Hello!
I am sketching out our patio. It will be 6 * 3.90 meters.
I plan to use a plastic roof like JärboLite opal, and according to the description, you should use 45*145 mm for the roof beam and 45*195 mm for the roof rafters. We live in Södermanland, snow zone 1.5 - 2.0.
The problem is that the dimension of the roof rafters is too high for my house to fit.
What do you think about reducing it to 45*170 mm for the roof rafters?
Will it hold, or do I need to reduce the depth of the patio?
Does anyone have experience with this type of roof?
Best regards, Magnus
I am sketching out our patio. It will be 6 * 3.90 meters.
I plan to use a plastic roof like JärboLite opal, and according to the description, you should use 45*145 mm for the roof beam and 45*195 mm for the roof rafters. We live in Södermanland, snow zone 1.5 - 2.0.
The problem is that the dimension of the roof rafters is too high for my house to fit.
What do you think about reducing it to 45*170 mm for the roof rafters?
Will it hold, or do I need to reduce the depth of the patio?
Does anyone have experience with this type of roof?
Best regards, Magnus
Hello again!
As mentioned, it will be tight to fit 45*195 trusses since the distance to the gutter is about 12 cm.
Is it possible to use construction brackets like fork brackets and hang the trusses in them? I was thinking of attaching the beam to the eaves of the house.
An alternative is to place the trusses against the house wall under the roof overhang, but then I lose height on the patio roof.
What should I do???
Feel free to give suggestions.
See the picture of the roof construction over the house and the overall view of the house with measurements.
As mentioned, it will be tight to fit 45*195 trusses since the distance to the gutter is about 12 cm.
Is it possible to use construction brackets like fork brackets and hang the trusses in them? I was thinking of attaching the beam to the eaves of the house.
An alternative is to place the trusses against the house wall under the roof overhang, but then I lose height on the patio roof.
What should I do???
Feel free to give suggestions.
See the picture of the roof construction over the house and the overall view of the house with measurements.
Hey. You can go down to 45X 145 if you add a bit of slope to it, like, integrate it into the existing roof. I see you have a hipped roof, with a good slope, so you could make a nice transition if you want.... I would think there will be a lot of pressure against your patio roof otherwise if you place the roof lower... It's a bit more work to attach the rafters to the existing structure, but in my opinion, it's worth it...
Hello!
Yes, the option to attach the roof underneath is probably not an option with a depth of 3.90 on the terrace. The height becomes so low, if you want to have an acceptable slope on the roof, that you would probably have to crawl onto the terrace.
Your suggestion to attach the terrace's rafters with the house's sounds good, but how do you go about it?
(I'm including a sketch of the terrace as we want it)
Yes, the option to attach the roof underneath is probably not an option with a depth of 3.90 on the terrace. The height becomes so low, if you want to have an acceptable slope on the roof, that you would probably have to crawl onto the terrace.
Your suggestion to attach the terrace's rafters with the house's sounds good, but how do you go about it?
(I'm including a sketch of the terrace as we want it)
Yes, you can reduce the size if you place the trusses closer together.
Yes, you can reduce the size if you choose a wood with higher tensile strength.
But you can also hang them with a notch at the top, so that at least half of the beam of the 195 in height remains to hang under the existing trusses with a plj. on each side and through bolts (or nail plates and anchor nails p.b.s.) in both the existing truss and the new trusses.
The new beams will experience different material stresses. They are considered as simply supported 2-span beams with evenly distributed load (in this case). They are exposed to a bending moment which is greatest in the middle and results in compression at the top and tension at the bottom. At the ends, the moment is zero. In between, it decreases (parabolically). The sizing is done for the maximum bending moment (tensile and compressive forces in the material).
Out at the ends, however, there is a shear force that is half of what each beam carries along its entire length. A shear force arises, which is managed with a smaller dimension than a bending moment, provided that the support/suspension can handle it, and the shear force does not come too far 'to the side of' the suspension (then a new moment is introduced). Say a maximum of 25 cm into the paw of the existing truss.
So you have three options to manage it. There are more, but these are the simplest.
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Builder
Yes, you can reduce the size if you choose a wood with higher tensile strength.
But you can also hang them with a notch at the top, so that at least half of the beam of the 195 in height remains to hang under the existing trusses with a plj. on each side and through bolts (or nail plates and anchor nails p.b.s.) in both the existing truss and the new trusses.
The new beams will experience different material stresses. They are considered as simply supported 2-span beams with evenly distributed load (in this case). They are exposed to a bending moment which is greatest in the middle and results in compression at the top and tension at the bottom. At the ends, the moment is zero. In between, it decreases (parabolically). The sizing is done for the maximum bending moment (tensile and compressive forces in the material).
Out at the ends, however, there is a shear force that is half of what each beam carries along its entire length. A shear force arises, which is managed with a smaller dimension than a bending moment, provided that the support/suspension can handle it, and the shear force does not come too far 'to the side of' the suspension (then a new moment is introduced). Say a maximum of 25 cm into the paw of the existing truss.
So you have three options to manage it. There are more, but these are the simplest.
______________
Builder
To simplify it a bit, you do it roughly like this... Tear off the roof tiles to an appropriate height, cut into the roof battens... (The boarding, that is) about 3 boards' height the entire length of the deck plus to the nearest rafter extra.... Place the removed battens there... a 45X 145, along with all relevant rafters... Then attach the new deck roof rafters to it.... I would have built them loose on site, not pre-made.... If possible, then add some anchors in the form of supports under the new beams.... All of this may sound advanced, but it will be nice and safe for snow and water for many years to come...
