Building Patio for a Glass Pergola

Hi,

I intend to have a glass pergola installed on the back of the house, something like this;

At the moment the back of the house has a step outside the French doors and then a path. The step looks good but the path has the typical slop away from the house and various cracking which indicates that it was simply poured onto grade with no prep. The total rise to the back door is about 20cm.

So I need to build the patio first, and my intention is to just build slightly bigger than the 3x4 m area of the pergola.

I have a few alternatives here;

• get in a ground works team to do the patio (expensive and could introduce considerable delays)
• hire a mini-digger and dig out found to around a meter before shuttering and readymix pour. This is a lot of work and quite expensive to boot and then there is the spoil to haul away.
• Use Screwpiles in place of footings.

The last option is my preferred choice as it is relatively simple and minimally disruptive.

The procedure would be;

• Dig out the topsoil down to about 30cm
• Put in a layer of gravel
• Use 1m long scewpiles, at this length they can be screwed in by hand. They are rated at about 2tonnes per pile. I would use a grid of piles at 1m spacings. I would also attach something like joist hangers along the back wall of the house to help bind the slab onto the house.
• Tie rebar grid between the piles and back to the wall ties.
• Shutter up to the level of top of the step at the back door.
• Pour readymix.

My guestimate is that the slab would load about 300kg per m2 and the glass pergola another 200kg say. That gives the screwpiles a roughly 4x margin of safety on their rated loading of 2tonnes and a lot of the loads would in fact be taken by the gravel grade meaning that the piles would be well within their specification.

Obviously since the glass pergola has sliding glass doors, any slip in the slab would be an absolute catastrophe with a high probability of shattered panes.

What do you think ?

FJMC

Difficult to comment on this as much depends on the ground, existing levels, previous excavation, drains, services, etc.

You will have to excavate about 400mm anyway to get in your base, sub-floor, insulation, etc. for a solid floor - a little extra around the perimeter for conventional foundations or a thickening of an RC raft type slab shouldn't be that much extra work?

Mindful of fully glazed extensions and how useful they are - and you are supposed to retain external patio doors between the extension and the main dwelling if you are not getting a good thermal performance from the extension. A garden room design with a mixture of walls and glazing can often be a lot more attractive in design and be a lot more useful as a room throughout the year?

My understanding is that the screwpiles are normally used for framed structures - floors and wall - so may have an impact on your design choices for the floor and for the walls, etc. of the extension.

F

https://www.boards.ie/discussion/comment/121706353#Comment_121706353

I originally intended to build a SIP based extension in the same location, but light into the existing patio doors put me off that idea. We built an extension onto the back of our previous house and it turned the living room into a cave. Not a mistake I want to repeat. Also there is only the two of us in a three bed semi so we really do not need any sort of an extension for extra space - the glass box offers other services.

The patio doors are newly installed triple glazed French doors so there is no issue with the level of insulation they offer. The idea is that in spring and autumn the closed up glass pergola will provide most of the heating for the house by leaving the French door open but having the sliding doors of the pergola closed. In the summer the sliding doors will be open and the patio will be open into the covered patio. We also have MVHR which will extract the incoming air from the glazed box so that it preheats the incoming air in spring and autumn.

I could just go with screw piles and PVC patio decking for an easy life - but I was hoping for a bit of thermal mass from the slab.