Replace some plinths with a beam. Dimension?

[Lasse Morin]

I am going to extend my summer cottage and need some good advice. The cottage is situated on a rocky outcrop and the extension will span a wide crack in the rock. I want to lay beams over the "ditch" instead of pouring and building pier foundations. I initially considered casting concrete beams but prefer to work with wood. How sturdy should the lumber be?

Would it be sufficient to use the same dimensions as in the roof? It's a single-pitch roof and the span is 3 meters, about the same as the "ditch" at its widest point. An argument against that idea could be that the roof is loaded more evenly (by snow) than the foundation. Someone might decide to put a piano on the floor.

A little background: I bought a kit for a garden shed and plan to place it one meter from the existing house. The space in between will be filled with closets and a passage to the existing house.

Those who have seen the cheapest garden sheds at building markets know that they are made with flimsy wood in the walls and roof. But this kit is special. The designer intended for a terrace on the roof. The slope is very slight (to be adjusted). A spiral steel staircase for the roof was also sold (I didn't buy it).

The roof should withstand 2 tons and the walls are constructed with almost as sturdy beams as the existing house.



The pier plan that came with the kit assumes 15 piers, some of which go where the ditch is (diagonally, from corner to corner of the extension). The plan suggests carrying beams of 100x100 mm on the piers, which are not included in the kit, with the reasoning that a full foundation slab could be poured instead.

Pouring for piers at the bottom of the ditch, with cracks in the rock, is not appealing. I have had blasters widen the crack (for drainage) and I don’t know if I can reach crack-free bedrock.

I want the floor to rest on stable structures. In the existing house, I can sit on the sofa and stomp on the floor if I want to hear the china rattle in the kitchen cabinets.

The image shows approximately how the foundation looks, with the piers and pier replacements marked. In the upper left corner, the pier replacement extends outside the house due to the shape of the ditch, which is a pure advantage because there will be a small deck there. At the opposite end, the ditch runs under the existing house and widens so that I have standing height under about a quarter of the cottage.

The ditch continues outside the extension with a suitable slope for drainage. It is about one meter deep.

So the question is: how should I dimension it?

 

[Dowser4711]

It should work excellently, with the right dimensions, someone else can calculate that. However, in that situation, I would probably have used beams straight off and reduced the number of posts, a bit more expensive but more enjoyable work and simpler with uniformity. But, keep in mind that you've drawn one of the beams outside the building structure. You'll need to provide some weather protection for it, partly so it’s not exposed to moisture, etc., but mainly so it doesn't lead rain into the building structure.

 

[Lasse Morin]

But, keep in mind that you have drawn one of the beams outside the house structure. You need to arrange some weather protection for it, partly so that it is not exposed to moisture itself, but mainly so that it doesn't channel rain under the house structure.

Yes, I am thinking about that too. I'll solve it when I build the deck.