Roofing options for bungalow with "room in roof" potential

[howplum]

Despite the hiccups with acquiring the land, I am still being optimistic and playing around with various floor plans. However, I have paid no attention to the roof design or construction, other than to include a proper staircase for access to the roof space, which would possibly be used for a bedroom and/or hobby room and/or storage and a shower room. In other words, should I be considering placing internal walls to support the bottom chord, or can the truss accommodate a wide span?

 

The footprint will be no more than 12 metres wide, but more likely 10, and 14 metres long, with a stepped design at the front to suit the curved plot frontage. The maximum ridge height allowed is 6 metres. Although the existing approved design includes a double hipped roof, I favour a simpler design with gables either end.

 

If I understand correctly, the main options are:

 

1. Attic trusses

2. Cut and pitch on site

3. SIPs

 

I did notice in an episode "The House that £100k Built" that a steel ridge beam was used, in conjunction with rafters cut on site, and I wondered what the pros and cons of that system are.

 

I am still undecided on the build system yet, which may be affected by the best roof option, or is that the tail wagging the dog?

 

In any event the roof space needs to be insulated, and heated, but as it would probably only be used occasionally, I was thinking of perhaps boxing in the stairs and fitting a door to avoid heat from downstairs rising to the roof space when it's not in use.

 

Any helpful information and opinions would be appreciated

 

 

 

 

[Mr Punter]

This is all so dependant on the design of the ground floor walls as you will need something load bearing for a 9.4m span or steel supports.  Maybe best to get the plot sorted, then a design for the house, then think about how best to build it.  Dormers will be far better for your resale value or even a 1 1/2 storey.

[ProDave]

Our house is built with a ridge beam (Kerto not steel) and the main advantage is it allows a warm roof design, so the whole of the interior space is inside the insulated envelope including any loft space not used as room in roof.

[Crofter]

12m is a pretty big span. Playing around with the interactive span calculator on the JJI website (https://www.jamesjones.co.uk/interactive-span-table) shows that you can only go up to 8m span, and that requires 450 deep joists at only 300 centres. Unless you have a completely open first floor it will almost certainly be worth having a supporting wall.

[Alexphd1]

We used a steel ridge beam with rafters cut on site all DIY.

advantage:- very cheap

disadvantage:- VERY time consuming

 

If paying a company to cut, fabricate and erect the steel, I cant see it being a cheap option.

 

Edited October 27, 2018 by Alexphd1

[Jeremy Harris]

Our house also has a big laminated timber ridge beam (~450mm deep) that runs the length of the house, supported by both end gable walls and two internal walls, 2m apart, 1m either side of the longitudinal centreline.  We have 403mm deep rafters, hung from the ridge beam down to the top of the walls.  The rafter pitch is 45 deg, which gives us an external appearance that's close to that of a bungalow (which is all the planners were inclined to approve), yet gives us around 4m of height in the vaulted ceiling first floor rooms, plus is gives us a 6.5m high vaulted ceiling in the entrance hall, that definitely makes a relatively small house seem a lot larger and more airy inside.  There are some photos on our blog, along with the whole build tale: http://www.mayfly.eu/

[Thedreamer]

We have prefabricated attic trusses for the two gable rooms upstairs.

 

In the middle we have two structural timber walls with a Kerto ridge beam in the ceiling and a steel beam below which allows for a partly vaulted living room ceiling.

 

With the use of a telehandler both the timber beam and steel beam were fitted very quickly perhaps a day or two.

 

As mentioned by others above, the span dimension seems a bit to long?

 

Here is a photo which shows the structural bones of the house.

 

 

[Ferdinand]

29 minutes ago, Thedreamer said:

We have prefabricated attic trusses for the two gable rooms upstairs.

 

In the middle we have two structural timber walls with a Kerto ridge beam in the ceiling and a steel beam below which allows for a partly vaulted living room ceiling.

 

With the use of a telehandler both the timber beam and steel beam were fitted very quickly perhaps a day or two.

 

As mentioned by others above, the span dimension seems a bit to long?

 

Here is a photo which shows the structural bones of the house.

 

 

 

You  don't appear to have much confidence in them staying up !

