and probably more usual to find in this sort of position. I know that it behaves well with mortar too.

CompacFoam appears to be an option when you have a thicker slab rather than just a screed. Included here for anyone who's interested. https://www.greenbuildingstore.co.uk/products/compacfoam-200/#prettyPhoto

I can't do that unfortunately as my insulation is underneath the concrete subfloor (precast Thermabeam). Looking at the datasheet, HardieBacker looks like a good option. Below is the table relevant to the HardieBacker 500 Cement Board which is 12mm thick. It has pretty high flexural strength, 2x or 3x that or cured screed. Purenit (datasheet) has a compressive strength which is only 4% of HardieBacker, so it's flexural strength will be poorer as well. Halving the thickness of a bending member reduces is stiffness by a factor of 8 and its strength by a factor or 4, so even though the HardieBacker is half the thickness of 25mm Purenit, it would have about 6x the strength. Rearranging the equation for Flexural Strength, the fracture force is F = (2 * width * thickness^2 * flexural_strength) / (3 * span). Popping some numbers in: thickness = 12mm *(100% - 6%) = 0.01128m width = 100mm = 0.1m (complicated this as there is obviously more to the side of the loaded the area, but if there is load spreading under a point load...) span = 100mm = 0.1m flexural strength = 10MPa (this was the stated requirement and the result was higher) This gives a fracture force of F = 850N = 85kgf. Great, that should strong enough! A bloke in high heels can stand on the centre of the cavity and not crack it. Thermally, it has half the thickness of 25mm Purenit and about twice the thermal conductivity, so same result. The Psi-value would probably be something like 0.19W/mK * 0.012m / 0.030m = 0.08W/mK. That's better than most lintels (0.3 to 0.5 W/mK). So if 2.4m long and a 20C temperature difference, it will conduct 0.08W/mK * 2.4m * 20K = 3.8W of heat. I used 30mm as the length of the bridge as that is the distance between the warm screed and opposite cavity edge. It's even thin enough that I can get a nice bed of mortar under it on both sides. Would be cool if you could get it in ~225mm wide strips or ~225mm square tiles, but it should cut easily.

I think I may have enough height for something sensible to bridge the cavity. Not sure what, but the image below shows the structural material in RED bridging the cavity. Some candidates: Purenit, ideally 20mm or less bedded on mortar on the Subfloor and outer leaf brickwork. Slate, plenty of space for slate. It forms a cold bridge but not much of one in comparison to all the people that just bridge the cavity with screed. Hardie backer type cement board A metal sheet ~3mm thick It would be great to be able to use something like wood that could also be cut to form the cill, but I think the rot risk with wood is way too high. Is there a nice structural, hydrophobic but paintable, easily machinable and cheap material that I could use here?

Interesting, so is that an Aluminium/Plastic/Aluminium sash with timber frame? Makes sense to have no sealant past the end of the timber frame. Does leave a bit of a spider home, but spiders are good.

But the cavity closer needs to support the frame. Would be easy if I was just trying to hold the closer itself up. Or do you think the frame would be self supporting enough from the side mountings?

This is getting annoying complicated now. The technical guy from Cavalok is on holiday, but I suspect he'll just say that the cavity closer has to be supported by a brick. Below is the original plan with the frame shown against the cavity closer, though in reality there will be a packed out 5mm gap there. The front face of the window is 54mm behind the face of brickwork. The point of support of the window frame will be the two barbed sections along the interior face of the window (circled in smaller extract below). An option that comes to mind is to use a piston brick to support the cavity closer. This would require the window to move outward and the tile angle to be steepened. It could perhaps be cut from a standard pistol brick or be fully custom. A benefit is it throws in an alternative to the batten that is usually used to get the tile angle. It places the second frame ridge (as you head from the interior) above the brick so Cavalok will probably prefer this. I have shown the arrangement with a standard pistol brick below in ORANGE. If we angled the cut on the standard pistol brick slightly it would make room for the window to move back about 10mm. I have spoken to a brick fabricator and to cut a pistol normally costs £1 each, but with the additional cut it would be more like £1.30. Pistol Brick:

It's Cemfloor Therm which is cement based liquid screed able to walk on after 24-48 hours. I had considered Thermio+ which is an Anhydrite screed, but I wasn't keen on it's durability against water and the latiance which I was quoted £720 to remove. I could have gone thinner with Thermio+ (above UFH pipes: 20mm vs 25mm), but I don't consider that an issue. Similar, but Cemfloor Therm has a better thermal conductivity (2.9 vs 2.5).