roof box fitting on Pollensa

I'm thinking of fixing a small roof box to the roof of my 2003 Pollensa. 
Rather than trying to find a way to attach it using the existing 'roof rack' I'm wondering if I can set some bolts right through the roof so it sits flush on the roof. I'm not sure of the roof built-up, and where the best place for fixings would be, does anyone know? Is there plywood sandwiched in there somewhere?

A risky undertaking, I think. Have you considered a back box?

Tinwheeler wrote:A risky undertaking, I think. Have you considered a back box?

I've considered a back box but prefer to keep the van as short as possible. There's a convenient space on the roof between the rooflights and a perfect spot for a box to store folding chairs which take up too much room eleswhere. I know it's risky, but done properly should be fine :-)

Your choice, obviously, but it’s not something I’d contemplate. Don’t forget you’d increase your height which could have drawbacks and it might also adversely affect your resale value.

Tinwheeler wrote:Your choice, obviously, but it’s not something I’d contemplate. Don’t forget you’d increase your height which could have drawbacks and it might also adversely affect your resale value.

But only very slightly, the front of the van (over cab) is taller so another good reason to sit it down onto the roof :-)

Do you have the rear ladder already?

Simplicity and minimalism....bin the chairs!
Going, even up to, far less on most motorhome roofs is dodgy at best.
The monocoque such as mine have a reinforced patch between the surrounding rails, actually intended for what you want. Best use is a solar panel, where mine is.
By adding height to the roof you do, imho 3 bad things...

You add height to the van, think ferries

You add weight as high as is possible weight of box and, I suspect more than just chairs, never a good thing.

You add aerodynamic drag to a vehicle already withe the aerodynamics of a house brick!

There are many easier to store chairs than folding ones, next post will show what I mean...

Sorry if being, uncharacteristically negative but wouldn't remotely consider this route......

We have similar, fit easily in the overcab loft and are light enough not to worry even weight obsessed me!

No reason why it couldn't be done. Definitely a job that requires you to consider some things like additional height and weight. 
Plenty people add AirCon and other things, just about what you need/want and get it done properly. There will be pros and cons to many personal preference modifications we all do. Personally I'd not want to be climbing up on my roof too often. Even ladders and professional roof racks are adding a "stress" to the structure of the van that it may not be designed to cope with. Saying that! plenty have done it, so it's a case of doing it right, if you must. And while accessing roof and loading the box etc, you just be considerate and not go overboard.

Thanks for all the comments. My van is 2.92m at its highest at the front. I can get a 35cm high box which will still keep the van below 3m, as the 2.92 is the front overcab, and lower where I want to fit the box. I can easily access the box (end catch) from the ladder. When you have 4 people you need all the storage you can get, and somewhere to simply stash a couple of folding chairs is a;ll I need, nothing heavy.
All I need to know, is what's in the roof section for me to bolt through, but not sure where I can get that info as Autosleeper don't seem to want to help and don't answer emails.

When we px-ed our previous van to buy our Pollensa, we had the dealer add the same extras as we had on the old one, i.e. bike rack, awning and top box. They couldn't easily swap the box as it had been Sikaflexed onto the roof of the old van, so they supplied a brand new one which they simply attached using the red Fiamma straps through the box's lugs and onto the side roof rails. 

However we removed the box after only using it for a couple of times since we no longer needed the extra storage once our daughter stopped coming with us on holidays and we also found it easier to use a back box on the cycle rack to save going up on the roof.

It's still sitting on a shelf in our garage in Berkshire in virtually brand new condition if you're interested - it's one of the sizes I believe they no longer make, see below. 

If you wanted a more permanent fixing, personally I'd go down the Sikaflex route rather than drilling through the roof shell, but the straps seemed to work fine and made it easier to remove when not required.

Thanks all for your comments.
I finally managed to get through to autosleeper and was told the roof consisted of 5mm gel coat, then 3mm ply sandwiching foam. Not a great deal to fix to but I decided to use some table leg bases, with an m8 thread. I used 4 x 25mm stainless screws on each, with a generous helping of CT1 adhesive. I bought a second hand rear opening Halfords roof box which at around 62cm wide fit snugly between the two rear roof lights as if it was made for the job. I can now store all my folding seats and get to them easily from standing on the rear ladder. Seems really solid, and has so far survived our trip all the way to Bulgaria. I'd show some pictures for anyone interested, but can't see a way of adding pics to the post....