Underpinning 1900's End Terrace for Loft Conversion


Postby Very Foncused » Mon Aug 08, 2016 12:27 pm

Hi All,

I have recently purchased a 1900s end of terrace which needs complete gutting. As part of the works we are refurbishing the property but have decided against a loft conversion (not enough funds).

We want to be able to convert the loft later, ideally from hipped roof to gable end.

We dug some trial holes and we can see that the foundations are very shallow. The house sits on a clay soil, so not the best start. There are some settlement cracks but there is no evidence of subsidence. The structural engineer has said not to add any additional load to the existing foundations and instead get them underpinned if we are to loft convert.

Given the stigma around underpinning and potential issues for insurance and resale I would like to get some answers:
- Is it common to underpin foundations for a loft conversion?
- If I went for a dormer conversion (i.e. no hip to gable conversion) would I really be adding much more load?
- Should I underpin all walls including party wall and internal support walls?
- Given that its an end of terrace, is underpinning a good idea? i.e. are all the houses not moving and could I potentially be a hard point?

Your help is very much appreciated.

Thanks
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Postby trent900 » Mon Jan 09, 2017 3:41 pm

I'm only a humble homeowner like you, and this is months after your question. I am not an expert but I have done some DIY structural engineering work which went OK, so I'll be brave:

I have no idea how common underpinning for a loft conversion is. As I say, not a tradesman.

I guess there will be several reasons for the engineer's opinion: first, the change of use from unloaded ceiling to domestic floor means there are loadings you have to take into account from the Eurocodes. Regardless of whether these ever happen...you have to take them into account. For a domestic floor it is 1.5kN/m2 or roughly two people standing in each and every square metre at the same time. Second, if you convert hip to gable the obvious solution is a vertical masonry wall, which is quite weighty compared to the replaced roof structure. Third there will need to be calculations for the new roof - again back to the Eurocodes - oh wait, you need to assume snow loads, wind loads, maintenance loads, all probably far in excess of the loads the footings were sized for back in the day.

Part of the problem is that the loads actually are increasing, don't fool yourself that they're not.

But part of the problem is that simply by making the change, you are invoking the rigour of modern building regulations on your old structure. If you asked the same structural engineer 'would this house pass current building regs if I make no changes' the answer is undoubtedly no. So any change requiring re-calculation automatically gets expensive because of modern regs. So it's only simple for you to make renovations, not structural alterations.

I have the same issue with my house - a late Victorian semi. My saving grace was that when the loft conversion was originally done in the 90s it was passed as OK without underpinning, so when I added a dormer I managed to argue the floor loading wasn't changing.

If your floor joists aren't bearing onto the party wall you might be able to avoid underpinning it, I guess.

In terms of the sense of underpinning one house in a terrace I see your point but I would suggest that it's unlikely the row does move together as the masonry walls will not be strong enough to make that happen. So as the ground underneath varies, so will the movement. My grandparents owned a large semi built onto concrete raft foundations and the loads generated from one side slipping were great enough to snap the raft and crack all the walls. My point being that if your row has all moved together so far I think this is more luck than design, so I don't think you're losing much by underpinning one end. If the other houses crack away from yours - well, that's manageable. But the whole row should be pretty much stable by now I would have thought. Is the row pretty much square and straight?

All in all I suspect the engineer is right, underpinning would be required by regs. There may be some super lightweight roof structure that avoids this but I doubt it because regardless of the mass of the structure the imposed loads that need to be assumed are significant.

Given that, I would be amazed if the cost of underpinning plus conversion worked out cost-effective. You'd be doing it for love, not money. But love is a good reason to do work, if you like the house and the location!

Hope you end up with something you're happy with.
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