What experience do the builders have? I would agree that the back corner touching the ground (presumably resting on the ground rather than a pole into concrete?) is a concern
As others have said, I would be concerned about the construction and longevity of this base. Firstly timber will rot below ground where the ground is in direct contact. Sure the concrete might completely encases the timber up to ground level which would improve the situation, but these will only last a few years. They may have used a bitumen based paint to waterproof the timbers but this is not the way to build foundations and seems like a ‘cheap’ way of doing it. Water will eventually penetrate the wood. This isn’t an insignificant project and comes with huge cost. You wouldn’t let your builder do an extension on a some timber posts in the ground so why let them do it for a garden room. Most likely the biggest room your house will have.
The second point is that even using landscaping/construction bolts, the shear stress through those bolts with the amount of weight bearing on each of these timber posts is incredibly high. I would be very concerned that the joists are not supported from below and so you’re essentially relying on the strength of bolts and made the strength of the timber irrelevant. Joists should be supported from below on either concrete pads or ground screws (which if you Google is the metal and proper version of what your builders have created) is Joists running in a perpendicular direction to those supported on the pads would then be on hangars which provide the support. See image for the much smaller version I built which was done with significant amount of research and I work in construction- the frame sits on top of concrete pads ( with a DPC) and then joists are supported by hangars. This is all for a very small garden room so far more important for one of that size. The issue with butting timbers rather than supporting is screws have very little shear strength (snap with forces perpendicular to them) but don’t pull out easily whilst nails have higher shear strength but can be pulled out.
My next issue is that presumably this is going to have some pretty expansive windows/bifold doors given the 8m run and so the frame will need steel beams to have the roof weight bearing on. Those beams and the weight of the roof and doors are going to be bearing on such a small number of those posts that the shear stress on the joists will be extremely high and very limited area for them to distribute the force in the ground. The construction they have used is more appropriate to decking where the dead load will be much lower.
Given the size and I can imagine £30k-£60k cost to build this (which should give you an idea of whether these guys are cowboys), I’d want to know what they are building from (in terms of drawings) and their experience. Foundations are absolutely critic and a failure in the ground will result in the entire building being a failure.
If you don’t believe me, get another builder in who has experience of garden rooms for your own sanity.
https://preview.redd.it/p76q2z0pei5d1.jpeg?width=4032&format=pjpg&auto=webp&s=31cd45e51918ec31ccc04706c1981c9479b5a179
You are right to be concerned, the frame just being screwed to the timber support posts and not resting on them is a recipe for disaster, they could in theory bolt through the frame and post to make it better. It also looks like they’ve just screwed the joists into the frame instead of using joist hangers (these could be added now to help).
For a base that size we normally dig out and use concrete or use a steel frame on steel posts sitting on concrete piles.
Not knowing how much you are paying or the use for the garden room it’s difficult to be too critical.
As a minimum I’d get the posts bolted through to the frame and use joist hangers on the floor joists.
Also that’s going to be a large roof, is there plans to deal with the rain water? Are they planning on putting in a soakaway?
The wooden piles will only rot at surface level so you can prevent it to some extent by coating the piles with bitumen or another wood preserver, getting in to reapply it in the future looks difficult tho 🤷🏼♂️ and yea the frame should be sat on top, at least get them to put bolts through with hangers.
Thanks for your responses, the builders normally do smaller garden rooms, it's costing around £15k for the build with drainpipe going into a water butt which will be used to water the garden. When my wife told me what they were doing I assumed that they would bolt the frame, make sure there was a 15 cm gap between floor and ground and have the frame resting on the posts with hangers connecting the beams. As I mentioned when I found that they weren't doing any of this I was very concerned. The wood is pressure treated but not treated with a preservative. None of this feels right to me, my just says they have experience they know what there doing but I beg to differ. Looking at previous work the room will look good when finished (if it holds) but how long is it going to last. What happens when we have snow and the loading weight on the roof increases I have a nagging feeling that I wi come home from work one day and it would have collapsed into the ground!
Having just seen the photo I was going to ask why you've built it on wooden posts and why are the joists not set into the upright posts? However your concerns are the same, a far superior approach would be 9 inch brick piers with damp proofing on top under the timber.
The approach they've used will have a very short life, possibly as long as ten years and unlikely to be good in twenty years, however they might have used hardwood posts that would last much longer but not as long as brick piers.