 

(Or are you due to do Hercules in amdram?)

[Thedreamer]

13 minutes ago, Ferdinand said:

 

You  don't appear to have much confidence in them staying up !

 

(Or are you due to do Hercules in amdram?)

 

Honesty,  did'nt enjoy walking on joists so holding on to the rafters!

 

That fear soon disappeared when painting the apex of the sofits and fascias.

[howplum]

Thank you everybody for your comments, information and insights. Having looked at the Kerto beam span tables it would seem there is a balance to be achieved between between span, beam depth and spacing, but a span of 5 - 6 metres would not require overly deep floor beams for the roof space, for instance. Obviously these issues will be addressed properly at the drawing stage, but at the moment I don't want to come up with an "ideal" plan to then find it is not technically viable, at least at a reasonable cost. Another option for the floor joists might be the metal web type, which at least make running services a lot easier.

 

As for the ridge beam, I can see that at least one load bearing support would be needed for a 14 metre length. However, I do have a further novice's question: what is the primary function of a ridge beam?

[PeterW]

A ridge beam takes partial loads from the roof structure and applies them to its bearing points - in this case the gable ends or any intermediate walls that reach roof height. 

 

The bigger issue you you will have with a wide low pitch roof is twofold - firstly you will have a problem with the amount of natural light in the centre of the building unless you open up to the roof and insert rooflights; and secondly you will need to work out how to stop the walls spreading out.

 

Where a ceiling joist acts as the bottom chord of a truss, it’s partially in tension to stop the rafter ends spreading. You can do this higher up with a collar tie, but the higher you go, the deeper the tie and the deeper the rafters needed too. 

 

You can get round the light issue using a celestory window and I’ve seen some really nice big bungalows converted in this way, or alternatively do as a number have done here and put full height atrium sections in the centre of the build. 

[howplum]

Thank you @Peterw, now I understand. I'm guessing the metal web joists could not work too well as the bottom chord of a truss, since I assume they might not be designed to work under tension.

 

A friend of mine suggested what I now know to be celestory windows, but the bungalow will be next to a conservation site, and from the comments on the current PP I suspect that nothing too eyecatching will be allowed.  However, the atrium, or light well, is an excellent idea which could illuminate both the ground floor and roof space without using too much space.

 

 

[Jeremy Harris]

We have Posijoists and a big ridge beam supporting deep I beam rafters for our room-in-roof design.  Works well, but does need a deep ridge beam to take the load of the rafters hanging from it.  With this design there are no tensile loads transmitted to the first floor joists, as the rafters hang from the ridge beam.  Some photos:

 

 

[Declan52]

The main difference in using attic truss and a cut roof is with a cut roof you will have the ability to bring your insulation and any membrane you are using from one wall plate up and down and  over to the other side with no breaks.  

With attic truss you will be left with the three triangle areas, red areas in pic,  which will be cold areas as they are outside the standard insulation area. Your membrane will go where the green line is on the pic. 

You can of course go for a warm roof and then all the insulation is between and above the truss. 

The  only other thing to remember is a cut roof is only as good as the  joiner cutting it. Attic truss are precision  factory made. 

Both types of roofs have their advantages and disadvantages and each will require specific detailing with regards insulation and membranes. 

 

 

[PeterW]

29 minutes ago, howplum said:

Thank you @Peterw, now I understand. I'm guessing the metal web joists could not work too well as the bottom chord of a truss, since I assume they might not be designed to work under tension.

 

A friend of mine suggested what I now know to be celestory windows, but the bungalow will be next to a conservation site, and from the comments on the current PP I suspect that nothing too eyecatching will be allowed.  However, the atrium, or light well, is an excellent idea which could illuminate both the ground floor and roof space without using too much space.

 

 

 

DWB (who I use) make trusses with metal web bottom chords as pretty much standard now for their attic trusses. It’s a useful detail and if I had known I would have had them make all the roof trusses with an integral stub wall. As it is they just provided the joists for the last build. 

[howplum]

Every question opens up a whole raft of new terminology and knowledge to a novice, which will all come in very useful when talking to a designer, so that hopefully I don't get blinded by science.

 

I can certainly see there are pros and cons for every system, so food for thought indeed.