MortarThePoint

I have a crane coming on Monday (07/12/20) and we only need it for an hour or two. Does anyone need the other half of the day?

So does that mean if you have Isokern pumice liner and leca infill like I do you can have trusses very close to the chimney's brickwork? That would be good as otherwise I have to solve a small truss that has gone in at 45mm from the outer skin of the chimney. (my chimney is 175mm Isokern, minimum 95mm Leca, blockwork, cavity, brickwork)

I'm not worried about it, that's what worried me 😁

Easy to find Australian guidance suggests up to 50mm out of plumb is acceptable. That's a bit much for my liking, but shows it's not very critical. http://www.hyne.com.au/documents/Feature Documents/TQ_26_TrussInstallation_final.pdf

I wondered what people's expectations were for truss verticality. The chippie has got the trusses roughly set and is fine tuning as he fits the trimmers. Being as I am, I can't help but get a level on it. I wanted to check my expectations though. My feeling is as long as the bubble is just touching or inside the lines it is OK. Trusses are made of wood that can be slightly twisted and warped so better than that feels unrealistic to me. You want to be using a 1800mm level for this. If you like numbers: My level is +/-0.5mm/m accuracy which I think means it is +/-2mm between the lines (https://www.leveldevelopments.com/2020/10/sensitivity-accuracy-of-spirit-level-vials/). 2mm/m (0.1degrees) would give a horizontal deviation of 6mm and a vertical deviation of 0.0mm on a 3m truss. To reach 1mm of vertical deviation the horizontal deviation needs to reach 77mm, or ~25mm/m (1.5 degrees). I expect a roof would actually be fine if the trusses were off by 1.5 degrees or more because the trusses are all braced so it's not so much about the trusses toppling. The profile of the roof isn't substantially affected even at 1.5 degrees. The exceptions to these are at hips, valleys and gables as such deviaton would move the positions of these elements that would then affect the profile of the roof. You'd be asking more out of the battens though with 1.5 degree error as the nominal 600mm could become 600+77+77=754mm so about 25% over what it should be.

I got complements from the brickie so I clearly did an alright job of it. The chippie hasn't turned his nose up at it so that's good too.

Get a drawing that shows the trimmer sizes and ideally the designer can generate the cut list for the infill timbers. The carpenter won't necessarily follow the cut list and the truss company will include extra provision for this, but it is useful to see. Double check the chimneys, >50mm clearance to all combustible materials. Check this on site as trusses are installed.

I did the lap cuts and it ends up being pretty quick and accurate once you get a technique going. A full depth circular saw cut into each side of the timber and then a half depth cross cut on the flat of the timber. Hammer and chisel to remove the chunk and clean up. Top tip: don't bother making bits for the gables. I got carried away and made a short section for one gable by mistake 🙃

I'm leave a 50mm gap between insulation in insulated pitches and the membrane for drape of the membrane (otherwise need counter battens). Does this rafter roll not prevent the required drape? The drape is need to allow water to pass between the outside of the membrane and the battens.

There's some debate on this as I understand it. I spoke to my architect and he said that NHBC now mandate ridge ventilation and that people are starting to doubt the breather membranes and install eave ventilation as well. He speculated that NHBC may mandate eave ventilation in due cause but was going to check LABC and NHBC for details.

What's that rippled black plastic called? I presume it's to create a ventilation gap between insulation and roofing membrane

This is part of the construction drawing. The loft insulation is to overlap the wall plate and cavity insulation.

The loft insulation will come down to the top of the beads, but I was planning to do that afterwards

We've gone the Isokern route. It is quite expensive compared to just a corrugated steel flue liner, but I was told by many that you can't just have a corrugated steel flue liner on a new build. There is a lot to be said for a simple straight run. Cool arrangement you are having with the BBQ. I always liked the idea of a pizza oven built in to the wall with a door in to the kitchen and a door on to the patio. That got forgotten about when the realities of the building arrived, but I still think it would be cool.

If your trusses are to come in two deliveries, get lists of which trusses are to be in each delivery. Don't just rely on a confirmed request, get the actual truss codes coming on the first lorry. You want the girder trusses on the first delivery.