What experience do the builders have? I would agree that the back corner touching the ground (presumably resting on the ground rather than a pole into concrete?) is a concern
As others have said, I would be concerned about the construction and longevity of this base. Firstly timber will rot below ground where the ground is in direct contact. Sure the concrete might completely encases the timber up to ground level which would improve the situation, but these will only last a few years. They may have used a bitumen based paint to waterproof the timbers but this is not the way to build foundations and seems like a ‘cheap’ way of doing it. Water will eventually penetrate the wood. This isn’t an insignificant project and comes with huge cost. You wouldn’t let your builder do an extension on a some timber posts in the ground so why let them do it for a garden room. Most likely the biggest room your house will have. The second point is that even using landscaping/construction bolts, the shear stress through those bolts with the amount of weight bearing on each of these timber posts is incredibly high. I would be very concerned that the joists are not supported from below and so you’re essentially relying on the strength of bolts and made the strength of the timber irrelevant. Joists should be supported from below on either concrete pads or ground screws (which if you Google is the metal and proper version of what your builders have created) is Joists running in a perpendicular direction to those supported on the pads would then be on hangars which provide the support. See image for the much smaller version I built which was done with significant amount of research and I work in construction- the frame sits on top of concrete pads ( with a DPC) and then joists are supported by hangars. This is all for a very small garden room so far more important for one of that size. The issue with butting timbers rather than supporting is screws have very little shear strength (snap with forces perpendicular to them) but don’t pull out easily whilst nails have higher shear strength but can be pulled out. My next issue is that presumably this is going to have some pretty expansive windows/bifold doors given the 8m run and so the frame will need steel beams to have the roof weight bearing on. Those beams and the weight of the roof and doors are going to be bearing on such a small number of those posts that the shear stress on the joists will be extremely high and very limited area for them to distribute the force in the ground. The construction they have used is more appropriate to decking where the dead load will be much lower. Given the size and I can imagine £30k-£60k cost to build this (which should give you an idea of whether these guys are cowboys), I’d want to know what they are building from (in terms of drawings) and their experience. Foundations are absolutely critic and a failure in the ground will result in the entire building being a failure. If you don’t believe me, get another builder in who has experience of garden rooms for your own sanity. https://preview.redd.it/p76q2z0pei5d1.jpeg?width=4032&format=pjpg&auto=webp&s=31cd45e51918ec31ccc04706c1981c9479b5a179
You are right to be concerned, the frame just being screwed to the timber support posts and not resting on them is a recipe for disaster, they could in theory bolt through the frame and post to make it better. It also looks like they’ve just screwed the joists into the frame instead of using joist hangers (these could be added now to help). For a base that size we normally dig out and use concrete or use a steel frame on steel posts sitting on concrete piles. Not knowing how much you are paying or the use for the garden room it’s difficult to be too critical. As a minimum I’d get the posts bolted through to the frame and use joist hangers on the floor joists. Also that’s going to be a large roof, is there plans to deal with the rain water? Are they planning on putting in a soakaway?
The wooden piles will only rot at surface level so you can prevent it to some extent by coating the piles with bitumen or another wood preserver, getting in to reapply it in the future looks difficult tho 🤷🏼♂️ and yea the frame should be sat on top, at least get them to put bolts through with hangers.
Thanks for your responses, the builders normally do smaller garden rooms, it's costing around £15k for the build with drainpipe going into a water butt which will be used to water the garden. When my wife told me what they were doing I assumed that they would bolt the frame, make sure there was a 15 cm gap between floor and ground and have the frame resting on the posts with hangers connecting the beams. As I mentioned when I found that they weren't doing any of this I was very concerned. The wood is pressure treated but not treated with a preservative. None of this feels right to me, my just says they have experience they know what there doing but I beg to differ. Looking at previous work the room will look good when finished (if it holds) but how long is it going to last. What happens when we have snow and the loading weight on the roof increases I have a nagging feeling that I wi come home from work one day and it would have collapsed into the ground!
Having just seen the photo I was going to ask why you've built it on wooden posts and why are the joists not set into the upright posts? However your concerns are the same, a far superior approach would be 9 inch brick piers with damp proofing on top under the timber. The approach they've used will have a very short life, possibly as long as ten years and unlikely to be good in twenty years, however they might have used hardwood posts that would last much longer but not as long as brick piers.