Loft conversion - calculating volume

[Claref]

Hi, I am planning a loft conversion - dormer - and have had plans drawn up which, depending on where I calculate measurements, either give me 49 cubic metres or 53.

The discrepancy arises because the house has had roof extensions by previous owners, who turned a sloping roof where two sloping sides met into a gable end. If I calculate the volume of this addition as if it was a cube then halve it and halve it again I get a greater volume than calculating the area as a triangle pyramid - which is the correct volume or calculation I should use?

Where I measure from - including the new dormer - will also make a difference - is it to the furthest point of the roof or where the point at which the roof meets walls?

Also, a garage which was attached to the house but not part of the house has been integrated into the house as an additional room, this involved raising the ceiling height of the garage and adding a pitched roof to match existing - do I include the additional roof space into my calculations for PD purposes? (FYI I have done so).

Furthermore, as both additional roof spaces had planning permission can they discounted from PD calculations anyway?

Be really grateful if someone can clarify my situation. There is no way my local planners (mid sussex) will grant me permission for a large flat roof dormer on my chalet bungalow - I've already discussed it with them -so its crucial I get my calculations right! (The plans were drawn up by an architect who says we are at the limit but as I'm the one with the bill at the end of the day just want to check and double double check!).

Many thanks....

 

[freddiemercurystwin]

I don't quite follow exactly what you're saying about where the two roofs meet.

The volume is the the additional volume created by you and any previous owners not just the new created by you. The volume is the habitable volume created.

I believe the loft volume can be in addition to any other space created eg the garage conversion.

In any case, if the rules appear to be slightly grey you should, without doubt, make no mistake, without a doubt, mad if you don't, get a certificate of lawful development.

The grey area you have delved into ie measuring loft volumes with regard to permitted development is without doubt an extremely grey area and you would be completely mad if you don't apply for a certificate of lawful development. The cost to yourself for having your architect draw up an application should be around £150 inc the £75 local authority fee. A certificate in your hand is priceless!

 

[Claref]

Thanks for this info. The planning dept are really unhelpful, I have been to visit them on three occasions but they manage to obfuscate every time!

If it is the case that the additional roof space created above the garage wont count in the 50 cubic metres allowed by PD then I have no problem at all since the garage roof space accounts for 22 cubic metres. Does it make any difference that this roof space now forms part of the house and has a shower room in it.

Can you confirm this is the case? Or tell me where I need to go to get confirmation?

Re the two sloping roofs - odd I know and it looks weird - but two sloping roofs joined above where the side wall of the house meets the rear wall of the house and kind of filled in where there would have been a gap - they are evident on my next door neighbours once identical bungalow.

thanks for prompt response

 

[freddiemercurystwin]

The grey area you have delved into ie measuring loft volumes with regard to permitted development is without doubt an extremely grey area and you would be completely mad if you don't apply for a certificate of lawful development. The cost to yourself for having your architect draw up an application should be around £150 inc the £75 local authority fee. A certificate in your hand is priceless!

If you were my client I would not be so bold as to categorically tell you that your development came under permitted development. It is a very grey area that you are questioning and a certificate of lawful development is the route to go. On the forms the architect will justify why the application is permitted development. The rules for permitted development are (unfortunately) in some respects very vague, as you have experienced the planners themselves have trouble understanding/explaining the rules hence there are often few other alternatives than applying for a certificate.

 

[Claref]

As far as I can establish PD is very clear about the allowance of 50 cubic metres. I am within this if I work out the roof space volume by using the mathematical formula for calculating the area of a pyramid. If planners use this calculation for working out the additional volume for adding a gable end onto a sloping roof then I have no problem.

My question then is, do planners use this formula when calculating changing a sloped roof to a gable end?

Many thanks,

 

[freddiemercurystwin]

The volume of a loft is all of the open air within the habitable loft space created by the conversion. They use the appropriate mathematical formulae required. If the correct mathematical process is followed the volume will be arrived at. If there are different methods of working out the overall volume as there can be and they are carried out correctly you will arrive at exactly the same figure.

