There will follow multiple posts on foundations for a tiny wooden building <groan>. It’s overkill, I know.
The normal caveats apply, namely that I have no qualifications in this area, so please do not do any actual building work based solely on the below.
Here is what I already knew about building conditions in the area: the subsoil in Redbourn is heavy clay (bad) and I am planning to build under a large ash tree (bad), hence I assumed that whatever I put up was sure to subside and eventually sink. It was this prospect that initiated my investigations into all things foundational.
Trees near houses get a lot of bad press, much of which is unwarranted.
It is true that tree roots can make already damaged drains worse by pushing into any gaps, but they do not possess the magical properties to pierce otherwise intact plastic and baked clay pipes.
It is true that the large structural roots near the surface can lift pavements and walls, but of course they need to be very close to the building for this to be a problem.
It is also true that trees can cause the moisture content of the ground to change, particularly in dry summers when they can take up considerable amounts of the available water in the area they occupy (the roots usual extend as a far as the canopy, but for some species much further). This is particularly a problem on clay soils because clay shrinks significantly when dry and this can cause changes in the height of the ground, resulting in cracks and ill fitting doors and windows and the like (and in some rare cases, worse).
Before you rush out and chop down all the trees in the vicinity, please keep in mind that the vast majority of trees growing near houses cause no problems at all, even on clay soils. Also note that if you chop down a tree you may well create a sudden change in ground conditions – this is because mature trees can retain a lot of water, even when not in leaf, and when this moisture content is eventually re-established the ground can heave upwards making existing problems worse. Consult an expert if you are worried.
Notwithstanding, all the above I happen to like trees, and besides Ash trees in Europe have enough problems of their own:
…so the Ash tree overhanging my garden is not going anywhere.
|An ash tree yesterday. In the past a naked child was passed through the split trunk of an ash in a ritual to cure a broken limb or rickets.|