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Worth making sure its under 15m2 internal floor area also, so an not to need to substantially non-combustible. (i.e. building regs).
Garden Room - Structural Design Advice
- Thread starter efunc
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I wish. It's 3 x 2.6m. So less than 8m2. This is a pretty modest build.
So, thanks to the advice in this thread I've tweaked the design a little bit to place a heavier 4x2 headplate orientated widthways forward at the front to support the rafters. There will be a second one behind it glued and bolted together. I've also tried to implement steel straps onto the right hand wall, as advised. Is this what you mean, or am I barking up the wrong tree?? Any feedback would be great:
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IMO that looks fine.
One small point - you might be better with 'L' straps. These are about 1m long and have a short leg at the end at 90 deg. about 100mm long. These could be hooked over the top of the end beam, and provide additional restraint, rather than just relying on the upward pull on the screws.
You would saw off the projecting 50mm where the hook overhangs the outer face of the beam.
One small point - you might be better with 'L' straps. These are about 1m long and have a short leg at the end at 90 deg. about 100mm long. These could be hooked over the top of the end beam, and provide additional restraint, rather than just relying on the upward pull on the screws.
You would saw off the projecting 50mm where the hook overhangs the outer face of the beam.
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Well yeah, it was a quick sketch really just to check if I'd understood the concept correctly, but a rush job, not to scale. I don't really want half my build cost to go on 4 strips of steel. Here is something proportioned a little better:
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I have some follow up questions now that the groundwork has been done. The builder has built one single course of engineering bricks for the plinth although I specified three courses. He says 1 course above the ground level is sufficient since the vertical cladding will shield the sole plate from rain splashback on the outside anyway. Is this correct? That puts the timber only 7cm off the ground. Also I thought cladding should be raised at least a couple of cms off the ground to prevent water getting to it by capillary action and debris accumulating around the base.
If I choose to do a 2 course plinth instead that would put the sole plate up at 15cm off the ground. My question is regarding door thresholds and the windows on my design. Is it permissible to sit the door directly on the plinth so that they're not too high up and then build the studwork up around them? ie the sole plate would terminate at the doors/windows. The frames would attach to the timber frame on the sides and the top but they would be positioned directly on the plinth. Is this OK to do, or is there a design problem with this?
If I choose to do a 2 course plinth instead that would put the sole plate up at 15cm off the ground. My question is regarding door thresholds and the windows on my design. Is it permissible to sit the door directly on the plinth so that they're not too high up and then build the studwork up around them? ie the sole plate would terminate at the doors/windows. The frames would attach to the timber frame on the sides and the top but they would be positioned directly on the plinth. Is this OK to do, or is there a design problem with this?
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Well, I'm back! And I have another question, but not quite sure if this should go in this part of the forum, the roofing section or the Planning/Building Regs bit. Perhaps most sensible in here. I've now updated my design again taking on advice gained here:
Principally I've changed the footings to 1 course of bricks under the doors/windows so there's not such a big step up, and 2 courses of bricks everywhere else. I've replaced the head plate with a 6x2 beam (150mm wide). I've added in the firrings for a 1:40 fall to the front, which I've set at 60mm at the rear, falling to 1mm at the front. The overall height from the base of the bricks now makes it 249cm at the back, but with the 18mm OSB decking and GRP that would increase to about 252cm. The problem is that this doesn't factor in the GPR roof raised edging which would run down the right and left sides of the roof. It also doesn't include any lead flashing chased into the wall above at the back.
What are my options for keeping it within the permitted development restrictions?
I could reduce the door frame height from 202cm to 200cm of course and gain 2cm height at the front. The 6x2 roof joists are going to use a rear wall for support at the back, and that's 227cm high, so fairly fixed. However, is it possible for me to lower the front by 2cm or more so the joists pitch forward? Then I can also use shallower firrings to achieve the same 1:40 fall. Or even make do with a 1:60 fall?
Any advice would be welcome as I presume this is a common dilemma and there are probably far cleverer solutions!
Thanks all.
Principally I've changed the footings to 1 course of bricks under the doors/windows so there's not such a big step up, and 2 courses of bricks everywhere else. I've replaced the head plate with a 6x2 beam (150mm wide). I've added in the firrings for a 1:40 fall to the front, which I've set at 60mm at the rear, falling to 1mm at the front. The overall height from the base of the bricks now makes it 249cm at the back, but with the 18mm OSB decking and GRP that would increase to about 252cm. The problem is that this doesn't factor in the GPR roof raised edging which would run down the right and left sides of the roof. It also doesn't include any lead flashing chased into the wall above at the back.
What are my options for keeping it within the permitted development restrictions?
I could reduce the door frame height from 202cm to 200cm of course and gain 2cm height at the front. The 6x2 roof joists are going to use a rear wall for support at the back, and that's 227cm high, so fairly fixed. However, is it possible for me to lower the front by 2cm or more so the joists pitch forward? Then I can also use shallower firrings to achieve the same 1:40 fall. Or even make do with a 1:60 fall?
Any advice would be welcome as I presume this is a common dilemma and there are probably far cleverer solutions!
Thanks all.
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Right, I lowered the height by 4cm now just to be safe, so it should now be fully compliant!