 

[Claref]

Thanks so much for your help. Can I pick your brains one last time? Re the calculation, you would think it would be the same whichever method used but its not so clearly I am doing something wrong. Perhaps you can help me again?

The roof space I am calculating has a dimension of 6m wide, the apex of the roof was brought forward from the pre-existing apex by 3m, no extra floor space has been added, the floor to apex height is 3.2m

Using the calculation for a three sided pyramid: 6m x 3m = 18m x .5 = 9 x 3.2 = 28.8 x .3 = 8.64 cubic metres.

However, if I calculate the area as a cube, ie 6 x 3 x 3.2 = 57.6
divide this by 2 then by half again (since half this roof space was already in existance) the answer is 14.4 cubic metres.

Can you tell me which is correct?!

 

[freddiemercurystwin]

Err, I'm a little confused, you're working out the volume for your loft and treating it as a pyramid like this:

Untitled

• freddiemercurystwin
• 13 Jun 2010
• 1

Using the calculation for a three sided pyramid: 6m x 3m = 18m x .5 = 9 x 3.2 = 28.8 x .3 = 8.64 cubic metres.

Yet here, you're working out the volume for your loft treating it as a shape like this:

Untitled

• freddiemercurystwin
• 13 Jun 2010
• 1

However, if I calculate the area as a cube, ie 6 x 3 x 3.2 = 57.6
divide this by 2 then by half again (since half this roof space was already in existance) the answer is 14.4 cubic metres.

and you wonder why you're getting different results?

Or have I lost the plot?

 

[Claref]

Hello again, thanks for your patience!

Sorry, this is all new to me! The calculation I gave was for the gable end addition created by the previous owners.

So, originally the roof was standard pitched and they altered it to make a gable end - which were the calculations I gave of 6 x 3 x 3.5 - basically the shape added is a three sided pyramid.

Does this make sense?

 

[freddiemercurystwin]

Err, still slightly confused anyhoo am I right in thinking you've added the bit in blue on my sketch and the dims?

Untitled

• freddiemercurystwin
• 13 Jun 2010
• 1

 

[Claref]

Wish I could do the drawings - anyway no its not like that!

Looking at your last diagram - if you remove the blue part, imagine drawing a line back along the ridge of the roof 3m, drawing a line from this point to where the edge of the floor and front wall would meet this makes the pyramid shape.

The widest point of 6m is from side to side as you look at the gable end.

Does this make it any clearer?

 

[freddiemercurystwin]

What about this one?

Untitled

• freddiemercurystwin
• 13 Jun 2010
• 1

 

[TicklyT]

Using the calculation for a three sided pyramid: 6m x 3m = 18m x .5 = 9 x 3.2 = 28.8 x .3 = 8.64 cubic metres

Not quite sure where that comes from. The formula for the volume of a pyramid is 1/3 X B X h, not 0.3 X B X h.

And it appears you are subtracting a tetrahedral section from a prism enclosing the roof to allow for the hip at one end. the dimensions are flawed unless you allow for the length (height?) of the triangular 'base' in three dimensions (the length of the sides will be longer when projected onto a plane parallel to the plane of the roof).

From the figures you have given, I suggest considering each half of the roof seperately.


As the volume of a pyramid is 1/3 B X h, where B is the area of the base, and h is the height - that works for a pyramid with any shaped base.

The original hipped roof would have had a volume of 1/3 X 6M X 6M X 3.2 M = 38.4M^3


Splitting the present roof into two volumes,we have:-
A pyramid, 3M X 6M X 3.2M
And a prism, 3M X 6M X 3.2M.

For the pyramid, B= (3M X 6M ), and h = 3.2M
the volume of the pyramid is 1/3 X 6M X 3M X 3.2M
=19.2M^3

For the prismatic section, the area of the triangular cross-section is 6M X 3.2M / 2 = 9.6 M^2 (the area of a triangle),
multiplied by the length, 9.6 M^2 X 3M
=28.8M^3

Adding the two volumes together gives a total volume for the roof of 28.8M^3 + 19.2M^3 = 48M^3