Ok! Thanks for the answers!
Now I've got something to think about. I'm not a carpentry pro, so I will probably try to make it as simple as possible if the whole summer vacation isn't going to be consumed.
The supplier's recommendations for the plastic roof, to have 45*195 for rafters, are based on a c/c measurement of 990 to 1200 mm. If I reduce to c/c 60, I should be able to reduce the dimension, but by how much?
Is there an easier table for roofing dimensioning?
Now I've got something to think about. I'm not a carpentry pro, so I will probably try to make it as simple as possible if the whole summer vacation isn't going to be consumed.
The supplier's recommendations for the plastic roof, to have 45*195 for rafters, are based on a c/c measurement of 990 to 1200 mm. If I reduce to c/c 60, I should be able to reduce the dimension, but by how much?
Is there an easier table for roofing dimensioning?
Hello again!
I've thought a bit more about your suggestions and I think the simplest solution is to hang the rafters on the house's existing rafters as "Byggaren" suggested. Making notches in the 195 beams halfway makes it fit perfectly with the other measurements.
I have enclosed eaves, and I understand that I need to remove some of the cladding to access the rafters, but how do you restore it neatly after installing the rafters?
If anyone has done something similar, please post pictures of it. As I said, I am an amateur and need some tips from those of you who know about this! and maybe there are others who have the same questions...
I've thought a bit more about your suggestions and I think the simplest solution is to hang the rafters on the house's existing rafters as "Byggaren" suggested. Making notches in the 195 beams halfway makes it fit perfectly with the other measurements.
I have enclosed eaves, and I understand that I need to remove some of the cladding to access the rafters, but how do you restore it neatly after installing the rafters?
If anyone has done something similar, please post pictures of it. As I said, I am an amateur and need some tips from those of you who know about this!
and maybe there are others who have the same questions...Nail a batten (22x45) on the inside of the roof trusses and nail up the eaves boards into it. You will need to cut out the existing eaves boards as needed for the new roof trusses. Therefore, take down the eaves boards whole and put them back up afterward with cutouts.lma said:
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Builder
OK!
That's how it will be. Thanks for the help!!!
Which is preferable, attaching rafters with a foundation iron on each side and a through bolt (or nail plates and anchor nails p.b.s.) on both the existing rafter and the new rafters?
Are anchor screws better than anchor nails?
That's how it will be. Thanks for the help!!!
Which is preferable, attaching rafters with a foundation iron on each side and a through bolt (or nail plates and anchor nails p.b.s.) on both the existing rafter and the new rafters?
Are anchor screws better than anchor nails?
If you make the hole for the through-bolt as high up on the paw under the under-roof panel and ditto as low as possible on the lower edge of the new beams, you will have maximum strength. The risk of shearing and crushing in the wood is then minimized because you have the most wood left in this way. It's about a pure shear force with a plj (flat iron) p.b.s. Bolt dimension preferably M10 or M12.lma said:
Nail plates with pre-drilled holes and anchor nails spread the load/shear force from several smaller nails with smaller contact areas in a single-shear joint instead of a bolt with a larger contact area and double-shear joint. This gives the bolt an advantage with the double-shear since the load becomes symmetrical. Theoretically, an anchor nail could be pulled out if the load per shear/nail becomes too large because it becomes eccentric, but if anchored with nails for the intended load, this risk is very marginal.
Anchor screw (which is screwed in, not driven in with an American screwdriver=hammer) is always better than anchor nail because there is no crushing of the fibers in the wood as with the driving in of anchor nails.
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The Builder
It is always the wood that is the weaker link in the chain when it comes to joints. Nails, screws, bolts, etc. have greater toughness and strength. Therefore, they are not dimensioning in the way that the wood becomes where compressive stresses from shear forces become dimensioning. This is, among other things, why you nail several nails to distribute the pressure. Not because the nail itself is the weak link.lma said:Thanks for many wise and thorough answers!
I am using M12 and flat iron.
You think an M12 bolt at the top & bottom should handle the load, adding more would weaken the wood's strength, I suppose.
If we assume that your trusses are 4.5 cm thick, we have: 1.2 x 4.5 = 5.4 cm^2 since an M12 bolt has a bearing surface = diameter 12 mm. Steel withstands pressure up to about 1470kp/cm^2 in normal cases. But wood can handle no more than approximately 20 kp/cm^2 (depending on wood quality) across the fibers. Then the M12 can theoretically carry 1.2x4.5x20=108kp, which constitutes half of the load you have on a beam. If you have a greater load, you will need to increase the bearing surface by using more bolts (or larger ones). Just make sure they preferably end up above the middle of the truss heel and not directly under each other.
Now an M12 carries far more than 108kp. (The value for the allowable stress in the wood, across the fibers, is taken from the building code that was in effect from 1980 until 1992, which may be around the decade when the house was built?) Considering that the values stated in today's building codes must be reduced with various partial coefficients to an allowable stress, it suffices as a guideline. You can also increase the 1980's standard by a factor of 1.4 for exceptional loads, i.e., if there should be drift snow on the roof or combined snow/wind loads.
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Byggaren