Before I forget again

I forgot to post some of the photos Ric took in his last week.

He just about got the east wall finished, but had a mishap with our only pointing trowel, which we use to rake out mortar. So it looks a bit gap-toothy at the moment, we will finish this off when we get up in September and get the raised wallhead blocked out – probably a days work, weather permitting. Meanwhile we are well pleased with how the stonework is turnng out!

One thing that puzzles us: Why is our rubble wall so pale compared to the untouched wall, when it is built from the stone we took out of the walls in the first place? Obviously the mortar is new, but not the stone. Answers on a postcard, please.

He got two more thresholds installed on doors opening onto the courtyard…

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He built up stonework to sill height in the last two window openings in the north wing, that started life as doors. So all 5 window openings are ready for sills, quoins and lintels. None of them will have stonework above the lintels.

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He heroically got all the openings with blockwork synthaprufed.

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He also carried on with his research into how to convert our piles of spoil into graded sand that we can use in our raised drainage mound.

Progress on roof trusses

We make progress. Stevenson & Kelly have designed the basic attic truss profile for the east wing and the raised-tie truss profile for the north wing, to the most accurate dimensions I have.
The attic truss has a 190mm bearing width either side of the upright timbers, this should give us enough space for a wallplate to bear fully on the wallhead. Where the wallhead goes outside the 195mm, we will either trim the blockwork back or can notch the rafters. So we are good to go. Each truss will weigh just short of 100kg, handling them will be a beast. On the plus side, the design allows the attic room to be 4m wide, 2.2m high and with the ceiling slope starting 1.4m above floor level, much better than I had expected.
The raised-tie truss has the top collar (the ceiling level) almost 3m wide and 900mm above the wallplate. The latter is 2.7m above finished floor level, so the north wing should be around 4.1m wide and 3.6m high.
We have asked S&K to design the full set of profiles (to accommodate our stairs and larger roof lights) and provide us the quote. At this moment, roof trusses in September look to be a thing!

Rethink on roof trusses

We need to order roof trusses now for the east and north wings, in time for September, and have changed our minds again. We will use attic trusses in our east and west wings, not the raised tie trusses the architect specified.

A quick history lesson: Our architect specified raised-tie trusses. The structural architect added extra bracing from each truss down onto the floor joists, because of the extra height we need in the upper floor. Reading around, this is because it is not practical to design trusses with the raised ties lifting the roof by more than a metre – where we need around 1.3m of extra height. Building Standards signed-off the modified design.
The truss designer said no and designed attic trusses. The architect had words with him and I thought he had relented and could design them.

However on further investigation, neither of our local truss manufacturers will design trusses that are not fully self-supporting, since they fear they would be blamed if they were installed as per the plans, but failed. I have some sympathy for them, but it left us in a bit of a bind. So we have just had a mad scramble to come up with attic trusses that work – without being able to get onsite to check & confirm measurements.

The least disruptive solution, for us, looks to be where:

1. The attic trusses bear on the wallheads, rather than be supported on timberwork underneath. So the timbers that form the attic walls and floor will hang down between the steading walls. We need studwork underneath anyway, to hold our wall insulation, and we can brace the trusses against them. But if they also had to bear the weight of the roof, we would a) have to go back to the structural engineer to check whether the thickened floor slab was sufficient foundation and b) would need 200mm lintels across our pre-existing external doorways, lifting the ceiling by that amount. This would be very bad for the attic room.
2. We allow 80mm gaps between the steading walls and the truss timbers, both sides, because the walls are not flat. I took 7 measurements between the walls at different heights and they generally slope outwards as they go up. The west wall also has a measurable bend outwards along its length. However, at least one piece of our concrete blockwork on both walls sticks proud of the rubble by up to 60mm. I really do not want to be trimming the blockwork and any stray lumps and bumps of granite just to squeeze the trusses in place. The earlier truss design gave us a comfortable 3.6m of width in the attic room.
3. We use 150mm solid foam roof insulation, in place of the specified 280mm frametherm panels, because the rafters cannot be more than 220mm deep and we must have a 50mm air gap above the insulation. The strength grade of timber used for trusses, T26 (or TR26) does not come bigger than 47 x 220mm and the truss designers really are not interested in composite rafters i.e. two lengths of timber, one on top of the other. This saves us the cost of extra timber on the trusses, but the premium on the insulation probably outweighs this: We will have to go for higher spec insulation, with a lambda value of 0.018 rather than the usual 0.022. The equivalent value for the frametherm is 0.032.

The one advantage of attic trusses, for us, is that it will be easier to support the trusses over the gap where the north wing joins the east wing. The foundation is inside of the line of the wallheads and we had worried about making sure that the raised tie trusses took this into account. Instead, with the 80mm airgap, we can simply support the trusses from underneath.

And we do not need to change the design of the trusses on the north wing. They stay as raised tie trusses, because the bottom chord (top collar) is low enough to keep them stable.

We now wait to see if what we have asked for is possible. And I will be chewing my finger nails until I know that my measurements are right and the trusses fit properly. We really do not want to be rebuilding our walls to fit the trusses!

Meanwhile, here is an earlier incarnation of our attic trusses…

And here is the equivalent rasied tie truss, for the north wing…