Quick question regarding insulation: The 4x2 timber being used for studs looks like it's roughly between 85 - 90mm deep. The roof joists are about 150mm deep. So in terms of PIR boards what should I be looking for? 90mm would be a very snug fit for the walls, but presumably I need to leave some space for airflow and even wiring, etc? What then is a suitable thickness that achieves good insulation, but leaves adequate space too? 70, 80mm, or less?
Also, for the roof, I need to allow airflow and also room for the LED spotlights. These are Integral EvoFire Fire Rated Downlight with Insulation guards on top which have a depth of 83mm. I guess they're more designed to push up into rockwool type insulation rather than rigid board. So should the PIR thickness be 150mm - 83mm = 70mm? That's relatively thin. Or is it better to go for up to 100mm and just cut bits out to provide clearance for lights where needed?
Also, is PIR more or less all the same? ie is it OK to use the same stuff in the floor, roof and walls?
Quick question regarding insulation: The 4x2 timber being used for studs looks like it's roughly between 85 - 90mm deep. The roof joists are about 150mm deep. So in terms of PIR boards what should I be looking for? 90mm would be a very snug fit for the walls, but presumably I need to leave some space for airflow and even wiring, etc? What then is a suitable thickness that achieves good insulation, but leaves adequate space too? 70, 80mm, or less?
Also, for the roof, I need to allow airflow and also room for the LED spotlights. These are Integral EvoFire Fire Rated Downlight with Insulation guards on top which have a depth of 83mm. I guess they're more designed to push up into rockwool type insulation rather than rigid board. So should the PIR thickness be 150mm - 83mm = 70mm? That's relatively thin. Or is it better to go for up to 100mm and just cut bits out to provide clearance for lights where needed?
Also, is PIR more or less all the same? ie is it OK to use the same stuff in the floor, roof and walls?
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4x2 CLS timber would normally be 96mm or thereabouts so yours is a bit thinner. 100mm is usually cheaper than 90mm due to it being a more common size, (likewise 50mm is cheaper per m3 than similar thicknesses)
You would need space on the cold side for draining any condensation but on the warm side it's up to you whether you want a service void for wiring or not. Channeling out a trough in the insulation is acceptable as long as you have the cable in contact with a timber/plasterboard as well, or take into account when sizing the cable.
The insulation is linear in terms of thermal resistance, even 50mm is pretty good, after that it's diminising returns.
PIR is more or less all the same. Most of it is foil faced.
You would need space on the cold side for draining any condensation but on the warm side it's up to you whether you want a service void for wiring or not. Channeling out a trough in the insulation is acceptable as long as you have the cable in contact with a timber/plasterboard as well, or take into account when sizing the cable.
The insulation is linear in terms of thermal resistance, even 50mm is pretty good, after that it's diminising returns.
PIR is more or less all the same. Most of it is foil faced.
I see, thanks John. So let me get my head around this. If I need space on the cold side is 10-20mm enough? And can the cable runs go on this side and then just poke through a hole to a socket or switch on the warm side where needed?
Regarding the cold roof, is rockwool more or less as good as PIR, or far less effective? I already have a lot of rockwool and can stuff a reasonable amount in the 150mm void, whereas there wouldn't be much more than 50mm of PIR given the need to allow for light fixtures and airflow. Thanks.
Regarding the cold roof, is rockwool more or less as good as PIR, or far less effective? I already have a lot of rockwool and can stuff a reasonable amount in the 150mm void, whereas there wouldn't be much more than 50mm of PIR given the need to allow for light fixtures and airflow. Thanks.
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space on the cold side you would have a breathable membrane and then only enough space to avoid trapping moisture. I'm not sure the manufadcturer's recommendation but I'm sure 10-15mm would be enough.
rockwool is about half as good as PIR for the same thickness. but it's less than half the price, so rockwool is more economical if you have the space. Stuffing more in a fixed space doesn't actually help, as it's the air gaps that do the insulating, not the wool itself. If you need a gap for airflow with PIR, you're likely to need the gap for wool as well.
rockwool is about half as good as PIR for the same thickness. but it's less than half the price, so rockwool is more economical if you have the space. Stuffing more in a fixed space doesn't actually help, as it's the air gaps that do the insulating, not the wool itself. If you need a gap for airflow with PIR, you're likely to need the gap for wool as well.
That different door and threshold level looks odd and wrong.
You're better off having the floor level at ground level and keep the door sizes to what they should be. 2m doors are crap, as will be that low ceiling.
You'll just need to detail the DPM/DPC accoringinly, and to keep a minimum 150mm (preferably 225mm) high brick plinth to keep the cladding out of the splash zone at ground level.
You're better off having the floor level at ground level and keep the door sizes to what they should be. 2m doors are crap, as will be that low ceiling.
You'll just need to detail the DPM/DPC accoringinly, and to keep a minimum 150mm (preferably 225mm) high brick plinth to keep the cladding out of the splash zone at ground level.
OK, another quick question. My framing is made up of 4x2 (38x89mm) timbers I believe. I'm purchasing 75mm Kingspan PIR for walls and floor and 100mm for the roof. When it comes to taping up the seams how wide do you usually need to go? Is 75mm tape enough, or do I really want 100mm tape to adequately span the timbers and tape across the PIR edges?
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