Fork anchors and load

[Torbjörn Lindholm]

Currently expanding the house. It will be 10x3.7 meters along one side of the house. The roof slopes away from the existing house. Roof beams 42x225 laminated wood and sheet metal roofing insulation and roof plasterboard, etc. I've attached a ledger to the existing wall where the rafters will rest. I've heard that fork anchors should be sufficient for the purpose, but once I bought them, I was struck by how flimsy they look. Will they really be able to handle this? I live outside Örebro, so it's snow zone 2.5.

 

[justusandersson]

Google "Beslagshandbok_2012_Gunnebo_Fastening.pdf" for all the information you need. A fork anchor can handle a lifting force of between 1.6 and 7 kN (approximately 160 and 700 kg) depending on the type, attachment, and duration of the load.

The largest lifting force you're likely to encounter is about 50 kg per square meter, which occurs when the wind speed is 30 m/s. A 37 sqm roof area corresponds to approximately 18.5 kN. Divide this number by the number of fork anchors you plan to use, and you can calculate the appropriate size for them yourself.

 

[Torbjörn Lindholm]

Google "Beslagshandbok_2012_Gunnebo_Fastening.pdf" where you will find all the information you need. A gaffelankare can handle a lifting force of between 1.6 and 7 kN (approximately 160 and 700 kg) depending on type, attachment, and load duration.

The highest lifting force you might encounter is about 50 kg per square meter, but this occurs at wind speeds of 30 m/s. 37 sqm area on the roof corresponds to approximately 18.5 kN. Divide this number by the number of gaffelankare you will use, and you can calculate the suitable dimension for them yourself

Thanks for the answer! So the biggest risk is the lifting force, right? That they should be able to hold down the roof rafters and resist the force of the roof and potential snow is less of a concern? I became unsure because when I tried to attach a truss, I realized that the design of the gaffelankare means the roof rafter does not rest against any part of the horizontal beam attached to the house, but only in the gaffelankare.

 

[Nötegårdsgubben]

It's called an anchor precisely because it is supposed to keep the roof in place if it is on its way somewhere (likely due to the uplifting force of the wind). The downward force should be absorbed by the truss/fastening in the house, so if the truss does not rest directly on the beam that is supposed to support it, it is done incorrectly. On the other hand, it is likely to be pushed down if the force becomes large. How much does it differ?

 

[justusandersson]

Almost all severe storm damage to houses occurs due to lift forces. Similarly, damage related to powerful explosions. It is when the shock wave retracts that the sheet metal roofs disappear. Lift forces due to wind are usually greatest on the leeward side. Furthermore, I agree with what Nötegårdsgubben writes about trusses.

 

[Torbjörn Lindholm]

It's called an anchor to keep the roof in place if it's heading somewhere (likely due to wind lift). The downward force should be taken by the roof truss/attachment to the house, so if the roof truss doesn't rest directly on the beam that's supposed to support it, then it's done incorrectly. On the other hand, it will likely be pressed down if the force becomes great. How much is the difference?

Thanks for the answer! The difference is 3.5 cm if I remember correctly. So, is it better to skip the anchor on the beam in the house and only have anchors in the outer wall of the extension?

The beam attached to the house has a contact surface of 4.5 cm (it's mounted on edge), so would it be sufficient to let the roof beam rest on that beam and fix it with some other type of bracket?

 

[Torbjörn Lindholm]

It's called an anchor precisely because it's supposed to hold the roof in place if it's about to go somewhere (likely due to the lift force of the wind). The downward force should be taken by the truss/attachment in the house, so if the truss doesn't rest directly on the beam that is supposed to support it, then it's done incorrectly. On the other hand, it will probably be pushed down if the force becomes large. How much does it differ?

Is it a ridge beam bracket (or two then, one right and one left) that is needed for each roof beam? And is a 170x45 beam sufficient as a bearer for them to rest on?

 

[Nötegårdsgubben]

I don't understand why it doesn't fit, or maybe what's not fitting. Place the roof rule on the wall mounts. Then nail the anchor with the fork upwards and the plate against the wall rule. But yes, if the forks don't fit, you can use roof beam brackets instead, they essentially do the same thing.

I'm not an expert in strength of materials, but it sounds like 170 should be more than enough when fastened to the wall. Just don't attach it too sparsely.